fix(security): classify audience failures by sentinel error

The lenient-audience path chose itself when the verify error text
contained the word audience. The key lookup error embeds the kid from the
unverified token header. A kid of audience made a failed signature look
like an audience-only failure. verifyAudience now returns a sentinel and
the caller uses errors.Is.

audience_sentinel_test.go

@@ -0,0 +1,24 @@
+package traefikoidc
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+// Only a real audience mismatch may select the lenient-audience path; an
+// error text that merely embeds the attacker-chosen kid must not.
+func TestAudienceErrorClassifiedBySentinel(t *testing.T) {
+	if !errors.Is(fmt.Errorf("standard claim verification failed: %w", verifyAudience("x", "y")), errInvalidAudience) {
+		t.Fatal("audience mismatch not recognized through wrapping")
+	}
+	if errors.Is(verifyAudience("y", "y"), errInvalidAudience) {
+		t.Fatal("matching audience classified as mismatch")
+	}
+	for _, kid := range []string{"audience", "invalid audience"} {
+		err := fmt.Errorf("failed to get public key: no matching public key found for kid: %s", kid)
+		if errors.Is(err, errInvalidAudience) {
+			t.Errorf("kid %q fooled the audience classifier", kid)
+		}
+	}
+}

jwt.go

@@ -6,6 +6,7 @@ import (
 	"crypto/ecdsa"
 	"crypto/rsa"
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"math"
 	"math/big"
@@ -195,11 +196,16 @@ func (j *JWT) Verify(issuerURL, expectedAudience string) error {
 //
 // Returns:
 //   - An error if the claim type is invalid or the expected audience is not present
+//
+// errInvalidAudience marks a token whose aud does not match; callers match it
+// with errors.Is, never by message text.
+var errInvalidAudience = errors.New("invalid audience")
+
 func verifyAudience(tokenAudience interface{}, expectedAudience string) error {
 	switch aud := tokenAudience.(type) {
 	case string:
 		if aud != expectedAudience {
-			return fmt.Errorf("invalid audience")
+			return errInvalidAudience
 		}
 	case []interface{}:
 		found := false
@@ -210,7 +216,7 @@ func verifyAudience(tokenAudience interface{}, expectedAudience string) error {
 			}
 		}
 		if !found {
-			return fmt.Errorf("invalid audience")
+			return errInvalidAudience
 		}
 	default:
 		return fmt.Errorf("invalid 'aud' claim type")

token_validation_rs.go

@@ -15,6 +15,7 @@ package traefikoidc
 import (
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"strings"
 	"time"
 )
@@ -266,8 +267,7 @@ func (t *TraefikOidc) validateStandardTokensRS(rs *requestState) (bool, bool, bo
 	var accessVerifyErr error
 	if err := t.verifyToken(rs.accessToken); err != nil {
 		accessVerifyErr = err
-		errMsg := err.Error()
-		if strings.Contains(errMsg, "invalid audience") || strings.Contains(errMsg, "audience") {
+		if errors.Is(err, errInvalidAudience) {
 			if t.strictAudienceValidation {
 				if rs.refreshToken != "" {
 					return false, true, false