metacrypt/internal/webserver/csrf.go

package webserver

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"fmt"
	"net/http"
	"strings"
	"sync"
)

const (
	csrfCookieName = "metacrypt_csrf"
	csrfFieldName  = "csrf_token"
	csrfTokenLen   = 32
)

// csrfProtect provides CSRF protection using the signed double-submit cookie
// pattern. A random secret is generated at startup. CSRF tokens are an HMAC of
// a random nonce, sent as both a cookie and a hidden form field. On POST the
// middleware verifies that the form field matches the cookie's HMAC.
type csrfProtect struct {
	secret []byte
	once   sync.Once
}

func newCSRFProtect() *csrfProtect {
	secret := make([]byte, 32)
	if _, err := rand.Read(secret); err != nil {
		panic("csrf: failed to generate secret: " + err.Error())
	}
	return &csrfProtect{secret: secret}
}

// generateToken creates a new CSRF token: base64(nonce) + "." + base64(hmac(nonce)).
func (c *csrfProtect) generateToken() (string, error) {
	nonce := make([]byte, csrfTokenLen)
	if _, err := rand.Read(nonce); err != nil {
		return "", fmt.Errorf("csrf: generate nonce: %w", err)
	}
	nonceB64 := base64.RawURLEncoding.EncodeToString(nonce)
	mac := hmac.New(sha256.New, c.secret)
	mac.Write(nonce)
	sigB64 := base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
	return nonceB64 + "." + sigB64, nil
}

// validToken checks that a token has a valid HMAC signature.
func (c *csrfProtect) validToken(token string) bool {
	parts := strings.SplitN(token, ".", 2)
	if len(parts) != 2 {
		return false
	}
	nonce, err := base64.RawURLEncoding.DecodeString(parts[0])
	if err != nil || len(nonce) != csrfTokenLen {
		return false
	}
	sig, err := base64.RawURLEncoding.DecodeString(parts[1])
	if err != nil {
		return false
	}
	mac := hmac.New(sha256.New, c.secret)
	mac.Write(nonce)
	return hmac.Equal(mac.Sum(nil), sig)
}

// setToken generates a new CSRF token, sets it as a cookie, and returns it
// for embedding in a form.
func (c *csrfProtect) setToken(w http.ResponseWriter) string {
	token, err := c.generateToken()
	if err != nil {
		return ""
	}
	http.SetCookie(w, &http.Cookie{
		Name:     csrfCookieName,
		Value:    token,
		Path:     "/",
		HttpOnly: true,
		Secure:   true,
		SameSite: http.SameSiteStrictMode,
	})
	return token
}

// middleware returns an HTTP middleware that enforces CSRF validation on
// mutation requests (POST, PUT, PATCH, DELETE). GET/HEAD/OPTIONS are passed
// through. The HTMX hx-post for /v1/seal is excluded since it hits the API
// server directly and uses token auth, not cookies.
func (c *csrfProtect) middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		switch r.Method {
		case http.MethodGet, http.MethodHead, http.MethodOptions:
			next.ServeHTTP(w, r)
			return
		}

		// Read token from form field (works for both regular forms and
		// multipart forms since ParseForm/ParseMultipartForm will have
		// been called or the field is available via FormValue).
		formToken := r.FormValue(csrfFieldName)

		// Read token from cookie.
		cookie, err := r.Cookie(csrfCookieName)
		if err != nil || cookie.Value == "" {
			http.Error(w, "CSRF validation failed", http.StatusForbidden)
			return
		}

		// Both tokens must be present, match each other, and be validly signed.
		if formToken == "" || formToken != cookie.Value || !c.validToken(formToken) {
			http.Error(w, "CSRF validation failed", http.StatusForbidden)
			return
		}

		next.ServeHTTP(w, r)
	})
}