CVE-2021-29456 — v4
MEDIUMCVE-2021-29456 is a medium-severity (CVSS 5.4) vulnerability in github.com/authelia/authelia/v4. A fix is available for github.com/authelia/authelia/v4 — see the affected versions and patch details below.
Authelia allows open redirects on the logout endpoint
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.
How urgent is this, really
CVE-2021-29456 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/authelia/authelia/v4Real-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
Impact
Utilizing a HTTP query parameter an attacker is able to redirect users from the web application to any domain. The URL of the intended redirect should always be checked for safety prior to forwarding the user. Other endpoints of the web application already do this, they check both that the domain is using the HTTPS protocol and that it exists on a domain associated with the application.
An attacker is able to use this unintended functionality to redirect users to malicious sites. This particular security issue allows the attacker to make a phishing attempt seem much more trustworthy to a user of the web application as the initial site before redirection is familiar to them, as well as the actual URL which they have theoretically visited frequently.
While this security issue does not directly impact the security of the web application, it is still not an acceptable scenario for the reasons mentioned above.
Patches
f0cb75e1e102f95f91e9254c66c797e821857690 fix(handlers): logout redirection validation (#1908) v4.28.0
Workarounds
Using a reverse proxy to strip the query parameter from the affected endpoint.
References
https://github.com/authelia/authelia/pull/1908
For more information
If you have any questions or comments about this advisory, please contact us. You may also contact us to request creating a back-ported fix for this if you are able to explain why you cannot upgrade; however upgrading is highly preferable.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/authelia/authelia/v4 | all versions | 4.28.0go get github.com/authelia/authelia/v4@v4.28.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/authelia/authelia/v4, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/authelia/authelia/v4 to 4.28.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-29456 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-29456 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-29456. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-29456 in your dependencies?
O3 Security finds CVE-2021-29456 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.