Update DefaultOsBrowserWebUi.cs

Author: ashok672

src/client/Microsoft.Identity.Client/Platforms/Features/DefaultOSBrowser/DefaultOsBrowserWebUi.cs

@@ -63,7 +63,6 @@ public async Task<AuthorizationResult> AcquireAuthorizationAsync(
         {
             try
             {
-                // Add response_mode=form_post for security (prevents auth code from appearing in browser history/logs)
                 var authUriBuilder = new UriBuilder(authorizationUri);
                 authUriBuilder.AppendOrReplaceQueryParameter(OAuth2Parameter.ResponseMode, "form_post");
                 authorizationUri = authUriBuilder.Uri;
@@ -87,18 +86,16 @@ public async Task<AuthorizationResult> AcquireAuthorizationAsync(
                             authResponse.RequestUri.AbsolutePath,
                             redirectUri.AbsolutePath));
                 }
-
-                // Use FromPostData for form_post responses (more secure - never constructs URI with auth code)
-                // Use FromUri for legacy GET responses (query string)
                 if (authResponse.IsFormPost)
                 {
                     _logger.Info(() => "[DefaultOsBrowser] Processing form_post response securely from POST data");
                     return AuthorizationResult.FromPostData(authResponse.PostData);
                 }
                 else
                 {
-                    _logger.Info(() => "[DefaultOsBrowser] Processing legacy GET response from query string");
-                    return AuthorizationResult.FromUri(authResponse.RequestUri.OriginalString);
+                    throw new MsalClientException(
+                        MsalError.AuthenticationFailed,
+                        "The authorization server did not honor response_mode=form_post");
                 }
             }
             catch (System.Net.HttpListenerException) // sometimes this exception sneaks out (see issue 1773)