GHSA-h95v-h523-3mw8: Guzzle: URI fragments disclosed in redirect Referer headers

When the optional referer redirect setting is enabled, affected versions of RedirectMiddleware can copy the fragment from the referring request URI into a generated Referer header. A URI fragment is the part after #. It is handled locally by the client and is not part of the HTTP request target, so the server handling the original request does not receive it. A generated Referer tells the redirect destination which URI led to the request. Guzzle correctly removes user information from that value, but retains the fragment when it follows a redirect to the same scheme, such as HTTPS to HTTPS. For example, an initial URI ending in #secret is sent without the fragment, but the redirect destination can receive a Referer ending in #secret. This behavior affects both the cURL and stream handlers.

An attacker who controls the redirect destination can read the fragment from the incoming header, including through request logs or application code. If the fragment contains a one-time login secret, access token, state value, or other private client data, it is disclosed to a server that was never meant to receive it. Exploitation requires the application to enable allow_redirects.referer, make a request to a URI with a sensitive fragment, and follow a same-scheme redirect to a less-trusted destination.

The referer setting is disabled by default. Applications that leave it disabled, do not put sensitive data in URI fragments, do not follow redirects, or only redirect within the same trust boundary are not affected. Guzzle already omits the generated header when the scheme changes. This issue is limited to the fragment’s inclusion. Reducing the path and query on cross-origin redirects is a separate privacy policy question.