GHSA-fvhj-4qfh-q2hm
MEDIUMTraefik incorrectly processes fragment in the URL, leads to Authorization Bypass
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
github.com/traefik/traefik/v2🐹github.com/traefik/traefik/v3Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
When a request is sent to Traefik with a URL fragment, Traefik automatically URL encodes and forwards the fragment to the backend server. This violates the RFC because in the origin-form the URL should only contain the absolute path and the query.
When this is combined with another frontend proxy like Nginx, it can be used to bypass frontend proxy URI-based access control restrictions.
Details
For example, we have this Nginx configuration:
location /admin {
deny all;
return 403;
}
This can be bypassed when the attacker is requesting to /#/../admin
This won’t be vulnerable if the backend server follows the RFC and ignores any characters after the fragment.
However, if Nginx is chained with another reverse proxy which automatically URL encode the character # (Traefik) the URL will become
/%23/../admin
And allow the attacker to completely bypass the Access Restriction from the Nginx Front-End proxy.
Here is a diagram to summarize the attack:
PoC
This is the POC docker I've set up. It contains Nginx, Traefik proxies and a backend server running PHP.
https://drive.google.com/file/d/1vLnA0g7N7ZKhLNmHmuJ4JJjV_J2akNMt/view?usp=sharing
Impact
This allows the attacker to completely bypass the Access Restriction from Front-End proxy.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/traefik/traefik/v2 | all versions | 2.10.6 |
| 🐹Go | github.com/traefik/traefik/v3 | all versions | 3.0.0-beta5 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/traefik/traefik/v2. 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 github.com/traefik/traefik/v2 to 2.10.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fvhj-4qfh-q2hm 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-fvhj-4qfh-q2hm 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-fvhj-4qfh-q2hm. 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-fvhj-4qfh-q2hm in your dependencies?
O3 detects GHSA-fvhj-4qfh-q2hm across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.