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

• Go github.com/gorilla/csrf: < 1.7.3 → fixed in 1.7.3

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) // // 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.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.