fix(auth): log why a passkey ceremony was refused

Every WebAuthn refusal answered "that passkey could not be verified" and
discarded the library's error, leaving a misconfigured relying party
undiagnosable. Log the stage, the derived RP ID and expected origin, the
request's Origin and X-Forwarded-Proto, and the library error with its
DevInfo. None of it is secret.

server/internal/auth/mfa_enrol.go

@@ -212,6 +212,7 @@ func HandleEnrolPasskeyFinish(c *gin.Context) {
 	}
 	cred, err := w.CreateCredential(waUser{handle: handle, name: t.Email}, *sessionData, parsed)
 	if err != nil {
+		logWebAuthnFailure(c, "enrolment registration", err)
 		c.JSON(http.StatusBadRequest, gin.H{"error": "that passkey could not be verified"})
 		return
 	}

server/internal/auth/passkey_login.go

@@ -85,6 +85,7 @@ func HandlePasskeyLoginFinish(c *gin.Context) {
 	// that owner's.
 	cred, err := finishAssertion(c, instanceID, "", "")
 	if err != nil {
+		logWebAuthnFailure(c, "passwordless assertion", err)
 		c.JSON(http.StatusUnauthorized, gin.H{
 			"error": "that passkey could not be verified", "code": "invalid_assertion",
 		})

server/internal/auth/webauthn.go

@@ -6,6 +6,7 @@ import (
 	"crypto/subtle"
 	"encoding/json"
 	"errors"
+	"log"
 	"net"
 	"net/http"
 	"time"
@@ -42,6 +43,23 @@ func rpConfig(c *gin.Context) (string, string) {
 	return rpID, scheme + "://" + host
 }
 
+// logWebAuthnFailure records why a ceremony was refused. The response stays
+// deliberately vague, so without this the only evidence of a misconfigured
+// relying party - most often a proxy that terminates TLS without passing
+// X-Forwarded-Proto: https - is a user reporting that their passkey "could not
+// be verified". Nothing logged here is secret: RP ID, origins, and the
+// library's error, whose DevInfo names expected and received values.
+func logWebAuthnFailure(c *gin.Context, stage string, err error) {
+	rpID, origin := rpConfig(c)
+	detail := err.Error()
+	var perr *protocol.Error
+	if errors.As(err, &perr) && perr.DevInfo != "" {
+		detail += " (" + perr.DevInfo + ")"
+	}
+	log.Printf("webauthn: %s refused: rp_id=%q expected_origin=%q request_origin=%q x_forwarded_proto=%q: %s",
+		stage, rpID, origin, c.GetHeader("Origin"), c.GetHeader("X-Forwarded-Proto"), detail)
+}
+
 func webAuthnFor(c *gin.Context) (*webauthn.WebAuthn, error) {
 	rpID, origin := rpConfig(c)
 	return webauthn.New(&webauthn.Config{
@@ -176,6 +194,7 @@ func HandleMFAWebAuthnFinish(c *gin.Context) {
 	}
 	cred, err := finishAssertion(c, t.InstanceID, t.UserID, t.Email)
 	if err != nil {
+		logWebAuthnFailure(c, "second-factor assertion", err)
 		left, ferr := FailTicket(c.Request.Context(), ticketID)
 		services.LogEvent(t.InstanceID, "mfa.failed", t.Email, "", "", "factor=webauthn")
 		if ferr != nil || left == 0 {
@@ -287,6 +306,7 @@ func HandleRegisterPasskeyFinish(c *gin.Context) {
 	}
 	cred, err := w.CreateCredential(waUser{handle: handle, name: sess.Email}, *sessionData, parsed)
 	if err != nil {
+		logWebAuthnFailure(c, "registration", err)
 		c.JSON(http.StatusBadRequest, gin.H{"error": "that passkey could not be verified"})
 		return
 	}
@@ -382,6 +402,7 @@ func HandleStepUpWebAuthnFinish(c *gin.Context) {
 	sess := GetSessionFromContext(c)
 	cred, err := finishAssertion(c, sess.InstanceID, sess.UserID, sess.Email)
 	if err != nil {
+		logWebAuthnFailure(c, "step-up assertion", err)
 		services.LogEvent(sess.InstanceID, "step_up.failed", sess.Email, "", "", "factor=webauthn")
 		c.JSON(http.StatusUnauthorized, gin.H{"error": "that passkey could not be verified", "code": "invalid_assertion"})
 		return