GHSA-h95v-h523-3mw8
MEDIUMGuzzle: URI fragments disclosed in redirect Referer headers
Blast Radius
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Description
Impact
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.
Patches
The issue is patched in 7.15.1 and later. Starting in that release, Guzzle removes both user information and the fragment before generating a redirect Referer value. Other redirect behavior does not change. Guzzle retains the referring path and query, and continues to omit the header when the scheme changes. Versions before 7.15.1 are affected when the optional referer setting is enabled.
Workarounds
If you cannot upgrade immediately, leave automatic Referer generation disabled. It is off by default. If redirect options are configured explicitly, ensure that referer is false:
$client->request('GET', $uri, [
'allow_redirects' => ['referer' => false],
]);
Alternatively, disable automatic redirects and follow trusted destinations manually, or remove the fragment from the request URI before sending a request that may redirect. Do not rely on the fact that fragments are absent from the initial HTTP request target, because the affected middleware can reintroduce them in the generated header.
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | guzzlehttp/guzzle | all versions | 7.15.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for guzzlehttp/guzzle. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update guzzlehttp/guzzle to 7.15.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-h95v-h523-3mw8 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-h95v-h523-3mw8 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-h95v-h523-3mw8. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-h95v-h523-3mw8 in your dependencies?
O3 detects GHSA-h95v-h523-3mw8 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.