Fix SEC-11: use json.Marshal for audit details

- Add internal/audit package with JSON() and JSONWithRoles() helpers
  that use json.Marshal instead of fmt.Sprintf with %q
- Replace all fmt.Sprintf audit detail construction in:
  - internal/server/server.go (10 occurrences)
  - internal/ui/handlers_auth.go (4 occurrences)
  - internal/grpcserver/auth.go (4 occurrences)
- Add tests for the helpers including edge-case Unicode,
  null bytes, special characters, and odd argument counts
- Fix broken {"roles":%v} formatting that produced invalid JSON

Security: Audit log detail strings are now constructed via
json.Marshal, which correctly handles all Unicode edge cases
(U+2028, U+2029, null bytes, etc.) that fmt.Sprintf with %q
may mishandle. This prevents potential log injection or parsing
issues in audit event consumers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/audit/detail.go

@@ -0,0 +1,33 @@
+// Package audit provides helpers for constructing audit log detail strings.
+package audit
+
+import "encoding/json"
+
+// JSON builds a JSON details string from key-value pairs for audit logging.
+// Uses json.Marshal for safe encoding rather than fmt.Sprintf with %q,
+// which is fragile for edge-case Unicode.
+func JSON(pairs ...string) string {
+	if len(pairs)%2 != 0 {
+		return "{}"
+	}
+	m := make(map[string]string, len(pairs)/2)
+	for i := 0; i < len(pairs); i += 2 {
+		m[pairs[i]] = pairs[i+1]
+	}
+	b, err := json.Marshal(m)
+	if err != nil {
+		return "{}"
+	}
+	return string(b)
+}
+
+// JSONWithRoles builds a JSON details string that includes a "roles" key
+// mapped to a string slice. This produces a proper JSON array for the value.
+func JSONWithRoles(roles []string) string {
+	m := map[string][]string{"roles": roles}
+	b, err := json.Marshal(m)
+	if err != nil {
+		return "{}"
+	}
+	return string(b)
+}

internal/audit/detail_test.go

@@ -0,0 +1,163 @@
+package audit
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestJSON(t *testing.T) {
+	tests := []struct {
+		name   string
+		pairs  []string
+		verify func(t *testing.T, result string)
+	}{
+		{
+			name:  "single pair",
+			pairs: []string{"username", "alice"},
+			verify: func(t *testing.T, result string) {
+				var m map[string]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON: %v", err)
+				}
+				if m["username"] != "alice" {
+					t.Fatalf("expected alice, got %s", m["username"])
+				}
+			},
+		},
+		{
+			name:  "multiple pairs",
+			pairs: []string{"jti", "abc-123", "reason", "logout"},
+			verify: func(t *testing.T, result string) {
+				var m map[string]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON: %v", err)
+				}
+				if m["jti"] != "abc-123" {
+					t.Fatalf("expected abc-123, got %s", m["jti"])
+				}
+				if m["reason"] != "logout" {
+					t.Fatalf("expected logout, got %s", m["reason"])
+				}
+			},
+		},
+		{
+			name:  "special characters in values",
+			pairs: []string{"username", "user\"with\\quotes"},
+			verify: func(t *testing.T, result string) {
+				var m map[string]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON for special chars: %v", err)
+				}
+				if m["username"] != "user\"with\\quotes" {
+					t.Fatalf("unexpected value: %s", m["username"])
+				}
+			},
+		},
+		{
+			name:  "unicode edge cases",
+			pairs: []string{"username", "user\u2028line\u2029sep"},
+			verify: func(t *testing.T, result string) {
+				var m map[string]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON for unicode: %v", err)
+				}
+				if m["username"] != "user\u2028line\u2029sep" {
+					t.Fatalf("unexpected value: %s", m["username"])
+				}
+			},
+		},
+		{
+			name:  "null bytes in value",
+			pairs: []string{"data", "before\x00after"},
+			verify: func(t *testing.T, result string) {
+				var m map[string]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON for null bytes: %v", err)
+				}
+				if m["data"] != "before\x00after" {
+					t.Fatalf("unexpected value: %q", m["data"])
+				}
+			},
+		},
+		{
+			name:  "odd number of args returns empty object",
+			pairs: []string{"key"},
+			verify: func(t *testing.T, result string) {
+				if result != "{}" {
+					t.Fatalf("expected {}, got %s", result)
+				}
+			},
+		},
+		{
+			name:  "no args returns empty object",
+			pairs: nil,
+			verify: func(t *testing.T, result string) {
+				if result != "{}" {
+					t.Fatalf("expected {}, got %s", result)
+				}
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := JSON(tc.pairs...)
+			tc.verify(t, result)
+		})
+	}
+}
+
+func TestJSONWithRoles(t *testing.T) {
+	tests := []struct {
+		name   string
+		roles  []string
+		verify func(t *testing.T, result string)
+	}{
+		{
+			name:  "multiple roles",
+			roles: []string{"admin", "editor"},
+			verify: func(t *testing.T, result string) {
+				var m map[string][]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON: %v", err)
+				}
+				if len(m["roles"]) != 2 || m["roles"][0] != "admin" || m["roles"][1] != "editor" {
+					t.Fatalf("unexpected roles: %v", m["roles"])
+				}
+			},
+		},
+		{
+			name:  "empty roles",
+			roles: []string{},
+			verify: func(t *testing.T, result string) {
+				var m map[string][]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON: %v", err)
+				}
+				if len(m["roles"]) != 0 {
+					t.Fatalf("expected empty roles, got %v", m["roles"])
+				}
+			},
+		},
+		{
+			name:  "roles with special characters",
+			roles: []string{"role\"special"},
+			verify: func(t *testing.T, result string) {
+				var m map[string][]string
+				if err := json.Unmarshal([]byte(result), &m); err != nil {
+					t.Fatalf("invalid JSON: %v", err)
+				}
+				if m["roles"][0] != "role\"special" {
+					t.Fatalf("unexpected role: %s", m["roles"][0])
+				}
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := JSONWithRoles(tc.roles)
+			tc.verify(t, result)
+		})
+	}
+}