webhooker/internal/middleware/csrf.go

package middleware

import (
	"net/http"

	"github.com/gorilla/csrf"
	"sneak.berlin/go/webhooker/internal/logfield"
	"sneak.berlin/go/webhooker/internal/reqtls"
)

// CSRFToken retrieves the CSRF token from the request context.
// Returns an empty string if the gorilla/csrf middleware has not run.
func CSRFToken(r *http.Request) string {
	return csrf.Token(r)
}

// CSRF returns middleware that provides CSRF protection using the
// gorilla/csrf library. The middleware uses the session authentication
// key to sign a CSRF cookie and validates a masked token submitted via
// the "csrf_token" form field (or the "X-CSRF-Token" header) on
// POST/PUT/PATCH/DELETE requests. Requests with an invalid or missing
// token receive a 403 Forbidden response.
//
// The middleware detects the client-facing transport protocol
// per-request via reqtls.IsTLS, the single TLS predicate the session
// cookie also uses. This allows correct behavior in all deployment
// scenarios:
//
//   - Direct HTTPS: strict Referer/Origin checks, Secure cookies.
//   - Behind a TLS-terminating reverse proxy: strict checks (the
//     browser is on HTTPS, so Origin/Referer headers use https://),
//     Secure cookies (the browser sees HTTPS from the proxy).
//   - Direct HTTP: relaxed Referer/Origin checks via PlaintextHTTPRequest,
//     non-Secure cookies so the browser sends them over HTTP.
//
// Two gorilla/csrf instances are maintained — one with Secure cookies
// (for TLS) and one without (for plaintext HTTP) — because the
// csrf.Secure option is set at creation time, not per-request.
func (m *Middleware) CSRF() func(http.Handler) http.Handler {
	csrfErrorHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// CSRF is registered ahead of RequireAuth on every route
		// group that uses it, so this WARN is reachable by an
		// unauthenticated client: a POST with no token to
		// /source/<any length of any text>/edit lands here. The
		// method and path are capped against the same budgets as
		// the access log. remote_addr is set by net/http from the
		// accepted connection rather than by the client, and
		// csrf.FailureReason returns one of gorilla/csrf's own
		// fixed error values, so neither is client-sized.
		m.log.Warn("csrf: token validation failed",
			"method", logfield.Truncate(
				r.Method, maxLogMethodBytes,
			),
			"path", logfield.Truncate(
				r.URL.Path, logfield.MaxBytes,
			),
			"remote_addr", r.RemoteAddr,
			"reason", csrf.FailureReason(r),
		)
		http.Error(w, "Forbidden - invalid CSRF token", http.StatusForbidden)
	})

	key := m.session.GetKey()
	baseOpts := []csrf.Option{
		csrf.FieldName("csrf_token"),
		csrf.SameSite(csrf.SameSiteLaxMode),
		csrf.Path("/"),
		csrf.ErrorHandler(csrfErrorHandler),
	}

	// Two middleware instances with different Secure flags but the
	// same signing key, so cookies are interchangeable between them.
	tlsProtect := csrf.Protect(key, append(baseOpts, csrf.Secure(true))...)
	httpProtect := csrf.Protect(key, append(baseOpts, csrf.Secure(false))...)

	return func(next http.Handler) http.Handler {
		tlsCSRF := tlsProtect(next)
		httpCSRF := httpProtect(next)

		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			if reqtls.IsTLS(r) {
				// Client is on TLS (directly or via reverse proxy).
				// Use Secure cookies and strict Origin/Referer checks.
				tlsCSRF.ServeHTTP(w, r)
			} else {
				// Plaintext HTTP: use non-Secure cookies and tell
				// gorilla/csrf to use "http" for scheme comparisons,
				// skipping the strict Referer check that assumes TLS.
				httpCSRF.ServeHTTP(w, csrf.PlaintextHTTPRequest(r))
			}
		})
	}
}