GHSA-fh3h-vg37-cc95
MEDIUMWebOb: Location header normalization during redirect leads to open redirect - again
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
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Description
Impact
When WebOb normalizes the HTTP Location header to include the request hostname, it does so by parsing the URL that the user is to be redirected to with Python's urllib.parse, and joining it to the base URL. urlsplit (called internally by urljoin) however treats a // at the start of a string as a URI without a scheme, and then treats the next part as the hostname. urljoin will then use that hostname from the second part as the hostname replacing the original one from the request.
In a previous advisory https://github.com/Pylons/webob/security/advisories/GHSA-mg3v-6m49-jhp3 an attempt to fix this was made by forcing the replacement of // with /%2f, however this did not take into account that since Python 3.10 urlsplit internally strips ASCII tab, carriage return, and newline characters from the string, so /\t/attacker.com gets turned into //attacker.com and the attacker is able to bypass the changes introduced in that previous advisory, thereby bringing back the problem that was attempted to be fixed.
>>> parse.urlparse("//attacker.com/some/path")
ParseResult(scheme='', netloc='attacker.com', path='/some/path', params='', query='', fragment='')
WebOb uses urljoin to take the request URI and join the redirect location to it, so assuming the request URI is https://example.org/ and the URL to redirect to is /\t/attacker.com/some/path/:
>>> parse.urljoin("https://example.org/", "/\t/attacker.com/some/path/")
'https://attacker.com/some/path/'
Which redirects from example.org where we want the user to stay to attacker.com.
Patches
This issue has been fixed in WebOb 1.8.10.
Workarounds
Any use of the Response class that includes a location can be rewritten to make sure to always pass a full URI that includes the hostname to redirect the user to, or to validate that the redirect target starts with a scheme (e.g. http:// or https://) before assigning to Response.location.
References
Thanks
- Caleb Brown of Google
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | webob | all versions | 1.8.10 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for webob. 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 webob to 1.8.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fh3h-vg37-cc95 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-fh3h-vg37-cc95 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-fh3h-vg37-cc95. 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-fh3h-vg37-cc95 in your dependencies?
O3 detects GHSA-fh3h-vg37-cc95 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.