GHSA-rq77-p4h8-4crw: gorilla/csrf CSRF vulnerability due to broken Referer validation

Summary

gorilla/csrf is vulnerable to CSRF via form submission from origins that share a top level domain with the target origin.

Details

gorilla/csrf does not validate the Origin header against an allowlist. Its executes its validation of the Referer header for cross-origin requests only when it believes the request is being served over TLS. It determines this by inspecting the r.URL.Scheme value. However, this value is never populated for “server” requests per the Go spec, and so this check does not run in practice.

    // URL specifies either the URI being requested (for server
    // requests) or the URL to access (for client requests).
    //
    // For server requests, the URL is parsed from the URI
    // supplied on the Request-Line as stored in RequestURI.  For
    // most requests, fields other than Path and RawQuery will be
    // empty. (See [RFC 7230, Section 5.3](https://rfc-editor.org/rfc/rfc7230.html#section-5.3))
    //
    // For client requests, the URL's Host specifies the server to
    // connect to, while the Request's Host field optionally
    // specifies the Host header value to send in the HTTP
    // request.
    URL *[url](https://pkg.go.dev/net/url).[URL](https://pkg.go.dev/net/url#URL)

PoC

• create trusted origin target.example.test protected with gorilla/csrf and served over TLS hosting form on /submit
• create attacker origin attack.example.test served over TLS
• attacker exfiltrates token & cookie combination from target.example.test
• attacker sets exfiltrated cookie with domain=.example.test and path=/submit
  • as the cookie has a more specific path than / (the default for CSRF cookies) it will be sent first by the browser on submit to our target origin
• submit form from attack.example.test with exfiltrated CSRF form token
• observe valid form submission as attack.example.test Origin / Referer headers are not validated.

Impact

This vulnerability allows an attacker who has gained XSS on a subdomain or top level domain to perform authenticated form submissions against gorilla/csrf protected targets that share the same top level domain.

This bug has existed in gorilla/csrf since its initial release in 2015.

References

• GHSA-rq77-p4h8-4crw
• gorilla/csrf@9dd6af1