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

Affected: Go: github.com/gorilla/csrf

### 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](https://pkg.go.dev/net/http#Request), 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…

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