Fix SSO redirect failing with htmx login form

The login form uses hx-post, so htmx sends the POST via fetch. A 302
redirect to the cross-origin service callback URL fails silently because
fetch follows the redirect but gets blocked by CORS. Use HX-Redirect
header instead, which tells htmx to perform a full page navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gen/mcias/v1/account.pb.go

@@ -4,7 +4,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/account.proto
 
 package mciasv1
@@ -1080,7 +1080,7 @@ const file_mcias_v1_account_proto_rawDesc = "" +
 	"\n" +
 	"GetPGCreds\x12\x1b.mcias.v1.GetPGCredsRequest\x1a\x1c.mcias.v1.GetPGCredsResponse\x12G\n" +
 	"\n" +
-	"SetPGCreds\x12\x1b.mcias.v1.SetPGCredsRequest\x1a\x1c.mcias.v1.SetPGCredsResponseB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"SetPGCreds\x12\x1b.mcias.v1.SetPGCredsRequest\x1a\x1c.mcias.v1.SetPGCredsResponseB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_account_proto_rawDescOnce sync.Once

gen/mcias/v1/account_grpc.pb.go

@@ -4,7 +4,7 @@
 // Code generated by protoc-gen-go-grpc. DO NOT EDIT.
 // versions:
 // - protoc-gen-go-grpc v1.6.1
-// - protoc             v3.20.3
+// - protoc             v6.32.1
 // source: mcias/v1/account.proto
 
 package mciasv1

gen/mcias/v1/admin.pb.go

@@ -4,7 +4,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/admin.proto
 
 package mciasv1
@@ -238,7 +238,7 @@ const file_mcias_v1_admin_proto_rawDesc = "" +
 	"\x01x\x18\x05 \x01(\tR\x01x2\x9a\x01\n" +
 	"\fAdminService\x12;\n" +
 	"\x06Health\x12\x17.mcias.v1.HealthRequest\x1a\x18.mcias.v1.HealthResponse\x12M\n" +
-	"\fGetPublicKey\x12\x1d.mcias.v1.GetPublicKeyRequest\x1a\x1e.mcias.v1.GetPublicKeyResponseB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"\fGetPublicKey\x12\x1d.mcias.v1.GetPublicKeyRequest\x1a\x1e.mcias.v1.GetPublicKeyResponseB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_admin_proto_rawDescOnce sync.Once

gen/mcias/v1/admin_grpc.pb.go

@@ -4,7 +4,7 @@
 // Code generated by protoc-gen-go-grpc. DO NOT EDIT.
 // versions:
 // - protoc-gen-go-grpc v1.6.1
-// - protoc             v3.20.3
+// - protoc             v6.32.1
 // source: mcias/v1/admin.proto
 
 package mciasv1

gen/mcias/v1/auth.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/auth.proto
 
 package mciasv1
@@ -919,7 +919,7 @@ const file_mcias_v1_auth_proto_rawDesc = "" +
 	"\n" +
 	"RemoveTOTP\x12\x1b.mcias.v1.RemoveTOTPRequest\x1a\x1c.mcias.v1.RemoveTOTPResponse\x12n\n" +
 	"\x17ListWebAuthnCredentials\x12(.mcias.v1.ListWebAuthnCredentialsRequest\x1a).mcias.v1.ListWebAuthnCredentialsResponse\x12q\n" +
-	"\x18RemoveWebAuthnCredential\x12).mcias.v1.RemoveWebAuthnCredentialRequest\x1a*.mcias.v1.RemoveWebAuthnCredentialResponseB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"\x18RemoveWebAuthnCredential\x12).mcias.v1.RemoveWebAuthnCredentialRequest\x1a*.mcias.v1.RemoveWebAuthnCredentialResponseB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_auth_proto_rawDescOnce sync.Once

gen/mcias/v1/auth_grpc.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go-grpc. DO NOT EDIT.
 // versions:
 // - protoc-gen-go-grpc v1.6.1
-// - protoc             v3.20.3
+// - protoc             v6.32.1
 // source: mcias/v1/auth.proto
 
 package mciasv1

gen/mcias/v1/common.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/common.proto
 
 package mciasv1
@@ -349,7 +349,7 @@ const file_mcias_v1_common_proto_rawDesc = "" +
 	"\x04port\x18\x05 \x01(\x05R\x04port\"5\n" +
 	"\x05Error\x12\x18\n" +
 	"\amessage\x18\x01 \x01(\tR\amessage\x12\x12\n" +
-	"\x04code\x18\x02 \x01(\tR\x04codeB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"\x04code\x18\x02 \x01(\tR\x04codeB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_common_proto_rawDescOnce sync.Once

gen/mcias/v1/policy.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/policy.proto
 
 package mciasv1
@@ -703,7 +703,7 @@ const file_mcias_v1_policy_proto_rawDesc = "" +
 	"\x10CreatePolicyRule\x12!.mcias.v1.CreatePolicyRuleRequest\x1a\".mcias.v1.CreatePolicyRuleResponse\x12P\n" +
 	"\rGetPolicyRule\x12\x1e.mcias.v1.GetPolicyRuleRequest\x1a\x1f.mcias.v1.GetPolicyRuleResponse\x12Y\n" +
 	"\x10UpdatePolicyRule\x12!.mcias.v1.UpdatePolicyRuleRequest\x1a\".mcias.v1.UpdatePolicyRuleResponse\x12Y\n" +
-	"\x10DeletePolicyRule\x12!.mcias.v1.DeletePolicyRuleRequest\x1a\".mcias.v1.DeletePolicyRuleResponseB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"\x10DeletePolicyRule\x12!.mcias.v1.DeletePolicyRuleRequest\x1a\".mcias.v1.DeletePolicyRuleResponseB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_policy_proto_rawDescOnce sync.Once

gen/mcias/v1/policy_grpc.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go-grpc. DO NOT EDIT.
 // versions:
 // - protoc-gen-go-grpc v1.6.1
-// - protoc             v3.20.3
+// - protoc             v6.32.1
 // source: mcias/v1/policy.proto
 
 package mciasv1

gen/mcias/v1/token.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.11
-// 	protoc        v3.20.3
+// 	protoc        v6.32.1
 // source: mcias/v1/token.proto
 
 package mciasv1
@@ -346,7 +346,7 @@ const file_mcias_v1_token_proto_rawDesc = "" +
 	"\fTokenService\x12P\n" +
 	"\rValidateToken\x12\x1e.mcias.v1.ValidateTokenRequest\x1a\x1f.mcias.v1.ValidateTokenResponse\x12\\\n" +
 	"\x11IssueServiceToken\x12\".mcias.v1.IssueServiceTokenRequest\x1a#.mcias.v1.IssueServiceTokenResponse\x12J\n" +
-	"\vRevokeToken\x12\x1c.mcias.v1.RevokeTokenRequest\x1a\x1d.mcias.v1.RevokeTokenResponseB2Z0git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
+	"\vRevokeToken\x12\x1c.mcias.v1.RevokeTokenRequest\x1a\x1d.mcias.v1.RevokeTokenResponseB0Z.git.wntrmute.dev/mc/mcias/gen/mcias/v1;mciasv1b\x06proto3"
 
 var (
 	file_mcias_v1_token_proto_rawDescOnce sync.Once

gen/mcias/v1/token_grpc.pb.go

@@ -3,7 +3,7 @@
 // Code generated by protoc-gen-go-grpc. DO NOT EDIT.
 // versions:
 // - protoc-gen-go-grpc v1.6.1
-// - protoc             v3.20.3
+// - protoc             v6.32.1
 // source: mcias/v1/token.proto
 
 package mciasv1

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/crypto v0.49.0
 	google.golang.org/grpc v1.79.3
-	google.golang.org/protobuf v1.36.10
+	google.golang.org/protobuf v1.36.11
 	modernc.org/sqlite v1.47.0
 )
 

go.sum

@@ -100,8 +100,8 @@ google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 h1:
 google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217/go.mod h1:7i2o+ce6H/6BluujYR+kqX3GKH+dChPTQU19wjRPiGk=
 google.golang.org/grpc v1.79.3 h1:sybAEdRIEtvcD68Gx7dmnwjZKlyfuc61Dyo9pGXXkKE=
 google.golang.org/grpc v1.79.3/go.mod h1:KmT0Kjez+0dde/v2j9vzwoAScgEPx/Bw1CYChhHLrHQ=
-google.golang.org/protobuf v1.36.10 h1:AYd7cD/uASjIL6Q9LiTjz8JLcrh/88q5UObnmY3aOOE=
+google.golang.org/protobuf v1.36.11 h1:fV6ZwhNocDyBLK0dj+fg8ektcVegBBuEolpbTQyBNVE=
-google.golang.org/protobuf v1.36.10/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
+google.golang.org/protobuf v1.36.11/go.mod h1:HTf+CrKn2C3g5S8VImy6tdcUvCska2kB7j23XfzDpco=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

internal/config/config.go

@@ -22,6 +22,24 @@ type Config struct { //nolint:govet // fieldalignment: TOML section order is mor
 	Tokens    TokensConfig    `toml:"tokens"`
 	Argon2    Argon2Config    `toml:"argon2"`
 	WebAuthn  WebAuthnConfig  `toml:"webauthn"`
+	SSO       SSOConfig       `toml:"sso"`
+}
+
+// SSOConfig holds registered SSO clients that may use the authorization code
+// flow to authenticate users via MCIAS. Omitting the [sso] section or leaving
+// clients empty disables SSO.
+type SSOConfig struct {
+	Clients []SSOClient `toml:"clients"`
+}
+
+// SSOClient is a registered relying-party application that may redirect users
+// to MCIAS for login. The redirect_uri is validated as an exact match (no
+// wildcards) to prevent open-redirect attacks.
+type SSOClient struct {
+	ClientID    string   `toml:"client_id"`    // unique identifier (e.g. "mcr")
+	RedirectURI string   `toml:"redirect_uri"` // exact callback URL, https required
+	ServiceName string   `toml:"service_name"` // passed to policy engine on login
+	Tags        []string `toml:"tags"`         // passed to policy engine on login
 }
 
 // WebAuthnConfig holds FIDO2/WebAuthn settings. Omitting the entire [webauthn]
@@ -246,9 +264,48 @@ func (c *Config) validate() error {
 		}
 	}
 
+	// SSO clients — if any are configured, each must have a unique client_id,
+	// a non-empty redirect_uri with the https:// scheme, and a non-empty
+	// service_name.
+	seen := make(map[string]bool, len(c.SSO.Clients))
+	for i, cl := range c.SSO.Clients {
+		prefix := fmt.Sprintf("sso.clients[%d]", i)
+		if cl.ClientID == "" {
+			errs = append(errs, fmt.Errorf("%s: client_id is required", prefix))
+		} else if seen[cl.ClientID] {
+			errs = append(errs, fmt.Errorf("%s: duplicate client_id %q", prefix, cl.ClientID))
+		} else {
+			seen[cl.ClientID] = true
+		}
+		if cl.RedirectURI == "" {
+			errs = append(errs, fmt.Errorf("%s: redirect_uri is required", prefix))
+		} else if !strings.HasPrefix(cl.RedirectURI, "https://") {
+			errs = append(errs, fmt.Errorf("%s: redirect_uri must use the https:// scheme (got %q)", prefix, cl.RedirectURI))
+		}
+		if cl.ServiceName == "" {
+			errs = append(errs, fmt.Errorf("%s: service_name is required", prefix))
+		}
+	}
+
 	return errors.Join(errs...)
 }
 
+// SSOClient looks up a registered SSO client by client_id.
+// Returns nil if no client with that ID is registered.
+func (c *Config) SSOClient(clientID string) *SSOClient {
+	for i := range c.SSO.Clients {
+		if c.SSO.Clients[i].ClientID == clientID {
+			return &c.SSO.Clients[i]
+		}
+	}
+	return nil
+}
+
+// SSOEnabled reports whether any SSO clients are registered.
+func (c *Config) SSOEnabled() bool {
+	return len(c.SSO.Clients) > 0
+}
+
 // DefaultExpiry returns the configured default token expiry duration.
 func (c *Config) DefaultExpiry() time.Duration { return c.Tokens.DefaultExpiry.Duration }
 

internal/config/config_test.go

@@ -244,6 +244,153 @@ func TestTrustedProxyValidation(t *testing.T) {
 	}
 }
 
+func TestSSOClientValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		extra   string
+		wantErr bool
+	}{
+		{
+			name: "valid single client",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+tags         = ["env:restricted"]
+`,
+			wantErr: false,
+		},
+		{
+			name: "valid multiple clients",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+
+[[sso.clients]]
+client_id    = "mcat"
+redirect_uri = "https://mcat.example.com/sso/callback"
+service_name = "mcat"
+`,
+			wantErr: false,
+		},
+		{
+			name: "missing client_id",
+			extra: `
+[[sso.clients]]
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "missing redirect_uri",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "http redirect_uri rejected",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "http://mcr.example.com/sso/callback"
+service_name = "mcr"
+`,
+			wantErr: true,
+		},
+		{
+			name: "missing service_name",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+`,
+			wantErr: true,
+		},
+		{
+			name: "duplicate client_id",
+			extra: `
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://other.example.com/sso/callback"
+service_name = "mcr2"
+`,
+			wantErr: true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			path := writeTempConfig(t, validConfig()+tc.extra)
+			_, err := Load(path)
+			if tc.wantErr && err == nil {
+				t.Error("expected validation error, got nil")
+			}
+			if !tc.wantErr && err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}
+
+func TestSSOClientLookup(t *testing.T) {
+	path := writeTempConfig(t, validConfig()+`
+[[sso.clients]]
+client_id    = "mcr"
+redirect_uri = "https://mcr.example.com/sso/callback"
+service_name = "mcr"
+tags         = ["env:restricted"]
+`)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	cl := cfg.SSOClient("mcr")
+	if cl == nil {
+		t.Fatal("SSOClient(mcr) returned nil")
+	}
+	if cl.RedirectURI != "https://mcr.example.com/sso/callback" {
+		t.Errorf("RedirectURI = %q", cl.RedirectURI)
+	}
+	if cl.ServiceName != "mcr" {
+		t.Errorf("ServiceName = %q", cl.ServiceName)
+	}
+	if len(cl.Tags) != 1 || cl.Tags[0] != "env:restricted" {
+		t.Errorf("Tags = %v", cl.Tags)
+	}
+
+	if cfg.SSOClient("nonexistent") != nil {
+		t.Error("SSOClient(nonexistent) should return nil")
+	}
+
+	if !cfg.SSOEnabled() {
+		t.Error("SSOEnabled() should return true")
+	}
+}
+
+func TestSSODisabledByDefault(t *testing.T) {
+	path := writeTempConfig(t, validConfig())
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.SSOEnabled() {
+		t.Error("SSOEnabled() should return false with no clients")
+	}
+}
+
 func TestDurationParsing(t *testing.T) {
 	var d duration
 	if err := d.UnmarshalText([]byte("1h30m")); err != nil {

internal/model/model.go

@@ -218,6 +218,9 @@ const (
 	EventWebAuthnRemoved   = "webauthn_removed"
 	EventWebAuthnLoginOK   = "webauthn_login_ok"
 	EventWebAuthnLoginFail = "webauthn_login_fail"
+
+	EventSSOAuthorize = "sso_authorize"
+	EventSSOLoginOK   = "sso_login_ok"
 )
 
 // ServiceAccountDelegate records that a specific account has been granted

internal/server/handlers_sso.go

@@ -0,0 +1,141 @@
+package server
+
+import (
+	"net/http"
+
+	"git.wntrmute.dev/mc/mcias/internal/audit"
+	"git.wntrmute.dev/mc/mcias/internal/middleware"
+	"git.wntrmute.dev/mc/mcias/internal/model"
+	"git.wntrmute.dev/mc/mcias/internal/policy"
+	"git.wntrmute.dev/mc/mcias/internal/sso"
+	"git.wntrmute.dev/mc/mcias/internal/token"
+)
+
+// ssoTokenRequest is the request body for POST /v1/sso/token.
+type ssoTokenRequest struct {
+	Code        string `json:"code"`
+	ClientID    string `json:"client_id"`
+	RedirectURI string `json:"redirect_uri"`
+}
+
+// handleSSOTokenExchange exchanges an SSO authorization code for a JWT token.
+//
+// Security design:
+//   - The authorization code is single-use (consumed via LoadAndDelete).
+//   - The client_id and redirect_uri must match the values stored when the code
+//     was issued, preventing a stolen code from being exchanged by a different
+//     service.
+//   - Policy evaluation uses the service_name and tags from the registered SSO
+//     client config (not from the request), preventing identity spoofing.
+//   - The code expires after 60 seconds to limit the interception window.
+func (s *Server) handleSSOTokenExchange(w http.ResponseWriter, r *http.Request) {
+	var req ssoTokenRequest
+	if !decodeJSON(w, r, &req) {
+		return
+	}
+
+	if req.Code == "" || req.ClientID == "" || req.RedirectURI == "" {
+		middleware.WriteError(w, http.StatusBadRequest, "code, client_id, and redirect_uri are required", "bad_request")
+		return
+	}
+
+	// Consume the authorization code (single-use).
+	ac, ok := sso.Consume(req.Code)
+	if !ok {
+		middleware.WriteError(w, http.StatusUnauthorized, "invalid or expired authorization code", "invalid_code")
+		return
+	}
+
+	// Security: verify client_id and redirect_uri match the stored values.
+	if ac.ClientID != req.ClientID || ac.RedirectURI != req.RedirectURI {
+		s.logger.Warn("sso: token exchange parameter mismatch",
+			"expected_client", ac.ClientID, "got_client", req.ClientID)
+		middleware.WriteError(w, http.StatusUnauthorized, "invalid or expired authorization code", "invalid_code")
+		return
+	}
+
+	// Look up the registered SSO client for policy context.
+	client := s.cfg.SSOClient(req.ClientID)
+	if client == nil {
+		// Should not happen if the authorize endpoint validated, but defend in depth.
+		middleware.WriteError(w, http.StatusUnauthorized, "unknown client", "invalid_code")
+		return
+	}
+
+	// Load account.
+	acct, err := s.db.GetAccountByID(ac.AccountID)
+	if err != nil {
+		s.logger.Error("sso: load account for token exchange", "error", err, "account_id", ac.AccountID)
+		middleware.WriteError(w, http.StatusInternalServerError, "internal error", "internal_error")
+		return
+	}
+
+	if acct.Status != model.AccountStatusActive {
+		middleware.WriteError(w, http.StatusForbidden, "account is not active", "account_inactive")
+		return
+	}
+
+	// Load roles for policy evaluation and expiry decision.
+	roles, err := s.db.GetRoles(acct.ID)
+	if err != nil {
+		middleware.WriteError(w, http.StatusInternalServerError, "internal error", "internal_error")
+		return
+	}
+
+	// Policy evaluation with the SSO client's service_name and tags.
+	{
+		input := policy.PolicyInput{
+			Subject:     acct.UUID,
+			AccountType: string(acct.AccountType),
+			Roles:       roles,
+			Action:      policy.ActionLogin,
+			Resource: policy.Resource{
+				ServiceName: client.ServiceName,
+				Tags:        client.Tags,
+			},
+		}
+		if effect, _ := s.polEng.Evaluate(input); effect == policy.Deny {
+			s.writeAudit(r, model.EventLoginFail, &acct.ID, nil,
+				audit.JSON("reason", "policy_deny", "service_name", client.ServiceName, "via", "sso"))
+			middleware.WriteError(w, http.StatusForbidden, "access denied by policy", "policy_denied")
+			return
+		}
+	}
+
+	// Determine expiry.
+	expiry := s.cfg.DefaultExpiry()
+	for _, rol := range roles {
+		if rol == "admin" {
+			expiry = s.cfg.AdminExpiry()
+			break
+		}
+	}
+
+	privKey, err := s.vault.PrivKey()
+	if err != nil {
+		middleware.WriteError(w, http.StatusServiceUnavailable, "vault sealed", "vault_sealed")
+		return
+	}
+	tokenStr, claims, err := token.IssueToken(privKey, s.cfg.Tokens.Issuer, acct.UUID, roles, expiry)
+	if err != nil {
+		s.logger.Error("sso: issue token", "error", err)
+		middleware.WriteError(w, http.StatusInternalServerError, "internal error", "internal_error")
+		return
+	}
+
+	if err := s.db.TrackToken(claims.JTI, acct.ID, claims.IssuedAt, claims.ExpiresAt); err != nil {
+		s.logger.Error("sso: track token", "error", err)
+		middleware.WriteError(w, http.StatusInternalServerError, "internal error", "internal_error")
+		return
+	}
+
+	s.writeAudit(r, model.EventSSOLoginOK, &acct.ID, nil,
+		audit.JSON("jti", claims.JTI, "client_id", client.ClientID))
+	s.writeAudit(r, model.EventTokenIssued, &acct.ID, nil,
+		audit.JSON("jti", claims.JTI, "via", "sso"))
+
+	writeJSON(w, http.StatusOK, loginResponse{
+		Token:     tokenStr,
+		ExpiresAt: claims.ExpiresAt.Format("2006-01-02T15:04:05Z"),
+	})
+}

internal/server/server.go

@@ -215,6 +215,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /v1/health", s.handleHealth)
 	mux.HandleFunc("GET /v1/keys/public", s.handlePublicKey)
 	mux.Handle("POST /v1/auth/login", loginRateLimit(http.HandlerFunc(s.handleLogin)))
+	mux.Handle("POST /v1/sso/token", loginRateLimit(http.HandlerFunc(s.handleSSOTokenExchange)))
 	mux.Handle("POST /v1/token/validate", loginRateLimit(http.HandlerFunc(s.handleTokenValidate)))
 
 	// API documentation: Swagger UI at /docs and raw spec at /docs/openapi.yaml.

internal/sso/session.go

@@ -0,0 +1,91 @@
+package sso
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"git.wntrmute.dev/mc/mcias/internal/crypto"
+)
+
+const (
+	sessionTTL   = 5 * time.Minute
+	sessionBytes = 16 // 128 bits of entropy for the nonce
+)
+
+// Session holds the SSO parameters between /sso/authorize and login completion.
+// The nonce is embedded as a hidden form field in the login page and carried
+// through the multi-step login flow (password → TOTP, or WebAuthn).
+type Session struct { //nolint:govet // fieldalignment: field order matches logical grouping
+	ClientID    string
+	RedirectURI string
+	State       string
+	ExpiresAt   time.Time
+}
+
+// pendingSessions stores SSO sessions created at /sso/authorize.
+var pendingSessions sync.Map //nolint:gochecknoglobals
+
+func init() {
+	go cleanupSessions()
+}
+
+func cleanupSessions() {
+	ticker := time.NewTicker(cleanupPeriod)
+	defer ticker.Stop()
+	for range ticker.C {
+		now := time.Now()
+		pendingSessions.Range(func(key, value any) bool {
+			s, ok := value.(*Session)
+			if !ok || now.After(s.ExpiresAt) {
+				pendingSessions.Delete(key)
+			}
+			return true
+		})
+	}
+}
+
+// StoreSession creates and stores a new SSO session, returning the hex-encoded
+// nonce that should be embedded in the login form.
+func StoreSession(clientID, redirectURI, state string) (string, error) {
+	raw, err := crypto.RandomBytes(sessionBytes)
+	if err != nil {
+		return "", fmt.Errorf("sso: generate session nonce: %w", err)
+	}
+	nonce := fmt.Sprintf("%x", raw)
+	pendingSessions.Store(nonce, &Session{
+		ClientID:    clientID,
+		RedirectURI: redirectURI,
+		State:       state,
+		ExpiresAt:   time.Now().Add(sessionTTL),
+	})
+	return nonce, nil
+}
+
+// ConsumeSession retrieves and deletes an SSO session by nonce.
+// Returns the Session and true if valid, or (nil, false) if unknown or expired.
+func ConsumeSession(nonce string) (*Session, bool) {
+	v, ok := pendingSessions.LoadAndDelete(nonce)
+	if !ok {
+		return nil, false
+	}
+	s, ok2 := v.(*Session)
+	if !ok2 || time.Now().After(s.ExpiresAt) {
+		return nil, false
+	}
+	return s, true
+}
+
+// GetSession retrieves an SSO session without consuming it (for read-only checks
+// during multi-step login). Returns nil if unknown or expired.
+func GetSession(nonce string) *Session {
+	v, ok := pendingSessions.Load(nonce)
+	if !ok {
+		return nil
+	}
+	s, ok2 := v.(*Session)
+	if !ok2 || time.Now().After(s.ExpiresAt) {
+		return nil
+	}
+	return s
+}

internal/sso/store.go

@@ -0,0 +1,93 @@
+// Package sso implements the authorization code store for the SSO redirect flow.
+//
+// MCIAS acts as the SSO provider: downstream services (MCR, MCAT, Metacrypt)
+// redirect users to MCIAS for login, and MCIAS issues a short-lived, single-use
+// authorization code that the service exchanges for a JWT token.
+//
+// Security design:
+//   - Authorization codes are 32 random bytes (256 bits), hex-encoded.
+//   - Codes are single-use: consumed via sync.Map LoadAndDelete on first exchange.
+//   - Codes expire after 60 seconds to limit the window for interception.
+//   - A background goroutine evicts expired codes every 5 minutes.
+//   - The code is bound to the client_id and redirect_uri presented at authorize
+//     time; the token exchange endpoint must verify both match.
+package sso
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"git.wntrmute.dev/mc/mcias/internal/crypto"
+)
+
+const (
+	codeTTL       = 60 * time.Second
+	codeBytes     = 32 // 256 bits of entropy
+	cleanupPeriod = 5 * time.Minute
+)
+
+// AuthCode is a pending authorization code awaiting exchange for a JWT.
+type AuthCode struct { //nolint:govet // fieldalignment: field order matches logical grouping
+	ClientID    string
+	RedirectURI string
+	State       string
+	AccountID   int64
+	ExpiresAt   time.Time
+}
+
+// pendingCodes stores issued authorization codes awaiting exchange.
+var pendingCodes sync.Map //nolint:gochecknoglobals
+
+func init() {
+	go cleanupCodes()
+}
+
+func cleanupCodes() {
+	ticker := time.NewTicker(cleanupPeriod)
+	defer ticker.Stop()
+	for range ticker.C {
+		now := time.Now()
+		pendingCodes.Range(func(key, value any) bool {
+			ac, ok := value.(*AuthCode)
+			if !ok || now.After(ac.ExpiresAt) {
+				pendingCodes.Delete(key)
+			}
+			return true
+		})
+	}
+}
+
+// Store creates and stores a new authorization code bound to the given
+// client_id, redirect_uri, state, and account. Returns the hex-encoded code.
+func Store(clientID, redirectURI, state string, accountID int64) (string, error) {
+	raw, err := crypto.RandomBytes(codeBytes)
+	if err != nil {
+		return "", fmt.Errorf("sso: generate authorization code: %w", err)
+	}
+	code := fmt.Sprintf("%x", raw)
+	pendingCodes.Store(code, &AuthCode{
+		ClientID:    clientID,
+		RedirectURI: redirectURI,
+		State:       state,
+		AccountID:   accountID,
+		ExpiresAt:   time.Now().Add(codeTTL),
+	})
+	return code, nil
+}
+
+// Consume retrieves and deletes an authorization code. Returns the AuthCode
+// and true if the code was valid and not expired, or (nil, false) otherwise.
+//
+// Security: LoadAndDelete ensures single-use; the code cannot be replayed.
+func Consume(code string) (*AuthCode, bool) {
+	v, ok := pendingCodes.LoadAndDelete(code)
+	if !ok {
+		return nil, false
+	}
+	ac, ok2 := v.(*AuthCode)
+	if !ok2 || time.Now().After(ac.ExpiresAt) {
+		return nil, false
+	}
+	return ac, true
+}

internal/sso/store_test.go

@@ -0,0 +1,132 @@
+package sso
+
+import (
+	"testing"
+	"time"
+)
+
+func TestStoreAndConsume(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "state123", 42)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+	if code == "" {
+		t.Fatal("Store returned empty code")
+	}
+
+	ac, ok := Consume(code)
+	if !ok {
+		t.Fatal("Consume returned false for valid code")
+	}
+	if ac.ClientID != "mcr" {
+		t.Errorf("ClientID = %q, want %q", ac.ClientID, "mcr")
+	}
+	if ac.RedirectURI != "https://mcr.example.com/cb" {
+		t.Errorf("RedirectURI = %q", ac.RedirectURI)
+	}
+	if ac.State != "state123" {
+		t.Errorf("State = %q", ac.State)
+	}
+	if ac.AccountID != 42 {
+		t.Errorf("AccountID = %d, want 42", ac.AccountID)
+	}
+}
+
+func TestConsumeSingleUse(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "s", 1)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+
+	if _, ok := Consume(code); !ok {
+		t.Fatal("first Consume should succeed")
+	}
+	if _, ok := Consume(code); ok {
+		t.Error("second Consume should fail (single-use)")
+	}
+}
+
+func TestConsumeUnknownCode(t *testing.T) {
+	if _, ok := Consume("nonexistent"); ok {
+		t.Error("Consume should fail for unknown code")
+	}
+}
+
+func TestConsumeExpiredCode(t *testing.T) {
+	code, err := Store("mcr", "https://mcr.example.com/cb", "s", 1)
+	if err != nil {
+		t.Fatalf("Store: %v", err)
+	}
+
+	// Manually expire the code.
+	v, loaded := pendingCodes.Load(code)
+	if !loaded {
+		t.Fatal("code not found in pendingCodes")
+	}
+	ac, ok := v.(*AuthCode)
+	if !ok {
+		t.Fatal("unexpected type in pendingCodes")
+	}
+	ac.ExpiresAt = time.Now().Add(-1 * time.Second)
+
+	if _, ok := Consume(code); ok {
+		t.Error("Consume should fail for expired code")
+	}
+}
+
+func TestStoreSessionAndConsume(t *testing.T) {
+	nonce, err := StoreSession("mcr", "https://mcr.example.com/cb", "state456")
+	if err != nil {
+		t.Fatalf("StoreSession: %v", err)
+	}
+	if nonce == "" {
+		t.Fatal("StoreSession returned empty nonce")
+	}
+
+	// GetSession should return it without consuming.
+	s := GetSession(nonce)
+	if s == nil {
+		t.Fatal("GetSession returned nil")
+	}
+	if s.ClientID != "mcr" {
+		t.Errorf("ClientID = %q", s.ClientID)
+	}
+
+	// Still available after GetSession.
+	s2, ok := ConsumeSession(nonce)
+	if !ok {
+		t.Fatal("ConsumeSession returned false")
+	}
+	if s2.State != "state456" {
+		t.Errorf("State = %q", s2.State)
+	}
+
+	// Consumed — should be gone.
+	if _, ok := ConsumeSession(nonce); ok {
+		t.Error("second ConsumeSession should fail")
+	}
+	if GetSession(nonce) != nil {
+		t.Error("GetSession should return nil after consume")
+	}
+}
+
+func TestConsumeSessionExpired(t *testing.T) {
+	nonce, err := StoreSession("mcr", "https://mcr.example.com/cb", "s")
+	if err != nil {
+		t.Fatalf("StoreSession: %v", err)
+	}
+
+	v, loaded := pendingSessions.Load(nonce)
+	if !loaded {
+		t.Fatal("session not found in pendingSessions")
+	}
+	sess, ok := v.(*Session)
+	if !ok {
+		t.Fatal("unexpected type in pendingSessions")
+	}
+	sess.ExpiresAt = time.Now().Add(-1 * time.Second)
+
+	if _, ok := ConsumeSession(nonce); ok {
+		t.Error("ConsumeSession should fail for expired session")
+	}
+}

internal/ui/handlers_auth.go

@@ -15,6 +15,7 @@ import (
 func (u *UIServer) handleLoginPage(w http.ResponseWriter, r *http.Request) {
 	u.render(w, "login", LoginData{
 		WebAuthnEnabled: u.cfg.WebAuthnEnabled(),
+		SSONonce:        r.URL.Query().Get("sso"),
 	})
 }
 
@@ -97,6 +98,8 @@ func (u *UIServer) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	ssoNonce := r.FormValue("sso_nonce")
+
 	// TOTP required: issue a server-side nonce and show the TOTP step form.
 	// Security: the nonce replaces the password hidden field (F-02). The password
 	// is not stored anywhere after this point; only the account ID is retained.
@@ -110,11 +113,12 @@ func (u *UIServer) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 		u.render(w, "totp_step", LoginData{
 			Username: username,
 			Nonce:    nonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
 
-	u.finishLogin(w, r, acct)
+	u.finishLogin(w, r, acct, ssoNonce)
 }
 
 // handleTOTPStep handles the second POST when totp_step=1 is set.
@@ -129,6 +133,7 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 	username := r.FormValue("username")  //nolint:gosec // body already limited by caller
 	nonce := r.FormValue("totp_nonce")   //nolint:gosec // body already limited by caller
 	totpCode := r.FormValue("totp_code") //nolint:gosec // body already limited by caller
+	ssoNonce := r.FormValue("sso_nonce") //nolint:gosec // body already limited by caller
 
 	// Security: consume the nonce (single-use); reject if unknown or expired.
 	accountID, ok := u.consumeTOTPNonce(nonce)
@@ -172,6 +177,7 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 			Error:    "invalid TOTP code",
 			Username: username,
 			Nonce:    newNonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
@@ -189,15 +195,36 @@ func (u *UIServer) handleTOTPStep(w http.ResponseWriter, r *http.Request) {
 			Error:    "invalid TOTP code",
 			Username: username,
 			Nonce:    newNonce,
+			SSONonce: ssoNonce,
 		})
 		return
 	}
 
-	u.finishLogin(w, r, acct)
+	u.finishLogin(w, r, acct, ssoNonce)
 }
 
 // finishLogin issues a JWT, sets the session cookie, and redirects to dashboard.
-func (u *UIServer) finishLogin(w http.ResponseWriter, r *http.Request, acct *model.Account) {
+// When ssoNonce is non-empty, the login is part of an SSO redirect flow: instead
+// of setting a session cookie, an authorization code is issued and the user is
+// redirected back to the service's callback URL.
+func (u *UIServer) finishLogin(w http.ResponseWriter, r *http.Request, acct *model.Account, ssoNonce string) {
+	// SSO redirect flow: issue authorization code and redirect to service.
+	if ssoNonce != "" {
+		if callbackURL, ok := u.buildSSOCallback(r, ssoNonce, acct.ID); ok {
+			// Security: htmx follows 302 redirects via fetch, which fails
+			// cross-origin (no CORS on the service callback). Use HX-Redirect
+			// so htmx performs a full page navigation instead.
+			if isHTMX(r) {
+				w.Header().Set("HX-Redirect", callbackURL)
+				w.WriteHeader(http.StatusOK)
+				return
+			}
+			http.Redirect(w, r, callbackURL, http.StatusFound)
+			return
+		}
+		// SSO session expired/consumed — fall through to normal login.
+	}
+
 	// Determine token expiry based on admin role.
 	expiry := u.cfg.DefaultExpiry()
 	roles, err := u.db.GetRoles(acct.ID)

internal/ui/handlers_sso.go

@@ -0,0 +1,84 @@
+package ui
+
+import (
+	"net/http"
+	"net/url"
+
+	"git.wntrmute.dev/mc/mcias/internal/audit"
+	"git.wntrmute.dev/mc/mcias/internal/model"
+	"git.wntrmute.dev/mc/mcias/internal/sso"
+)
+
+// handleSSOAuthorize validates the SSO request parameters against registered
+// clients, creates an SSO session, and redirects to /login with the SSO nonce.
+//
+// Security: the client_id and redirect_uri are validated against the MCIAS
+// config (exact match). The state parameter is opaque and carried through
+// unchanged. An SSO session is created server-side so the nonce is the only
+// value embedded in the login form.
+func (u *UIServer) handleSSOAuthorize(w http.ResponseWriter, r *http.Request) {
+	clientID := r.URL.Query().Get("client_id")
+	redirectURI := r.URL.Query().Get("redirect_uri")
+	state := r.URL.Query().Get("state")
+
+	if clientID == "" || redirectURI == "" || state == "" {
+		http.Error(w, "missing required parameters: client_id, redirect_uri, state", http.StatusBadRequest)
+		return
+	}
+
+	// Security: validate client_id against registered SSO clients.
+	client := u.cfg.SSOClient(clientID)
+	if client == nil {
+		u.logger.Warn("sso: unknown client_id", "client_id", clientID)
+		http.Error(w, "unknown client_id", http.StatusBadRequest)
+		return
+	}
+
+	// Security: redirect_uri must exactly match the registered URI to prevent
+	// open-redirect attacks.
+	if redirectURI != client.RedirectURI {
+		u.logger.Warn("sso: redirect_uri mismatch",
+			"client_id", clientID,
+			"expected", client.RedirectURI,
+			"got", redirectURI)
+		http.Error(w, "redirect_uri does not match registered URI", http.StatusBadRequest)
+		return
+	}
+
+	nonce, err := sso.StoreSession(clientID, redirectURI, state)
+	if err != nil {
+		u.logger.Error("sso: store session", "error", err)
+		http.Error(w, "internal error", http.StatusInternalServerError)
+		return
+	}
+
+	u.writeAudit(r, model.EventSSOAuthorize, nil, nil,
+		audit.JSON("client_id", clientID))
+
+	http.Redirect(w, r, "/login?sso="+url.QueryEscape(nonce), http.StatusFound)
+}
+
+// buildSSOCallback consumes the SSO session, generates an authorization code,
+// and returns the callback URL with code and state parameters. Returns ("", false)
+// if the SSO session is expired or already consumed.
+//
+// Security: the SSO session is consumed (single-use) and the authorization code
+// is stored server-side for exchange via POST /v1/sso/token. The state parameter
+// is carried through unchanged for the service to validate.
+func (u *UIServer) buildSSOCallback(r *http.Request, ssoNonce string, accountID int64) (string, bool) {
+	sess, ok := sso.ConsumeSession(ssoNonce)
+	if !ok {
+		return "", false
+	}
+
+	code, err := sso.Store(sess.ClientID, sess.RedirectURI, sess.State, accountID)
+	if err != nil {
+		u.logger.Error("sso: store auth code", "error", err)
+		return "", false
+	}
+
+	u.writeAudit(r, model.EventSSOLoginOK, &accountID, nil,
+		audit.JSON("client_id", sess.ClientID))
+
+	return sess.RedirectURI + "?code=" + url.QueryEscape(code) + "&state=" + url.QueryEscape(sess.State), true
+}

internal/ui/handlers_webauthn.go

@@ -27,10 +27,11 @@ const (
 )
 
 // webauthnCeremony holds a pending WebAuthn ceremony.
-type webauthnCeremony struct {
+type webauthnCeremony struct { //nolint:govet // fieldalignment: field order matches logical grouping
 	expiresAt time.Time
 	session   *libwebauthn.SessionData
 	accountID int64
+	ssoNonce  string // non-empty when login is part of an SSO redirect flow
 }
 
 // pendingWebAuthnCeremonies stores in-flight WebAuthn ceremonies for the UI.
@@ -55,7 +56,7 @@ func cleanupUIWebAuthnCeremonies() {
 	}
 }
 
-func storeUICeremony(session *libwebauthn.SessionData, accountID int64) (string, error) {
+func storeUICeremony(session *libwebauthn.SessionData, accountID int64, ssoNonce string) (string, error) {
 	raw, err := crypto.RandomBytes(webauthnNonceBytes)
 	if err != nil {
 		return "", fmt.Errorf("webauthn: generate ceremony nonce: %w", err)
@@ -64,6 +65,7 @@ func storeUICeremony(session *libwebauthn.SessionData, accountID int64) (string,
 	pendingUIWebAuthnCeremonies.Store(nonce, &webauthnCeremony{
 		session:   session,
 		accountID: accountID,
+		ssoNonce:  ssoNonce,
 		expiresAt: time.Now().Add(webauthnCeremonyTTL),
 	})
 	return nonce, nil
@@ -170,7 +172,7 @@ func (u *UIServer) handleWebAuthnBegin(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	nonce, err := storeUICeremony(session, acct.ID)
+	nonce, err := storeUICeremony(session, acct.ID, "")
 	if err != nil {
 		writeJSONError(w, http.StatusInternalServerError, "internal error")
 		return
@@ -352,6 +354,7 @@ func (u *UIServer) handleWebAuthnLoginBegin(w http.ResponseWriter, r *http.Reque
 	r.Body = http.MaxBytesReader(w, r.Body, maxFormBytes)
 	var req struct {
 		Username string `json:"username"`
+		SSONonce string `json:"sso_nonce"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeJSONError(w, http.StatusBadRequest, "invalid JSON")
@@ -413,7 +416,7 @@ func (u *UIServer) handleWebAuthnLoginBegin(w http.ResponseWriter, r *http.Reque
 		return
 	}
 
-	nonce, err := storeUICeremony(session, accountID)
+	nonce, err := storeUICeremony(session, accountID, req.SSONonce)
 	if err != nil {
 		writeJSONError(w, http.StatusInternalServerError, "internal error")
 		return
@@ -582,6 +585,17 @@ func (u *UIServer) handleWebAuthnLoginFinish(w http.ResponseWriter, r *http.Requ
 
 	_ = u.db.ClearLoginFailures(acct.ID)
 
+	// SSO redirect flow: issue authorization code and return redirect URL as JSON.
+	if ceremony.ssoNonce != "" {
+		if callbackURL, ok := u.buildSSOCallback(r, ceremony.ssoNonce, acct.ID); ok {
+			u.writeAudit(r, model.EventWebAuthnLoginOK, &acct.ID, nil, "")
+			w.Header().Set("Content-Type", "application/json")
+			_ = json.NewEncoder(w).Encode(map[string]string{"redirect": callbackURL})
+			return
+		}
+		// SSO session expired — fall through to normal login.
+	}
+
 	// Issue JWT and set session cookie.
 	expiry := u.cfg.DefaultExpiry()
 	roles, err := u.db.GetRoles(acct.ID)

internal/ui/ui.go

@@ -445,6 +445,9 @@ func (u *UIServer) Register(mux *http.ServeMux) {
 	uiMux.HandleFunc("GET /unseal", u.handleUnsealPage)
 	uiMux.Handle("POST /unseal", unsealRateLimit(http.HandlerFunc(u.handleUnsealPost)))
 
+	// SSO authorize route (no session required, rate-limited).
+	uiMux.Handle("GET /sso/authorize", loginRateLimit(http.HandlerFunc(u.handleSSOAuthorize)))
+
 	// Auth routes (no session required).
 	uiMux.HandleFunc("GET /login", u.handleLoginPage)
 	uiMux.Handle("POST /login", loginRateLimit(http.HandlerFunc(u.handleLoginPost)))
@@ -810,6 +813,7 @@ type PageData struct {
 type LoginData struct {
 	Error    string
 	Username string // pre-filled on TOTP step
+	SSONonce string // SSO session nonce (hidden field for SSO redirect flow)
 	// Security (F-02): Password is no longer carried in the HTML form. Instead
 	// a short-lived server-side nonce is issued after successful password
 	// verification, and only the nonce is embedded in the TOTP step form.

vendor/google.golang.org/protobuf/internal/encoding/tag/tag.go

@@ -32,7 +32,7 @@ var byteType = reflect.TypeOf(byte(0))
 func Unmarshal(tag string, goType reflect.Type, evs protoreflect.EnumValueDescriptors) protoreflect.FieldDescriptor {
 	f := new(filedesc.Field)
 	f.L0.ParentFile = filedesc.SurrogateProto2
-	f.L1.EditionFeatures = f.L0.ParentFile.L1.EditionFeatures
+	packed := false
 	for len(tag) > 0 {
 		i := strings.IndexByte(tag, ',')
 		if i < 0 {
@@ -108,7 +108,7 @@ func Unmarshal(tag string, goType reflect.Type, evs protoreflect.EnumValueDescri
 				f.L1.StringName.InitJSON(jsonName)
 			}
 		case s == "packed":
-			f.L1.EditionFeatures.IsPacked = true
+			packed = true
 		case strings.HasPrefix(s, "def="):
 			// The default tag is special in that everything afterwards is the
 			// default regardless of the presence of commas.
@@ -121,6 +121,13 @@ func Unmarshal(tag string, goType reflect.Type, evs protoreflect.EnumValueDescri
 		tag = strings.TrimPrefix(tag[i:], ",")
 	}
 
+	// Update EditionFeatures after the loop and after we know whether this is
+	// a proto2 or proto3 field.
+	f.L1.EditionFeatures = f.L0.ParentFile.L1.EditionFeatures
+	if packed {
+		f.L1.EditionFeatures.IsPacked = true
+	}
+
 	// The generator uses the group message name instead of the field name.
 	// We obtain the real field name by lowercasing the group name.
 	if f.L1.Kind == protoreflect.GroupKind {

vendor/google.golang.org/protobuf/internal/encoding/text/decode.go

@@ -424,27 +424,34 @@ func (d *Decoder) parseFieldName() (tok Token, err error) {
 	return Token{}, d.newSyntaxError("invalid field name: %s", errId(d.in))
 }
 
-// parseTypeName parses Any type URL or extension field name. The name is
+// parseTypeName parses an Any type URL or an extension field name. The name is
-// enclosed in [ and ] characters. The C++ parser does not handle many legal URL
+// enclosed in [ and ] characters. We allow almost arbitrary type URL prefixes,
-// strings. This implementation is more liberal and allows for the pattern
+// closely following the text-format spec [1,2]. We implement "ExtensionName |
-// ^[-_a-zA-Z0-9]+([./][-_a-zA-Z0-9]+)*`). Whitespaces and comments are allowed
+// AnyName" as follows (with some exceptions for backwards compatibility):
-// in between [ ], '.', '/' and the sub names.
+//
+// char      = [-_a-zA-Z0-9]
+// url_char  = char | [.~!$&'()*+,;=] | "%", hex, hex
+//
+// Ident         = char, { char }
+// TypeName      = Ident, { ".", Ident } ;
+// UrlPrefix     = url_char, { url_char | "/" } ;
+// ExtensionName = "[", TypeName, "]" ;
+// AnyName       = "[", UrlPrefix, "/", TypeName, "]" ;
+//
+// Additionally, we allow arbitrary whitespace and comments between [ and ].
+//
+// [1] https://protobuf.dev/reference/protobuf/textformat-spec/#characters
+// [2] https://protobuf.dev/reference/protobuf/textformat-spec/#field-names
 func (d *Decoder) parseTypeName() (Token, error) {
-	startPos := len(d.orig) - len(d.in)
 	// Use alias s to advance first in order to use d.in for error handling.
-	// Caller already checks for [ as first character.
+	// Caller already checks for [ as first character (d.in[0] == '[').
 	s := consume(d.in[1:], 0)
 	if len(s) == 0 {
 		return Token{}, ErrUnexpectedEOF
 	}
 
+	// Collect everything between [ and ] in name.
 	var name []byte
-	for len(s) > 0 && isTypeNameChar(s[0]) {
-		name = append(name, s[0])
-		s = s[1:]
-	}
-	s = consume(s, 0)
-
 	var closed bool
 	for len(s) > 0 && !closed {
 		switch {
@@ -452,23 +459,20 @@ func (d *Decoder) parseTypeName() (Token, error) {
 			s = s[1:]
 			closed = true
 
-		case s[0] == '/', s[0] == '.':
+		case s[0] == '/' || isTypeNameChar(s[0]) || isUrlExtraChar(s[0]):
-			if len(name) > 0 && (name[len(name)-1] == '/' || name[len(name)-1] == '.') {
-				return Token{}, d.newSyntaxError("invalid type URL/extension field name: %s",
-					d.orig[startPos:len(d.orig)-len(s)+1])
-			}
 			name = append(name, s[0])
-			s = s[1:]
+			s = consume(s[1:], 0)
-			s = consume(s, 0)
+
-			for len(s) > 0 && isTypeNameChar(s[0]) {
+		// URL percent-encoded chars
-				name = append(name, s[0])
+		case s[0] == '%':
-				s = s[1:]
+			if len(s) < 3 || !isHexChar(s[1]) || !isHexChar(s[2]) {
+				return Token{}, d.parseTypeNameError(s, 3)
 			}
-			s = consume(s, 0)
+			name = append(name, s[0], s[1], s[2])
+			s = consume(s[3:], 0)
 
 		default:
-			return Token{}, d.newSyntaxError(
+			return Token{}, d.parseTypeNameError(s, 1)
-				"invalid type URL/extension field name: %s", d.orig[startPos:len(d.orig)-len(s)+1])
 		}
 	}
 
@@ -476,15 +480,38 @@ func (d *Decoder) parseTypeName() (Token, error) {
 		return Token{}, ErrUnexpectedEOF
 	}
 
-	// First character cannot be '.'. Last character cannot be '.' or '/'.
+	// Split collected name on last '/' into urlPrefix and typeName (if '/' is
-	size := len(name)
+	// present).
-	if size == 0 || name[0] == '.' || name[size-1] == '.' || name[size-1] == '/' {
+	typeName := name
-		return Token{}, d.newSyntaxError("invalid type URL/extension field name: %s",
+	if i := bytes.LastIndexByte(name, '/'); i != -1 {
-			d.orig[startPos:len(d.orig)-len(s)])
+		urlPrefix := name[:i]
+		typeName = name[i+1:]
+
+		// urlPrefix may be empty (for backwards compatibility).
+		// If non-empty, it must not start with '/'.
+		if len(urlPrefix) > 0 && urlPrefix[0] == '/' {
+			return Token{}, d.parseTypeNameError(s, 0)
+		}
 	}
 
+	// typeName must not be empty (note: "" splits to [""]) and all identifier
+	// parts must not be empty.
+	for _, ident := range bytes.Split(typeName, []byte{'.'}) {
+		if len(ident) == 0 {
+			return Token{}, d.parseTypeNameError(s, 0)
+		}
+	}
+
+	// typeName must not contain any percent-encoded or special URL chars.
+	for _, b := range typeName {
+		if b == '%' || (b != '.' && isUrlExtraChar(b)) {
+			return Token{}, d.parseTypeNameError(s, 0)
+		}
+	}
+
+	startPos := len(d.orig) - len(d.in)
+	endPos := len(d.orig) - len(s)
 	d.in = s
-	endPos := len(d.orig) - len(d.in)
 	d.consume(0)
 
 	return Token{
@@ -496,16 +523,32 @@ func (d *Decoder) parseTypeName() (Token, error) {
 	}, nil
 }
 
-func isTypeNameChar(b byte) bool {
+func (d *Decoder) parseTypeNameError(s []byte, numUnconsumedChars int) error {
-	return (b == '-' || b == '_' ||
+	return d.newSyntaxError(
-		('0' <= b && b <= '9') ||
+		"invalid type URL/extension field name: %s",
-		('a' <= b && b <= 'z') ||
+		d.in[:len(d.in)-len(s)+min(numUnconsumedChars, len(s))],
-		('A' <= b && b <= 'Z'))
+	)
 }
 
-func isWhiteSpace(b byte) bool {
+func isHexChar(b byte) bool {
+	return ('0' <= b && b <= '9') ||
+		('a' <= b && b <= 'f') ||
+		('A' <= b && b <= 'F')
+}
+
+func isTypeNameChar(b byte) bool {
+	return b == '-' || b == '_' ||
+		('0' <= b && b <= '9') ||
+		('a' <= b && b <= 'z') ||
+		('A' <= b && b <= 'Z')
+}
+
+// isUrlExtraChar complements isTypeNameChar with extra characters that we allow
+// in URLs but not in type names. Note that '/' is not included so that it can
+// be treated specially.
+func isUrlExtraChar(b byte) bool {
 	switch b {
-	case ' ', '\n', '\r', '\t':
+	case '.', '~', '!', '$', '&', '(', ')', '*', '+', ',', ';', '=':
 		return true
 	default:
 		return false

vendor/google.golang.org/protobuf/internal/filedesc/desc.go

@@ -32,6 +32,7 @@ const (
 	EditionProto3      Edition = 999
 	Edition2023        Edition = 1000
 	Edition2024        Edition = 1001
+	EditionUnstable    Edition = 9999
 	EditionUnsupported Edition = 100000
 )
 

vendor/google.golang.org/protobuf/internal/filedesc/desc_lazy.go

@@ -330,7 +330,6 @@ func (md *Message) unmarshalFull(b []byte, sb *strs.Builder) {
 				md.L1.Extensions.List[extensionIdx].unmarshalFull(v, sb)
 				extensionIdx++
 			case genid.DescriptorProto_Options_field_number:
-				md.unmarshalOptions(v)
 				rawOptions = appendOptions(rawOptions, v)
 			}
 		default:
@@ -356,27 +355,6 @@ func (md *Message) unmarshalFull(b []byte, sb *strs.Builder) {
 	md.L2.Options = md.L0.ParentFile.builder.optionsUnmarshaler(&descopts.Message, rawOptions)
 }
 
-func (md *Message) unmarshalOptions(b []byte) {
-	for len(b) > 0 {
-		num, typ, n := protowire.ConsumeTag(b)
-		b = b[n:]
-		switch typ {
-		case protowire.VarintType:
-			v, m := protowire.ConsumeVarint(b)
-			b = b[m:]
-			switch num {
-			case genid.MessageOptions_MapEntry_field_number:
-				md.L1.IsMapEntry = protowire.DecodeBool(v)
-			case genid.MessageOptions_MessageSetWireFormat_field_number:
-				md.L1.IsMessageSet = protowire.DecodeBool(v)
-			}
-		default:
-			m := protowire.ConsumeFieldValue(num, typ, b)
-			b = b[m:]
-		}
-	}
-}
-
 func unmarshalMessageReservedRange(b []byte) (r [2]protoreflect.FieldNumber) {
 	for len(b) > 0 {
 		num, typ, n := protowire.ConsumeTag(b)

vendor/google.golang.org/protobuf/internal/genid/descriptor_gen.go

@@ -26,6 +26,7 @@ const (
 	Edition_EDITION_PROTO3_enum_value          = 999
 	Edition_EDITION_2023_enum_value            = 1000
 	Edition_EDITION_2024_enum_value            = 1001
+	Edition_EDITION_UNSTABLE_enum_value        = 9999
 	Edition_EDITION_1_TEST_ONLY_enum_value     = 1
 	Edition_EDITION_2_TEST_ONLY_enum_value     = 2
 	Edition_EDITION_99997_TEST_ONLY_enum_value = 99997

vendor/google.golang.org/protobuf/internal/impl/codec_map.go

@@ -113,6 +113,9 @@ func sizeMap(mapv reflect.Value, mapi *mapInfo, f *coderFieldInfo, opts marshalO
 }
 
 func consumeMap(b []byte, mapv reflect.Value, wtyp protowire.Type, mapi *mapInfo, f *coderFieldInfo, opts unmarshalOptions) (out unmarshalOutput, err error) {
+	if opts.depth--; opts.depth < 0 {
+		return out, errRecursionDepth
+	}
 	if wtyp != protowire.BytesType {
 		return out, errUnknown
 	}
@@ -170,6 +173,9 @@ func consumeMap(b []byte, mapv reflect.Value, wtyp protowire.Type, mapi *mapInfo
 }
 
 func consumeMapOfMessage(b []byte, mapv reflect.Value, wtyp protowire.Type, mapi *mapInfo, f *coderFieldInfo, opts unmarshalOptions) (out unmarshalOutput, err error) {
+	if opts.depth--; opts.depth < 0 {
+		return out, errRecursionDepth
+	}
 	if wtyp != protowire.BytesType {
 		return out, errUnknown
 	}

vendor/google.golang.org/protobuf/internal/impl/decode.go

@@ -102,8 +102,7 @@ var errUnknown = errors.New("unknown")
 
 func (mi *MessageInfo) unmarshalPointer(b []byte, p pointer, groupTag protowire.Number, opts unmarshalOptions) (out unmarshalOutput, err error) {
 	mi.init()
-	opts.depth--
+	if opts.depth--; opts.depth < 0 {
-	if opts.depth < 0 {
 		return out, errRecursionDepth
 	}
 	if flags.ProtoLegacy && mi.isMessageSet {

vendor/google.golang.org/protobuf/internal/impl/validate.go

@@ -68,9 +68,13 @@ func Validate(mt protoreflect.MessageType, in protoiface.UnmarshalInput) (out pr
 	if in.Resolver == nil {
 		in.Resolver = protoregistry.GlobalTypes
 	}
+	if in.Depth == 0 {
+		in.Depth = protowire.DefaultRecursionLimit
+	}
 	o, st := mi.validate(in.Buf, 0, unmarshalOptions{
 		flags:    in.Flags,
 		resolver: in.Resolver,
+		depth:    in.Depth,
 	})
 	if o.initialized {
 		out.Flags |= protoiface.UnmarshalInitialized
@@ -257,6 +261,9 @@ func (mi *MessageInfo) validate(b []byte, groupTag protowire.Number, opts unmars
 		states[0].typ = validationTypeGroup
 		states[0].endGroup = groupTag
 	}
+	if opts.depth--; opts.depth < 0 {
+		return out, ValidationInvalid
+	}
 	initialized := true
 	start := len(b)
 State:
@@ -451,6 +458,13 @@ State:
 						mi:      vi.mi,
 						tail:    b,
 					})
+					if vi.typ == validationTypeMessage ||
+						vi.typ == validationTypeGroup ||
+						vi.typ == validationTypeMap {
+						if opts.depth--; opts.depth < 0 {
+							return out, ValidationInvalid
+						}
+					}
 					b = v
 					continue State
 				case validationTypeRepeatedVarint:
@@ -499,6 +513,9 @@ State:
 						mi:       vi.mi,
 						endGroup: num,
 					})
+					if opts.depth--; opts.depth < 0 {
+						return out, ValidationInvalid
+					}
 					continue State
 				case flags.ProtoLegacy && vi.typ == validationTypeMessageSetItem:
 					typeid, v, n, err := messageset.ConsumeFieldValue(b, false)
@@ -521,6 +538,13 @@ State:
 							mi:   xvi.mi,
 							tail: b[n:],
 						})
+						if xvi.typ == validationTypeMessage ||
+							xvi.typ == validationTypeGroup ||
+							xvi.typ == validationTypeMap {
+							if opts.depth--; opts.depth < 0 {
+								return out, ValidationInvalid
+							}
+						}
 						b = v
 						continue State
 					}
@@ -547,12 +571,14 @@ State:
 		switch st.typ {
 		case validationTypeMessage, validationTypeGroup:
 			numRequiredFields = int(st.mi.numRequiredFields)
+			opts.depth++
 		case validationTypeMap:
 			// If this is a map field with a message value that contains
 			// required fields, require that the value be present.
 			if st.mi != nil && st.mi.numRequiredFields > 0 {
 				numRequiredFields = 1
 			}
+			opts.depth++
 		}
 		// If there are more than 64 required fields, this check will
 		// always fail and we will report that the message is potentially

vendor/google.golang.org/protobuf/internal/version/version.go

@@ -52,7 +52,7 @@ import (
 const (
 	Major      = 1
 	Minor      = 36
-	Patch      = 10
+	Patch      = 11
 	PreRelease = ""
 )
 

vendor/google.golang.org/protobuf/proto/decode.go

@@ -121,9 +121,8 @@ func (o UnmarshalOptions) unmarshal(b []byte, m protoreflect.Message) (out proto
 
 		out, err = methods.Unmarshal(in)
 	} else {
-		o.RecursionLimit--
+		if o.RecursionLimit--; o.RecursionLimit < 0 {
-		if o.RecursionLimit < 0 {
+			return out, errRecursionDepth
-			return out, errors.New("exceeded max recursion depth")
 		}
 		err = o.unmarshalMessageSlow(b, m)
 	}
@@ -220,6 +219,9 @@ func (o UnmarshalOptions) unmarshalSingular(b []byte, wtyp protowire.Type, m pro
 }
 
 func (o UnmarshalOptions) unmarshalMap(b []byte, wtyp protowire.Type, mapv protoreflect.Map, fd protoreflect.FieldDescriptor) (n int, err error) {
+	if o.RecursionLimit--; o.RecursionLimit < 0 {
+		return 0, errRecursionDepth
+	}
 	if wtyp != protowire.BytesType {
 		return 0, errUnknown
 	}
@@ -305,3 +307,5 @@ func (o UnmarshalOptions) unmarshalMap(b []byte, wtyp protowire.Type, mapv proto
 var errUnknown = errors.New("BUG: internal error (unknown)")
 
 var errDecode = errors.New("cannot parse invalid wire-format data")
+
+var errRecursionDepth = errors.New("exceeded maximum recursion depth")

vendor/google.golang.org/protobuf/types/known/timestamppb/timestamp.pb.go

@@ -172,13 +172,14 @@ import (
 // ) to obtain a formatter capable of generating timestamps in this format.
 type Timestamp struct {
 	state protoimpl.MessageState `protogen:"open.v1"`
-	// Represents seconds of UTC time since Unix epoch
+	// Represents seconds of UTC time since Unix epoch 1970-01-01T00:00:00Z. Must
-	// 1970-01-01T00:00:00Z. Must be from 0001-01-01T00:00:00Z to
+	// be between -315576000000 and 315576000000 inclusive (which corresponds to
-	// 9999-12-31T23:59:59Z inclusive.
+	// 0001-01-01T00:00:00Z to 9999-12-31T23:59:59Z).
 	Seconds int64 `protobuf:"varint,1,opt,name=seconds,proto3" json:"seconds,omitempty"`
-	// Non-negative fractions of a second at nanosecond resolution. Negative
+	// Non-negative fractions of a second at nanosecond resolution. This field is
-	// second values with fractions must still have non-negative nanos values
+	// the nanosecond portion of the duration, not an alternative to seconds.
-	// that count forward in time. Must be from 0 to 999,999,999
+	// Negative second values with fractions must still have non-negative nanos
+	// values that count forward in time. Must be between 0 and 999,999,999
 	// inclusive.
 	Nanos         int32 `protobuf:"varint,2,opt,name=nanos,proto3" json:"nanos,omitempty"`
 	unknownFields protoimpl.UnknownFields

vendor/modules.txt

@@ -172,7 +172,7 @@ google.golang.org/grpc/stats
 google.golang.org/grpc/status
 google.golang.org/grpc/tap
 google.golang.org/grpc/test/bufconn
-# google.golang.org/protobuf v1.36.10
+# google.golang.org/protobuf v1.36.11
 ## explicit; go 1.23
 google.golang.org/protobuf/encoding/protojson
 google.golang.org/protobuf/encoding/prototext

web/static/webauthn.js

@@ -110,18 +110,22 @@
   };
 
   // mciasWebAuthnLogin initiates a passkey login.
-  window.mciasWebAuthnLogin = function (username, onSuccess, onError) {
+  // ssoNonce is optional — when non-empty, it is included in the begin/finish
+  // requests so the server can redirect back to the SSO client after login.
+  window.mciasWebAuthnLogin = function (username, ssoNonce, onSuccess, onError) {
     if (!window.PublicKeyCredential) {
       onError('WebAuthn is not supported in this browser.');
       return;
     }
 
     var savedNonce = '';
+    var beginBody = { username: username || '' };
+    if (ssoNonce) { beginBody.sso_nonce = ssoNonce; }
 
     fetch('/login/webauthn/begin', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
-      body: JSON.stringify({ username: username || '' })
+      body: JSON.stringify(beginBody)
     })
     .then(function (resp) {
       if (!resp.ok) return resp.text().then(function (t) { throw new Error(t || 'Login failed'); });
@@ -163,7 +167,7 @@
       if (!resp.ok) return resp.text().then(function (t) { throw new Error(t || 'Login failed'); });
       return resp.json();
     })
-    .then(function () { onSuccess(); })
+    .then(function (data) { onSuccess(data); })
     .catch(function (err) { onError(err.message || 'Login failed'); });
   };
 
@@ -208,11 +212,14 @@
         hideError('webauthn-login-error');
         var usernameInput = document.getElementById('username');
         var username = usernameInput ? usernameInput.value.trim() : '';
+        var ssoNonce = loginBtn.getAttribute('data-sso-nonce') || '';
         loginBtn.disabled = true;
         loginBtn.textContent = 'Waiting for authenticator...';
 
-        window.mciasWebAuthnLogin(username, function () {
+        window.mciasWebAuthnLogin(username, ssoNonce, function (data) {
-          window.location.href = '/dashboard';
+          // The server returns a redirect URL — either /dashboard for direct
+          // login, or the SSO client callback URL with code and state params.
+          window.location.href = (data && data.redirect) || '/dashboard';
         }, function (err) {
           loginBtn.disabled = false;
           loginBtn.textContent = 'Sign in with passkey';

web/templates/fragments/totp_step.html

@@ -5,6 +5,7 @@
   <input type="hidden" name="username" value="{{.Username}}">
   <input type="hidden" name="totp_nonce" value="{{.Nonce}}">
   <input type="hidden" name="totp_step" value="1">
+  {{if .SSONonce}}<input type="hidden" name="sso_nonce" value="{{.SSONonce}}">{{end}}
   <div class="form-group">
     <label for="totp_code">Authenticator Code</label>
     <input class="form-control" type="text" id="totp_code" name="totp_code"

web/templates/login.html

@@ -15,6 +15,7 @@
       {{if .Error}}<div class="alert alert-error" role="alert">{{.Error}}</div>{{end}}
       <form id="login-form" method="POST" action="/login"
             hx-post="/login" hx-target="#login-card" hx-swap="outerHTML" hx-select="#login-card">
+        {{if .SSONonce}}<input type="hidden" name="sso_nonce" value="{{.SSONonce}}">{{end}}
         <div class="form-group">
           <label for="username">Username</label>
           <input class="form-control" type="text" id="username" name="username"
@@ -37,7 +38,8 @@
           <hr style="flex:1;border:0;border-top:1px solid #ddd">
         </div>
         <div id="webauthn-login-error" class="alert alert-error" style="display:none" role="alert"></div>
-        <button class="btn btn-secondary" type="button" id="webauthn-login-btn" style="width:100%">
+        <button class="btn btn-secondary" type="button" id="webauthn-login-btn" style="width:100%"
+                {{if .SSONonce}}data-sso-nonce="{{.SSONonce}}"{{end}}>
           Sign in with passkey
         </button>
       </div>