oauthex: require S256 PKCE support in GetAuthServerMeta

oauthex/auth_meta.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"slices"
 
 	"github.com/modelcontextprotocol/go-sdk/internal/authutil"
 )
@@ -131,8 +132,9 @@ type AuthServerMeta struct {
 //   - The metadataURL must use HTTPS or be a local address.
 //   - The Issuer field is checked against metadataURL.Issuer.
 //
-// It also verifies that the authorization server supports PKCE and that the URLs
-// in the metadata don't use dangerous schemes.
+// It also verifies that the authorization server supports PKCE with the S256
+// code challenge method, which is the one this SDK's clients use, and that the
+// URLs in the metadata don't use dangerous schemes.
 //
 // It returns an error if the request fails with a non-4xx status code or the fetched
 // metadata doesn't pass security validations.
@@ -161,6 +163,9 @@ func GetAuthServerMeta(ctx context.Context, metadataURL, issuer string, c *http.
 	if len(asm.CodeChallengeMethodsSupported) == 0 {
 		return nil, fmt.Errorf("authorization server at %s does not implement PKCE", issuer)
 	}
+	if !slices.Contains(asm.CodeChallengeMethodsSupported, "S256") {
+		return nil, fmt.Errorf("authorization server at %s does not support the S256 PKCE method", issuer)
+	}
 
 	// Validate endpoint URLs to prevent XSS attacks (see #526).
 	if err := validateAuthServerMetaURLs(asm); err != nil {

oauthex/auth_meta_test.go

@@ -37,6 +37,7 @@ func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 	tests := []struct {
 		name                    string
 		hasPKCESupport          bool
+		pkceMethods             []string // overrides the default ["S256"] when set
 		wantError               string
 		issuerWithTrailingSlash bool
 	}{
@@ -49,6 +50,19 @@ func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 			hasPKCESupport: false,
 			wantError:      "does not implement PKCE",
 		},
+		{
+			// The client always sends an S256 code challenge, so a server
+			// that offers only plain cannot complete the flow.
+			name:           "server_with_only_plain_pkce",
+			hasPKCESupport: true,
+			pkceMethods:    []string{"plain"},
+			wantError:      "does not support the S256 PKCE method",
+		},
+		{
+			name:           "server_with_plain_and_s256_pkce",
+			hasPKCESupport: true,
+			pkceMethods:    []string{"plain", "S256"},
+		},
 		{
 			// ProtectedResourceMetadata may contain AuthorizationServers with a trailing slash (see Issue #953)
 			name:                    "issuer_with_trailing_slash",
@@ -79,6 +93,9 @@ func TestGetAuthServerMetaPKCESupport(t *testing.T) {
 				// Add PKCE support based on test case
 				if tt.hasPKCESupport {
 					metadata.CodeChallengeMethodsSupported = []string{"S256"}
+					if tt.pkceMethods != nil {
+						metadata.CodeChallengeMethodsSupported = tt.pkceMethods
+					}
 				}
 				// If hasPKCESupport is false, CodeChallengeMethodsSupported remains empty