GHSA-hff2-gcpx-8f4p: Apollo Router Core: Browser Bug Enables Bypass of XS-Search Prevention via Read-Only Cross-Site Request Forgery

In a Cross-Site Request Forgery attack, untrusted web content causes browsers to send authenticated requests to web servers which use cookies for authentication. While the web content is prevented from reading the request’s response due to the Cross-Origin Request Sharing (CORS) protocol, the attacker may be able to cause side effects in the server (“CSRF” attack), or learn something about the response via timing analysis (“XS-Search” attack).

Apollo Router has a built-in feature which prevents CSRF and XS-Search attacks: it refuses to process GraphQL requests that could possibly have been sent by a spec-compliant web browser without a protective “preflight” step. See Apollo Router’s docs for more details on CORS, CSRF attacks, and Apollo Router’s CSRF prevention feature.

This feature is fully effective against attacks carried out against users of spec-compliant browsers. Unfortunately, a major browser introduced a bug in 2025 which meant in certain cases, it failed to follow the CORS spec. The browser’s maintainers have already committed to fixing the bug and making the browser spec-compliant again.

Even with this bug, Apollo Router’s CSRF prevention feature blocks “side effect” CSRF attacks: Apollo Router will still correctly refuse to execute mutations in requests that were not preflighted. However, some specially crafted authenticated GraphQL queries can be issued across origins without preflight in buggy versions of this browser, allowing for XS-Search attacks: an attacker can analyze response times to learn facts about the responses to requests such as whether fields return null or approximately how many list entries are returned from fields.

Apollo Router installations are only vulnerable if they rely on cookies (or HTTP Basic Auth) for authentication.