auth: add AllowMissingExpiration option to RequireBearerTokenOptions (#971)

## Summary

- Add an `AllowMissingExpiration bool` field to
`RequireBearerTokenOptions` that opts the middleware out of the existing
`tokenInfo.Expiration.IsZero()` reject.
- Default false preserves the existing strict behaviour: every TokenInfo
must carry an Expiration.

## Motivation

Some IdPs emit session-bound bearer tokens that do not carry a
standalone `exp` claim — the token's lifetime is bounded by an external
session and is not advertised in-band. Resource servers integrating with
such IdPs today have to synthesise a fake Expiration from their
TokenVerifier callback (dishonest data through the auth layer) or fork
`RequireBearerToken` to bypass the check.

This option lets the verifier honestly report "no expiration in the
token" via a zero `Expiration` and have the middleware accept it — while
still applying the elapsed-validity check for tokens that DO carry an
Expiration.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

auth/auth.go

@@ -47,6 +47,21 @@ type RequireBearerTokenOptions struct {
 	ResourceMetadataURL string
 	// The required scopes.
 	Scopes []string
+	// AllowMissingExpiration opts the middleware out of the
+	// `tokenInfo.Expiration.IsZero()` reject. Default false preserves the
+	// existing strict behaviour (every TokenInfo must carry an Expiration).
+	//
+	// Some IdPs emit session-bound bearer tokens that do not carry a standalone
+	// `exp` claim — the token's lifetime is bounded by an external session and
+	// is not advertised in-band. Resource servers integrating with such IdPs
+	// need to opt in to validating the rest of the token (scopes, signature
+	// via the verifier callback, etc.) without requiring the expiration field
+	// to be present.
+	//
+	// When enabled, the verifier is still responsible for any session-level
+	// validity check it can perform; this option only relaxes the middleware's
+	// own expiration enforcement.
+	AllowMissingExpiration bool
 }
 
 type tokenInfoKey struct{}
@@ -131,9 +146,10 @@ func verify(req *http.Request, verifier TokenVerifier, opts *RequireBearerTokenO
 
 	// Check expiration.
 	if tokenInfo.Expiration.IsZero() {
-		return nil, "token missing expiration", http.StatusUnauthorized
-	}
-	if tokenInfo.Expiration.Before(time.Now()) {
+		if opts == nil || !opts.AllowMissingExpiration {
+			return nil, "token missing expiration", http.StatusUnauthorized
+		}
+	} else if tokenInfo.Expiration.Before(time.Now()) {
 		return nil, "token expired", http.StatusUnauthorized
 	}
 	return tokenInfo, "", 0

auth/auth_test.go

@@ -62,6 +62,11 @@ func TestVerify(t *testing.T) {
 			"no expiration", nil, "Bearer noexp",
 			"token missing expiration", 401,
 		},
+		{
+			"no expiration with AllowMissingExpiration accepts",
+			&RequireBearerTokenOptions{AllowMissingExpiration: true}, "Bearer noexp",
+			"", 0,
+		},
 		{
 			"expired", nil, "Bearer expired",
 			"token expired", 401,