Summary
Authelia allows open redirects on the logout endpoint
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.
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.
Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.
CVE-2021-29456 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.28.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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f0cb75e1e102f95f91e9254c66c797e821857690 fix(handlers): logout redirection validation (#1908) v4.28.0
Frequently Asked Questions
- What is CVE-2021-29456? CVE-2021-29456 is a medium-severity open redirect vulnerability in github.com/authelia/authelia/v4 (go), affecting versions < 4.28.0. It is fixed in 4.28.0. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
- How severe is CVE-2021-29456? CVE-2021-29456 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/authelia/authelia/v4 are affected by CVE-2021-29456? github.com/authelia/authelia/v4 (go) versions < 4.28.0 is affected.
- Is there a fix for CVE-2021-29456? Yes. CVE-2021-29456 is fixed in 4.28.0. Upgrade to this version or later.
- Is CVE-2021-29456 exploitable, and should I be worried? Whether CVE-2021-29456 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-29456 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-29456? Upgrade
github.com/authelia/authelia/v4to 4.28.0 or later.