Compare auth server metadata issuer ignoring trailing slash (#955)

oauthex/auth_meta.go: fixes metadata issuer comparison to ignore
trailing slash difference

Google's gmail MCP server returns `https://accounts.google.com/` as the
Authorization Server in the Protected Resource Metadata, but returns
`https://accounts.google.com` in the Auth Server Metadata. This causes a
comparison failure per rfc8414 section-3.3: Authorization Server
Metadata Validation. The authorization server URL is meant to be a base
to build other URLs from, it does not seem necessary to enforce that the
trailing slash (or lack thereof) matches with the issuer URL. Please see
#953 for more details.

Fixes #953

---------

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

oauthex/auth_meta.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"strings"
 )
 
 // AuthServerMeta represents the metadata for an OAuth 2.0 authorization server,
@@ -145,7 +146,7 @@ func GetAuthServerMeta(ctx context.Context, metadataURL, issuer string, c *http.
 		}
 		return nil, fmt.Errorf("%v", err) // Do not expose error types.
 	}
-	if asm.Issuer != issuer {
+	if strings.TrimRight(asm.Issuer, "/") != strings.TrimRight(issuer, "/") {
 		// Validate the Issuer field (see RFC 8414, section 3.3).
 		return nil, fmt.Errorf("metadata issuer %q does not match issuer URL %q", asm.Issuer, issuer)
 	}

oauthex/auth_meta_test.go

@@ -35,9 +35,10 @@ func TestAuthMetaParse(t *testing.T) {
 func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 	ctx := context.Background()
 	tests := []struct {
-		name           string
-		hasPKCESupport bool
-		wantError      string
+		name                    string
+		hasPKCESupport          bool
+		wantError               string
+		issuerWithTrailingSlash bool
 	}{
 		{
 			name:           "server_with_pkce_support",
@@ -48,6 +49,12 @@ func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 			hasPKCESupport: false,
 			wantError:      "does not implement PKCE",
 		},
+		{
+			// ProtectedResourceMetadata may contain AuthorizationServers with a trailing slash (see Issue #953)
+			name:                    "issuer_with_trailing_slash",
+			hasPKCESupport:          true,
+			issuerWithTrailingSlash: true,
+		},
 	}
 
 	for _, tt := range tests {
@@ -85,6 +92,9 @@ func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 			u, _ := url.Parse(ts.URL)
 			issuer := "https://localhost:" + u.Port()
 			metadataURL := issuer + "/.well-known/oauth-authorization-server"
+			if tt.issuerWithTrailingSlash {
+				issuer += "/"
+			}
 
 			// The fake server presents a cert for example.com; set ServerName accordingly.
 			httpClient := ts.Client()