# CVE-2021-29456: Authelia allows open redirects on the logout endpoint

> Attackers can redirect users to malicious sites via the logout endpoint, potentially facilitating phishing attacks against Authelia users.

- **Severity:** Medium 5.7 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 21 Apr 2021
- **Updated:** 3 Aug 2024
- **Also known as:** GHSA-36f2-fcrx-fp4j

## What is CVE-2021-29456?

CVE-2021-29456 is a medium-severity vulnerability in authelia, published on 21 Apr 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Authelia is an open-source authentication and authorization server providing 2-factor authentication and single sign-on (SSO) for your applications via a web portal. In versions 4.27.4 and earlier, utilizing a HTTP query parameter an attacker is able to redirect users from the web application to any domain, including potentially malicious sites. This security issue does not directly impact the security of the web application itself. As a workaround, one can use a reverse proxy to strip the query parameter from the affected endpoint. There is a patch for version 4.28.0.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2021-29456?

2 open source packages ship an affected version, including github.com/authelia/authelia/v4. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/authelia/authelia/v4` | Go | All versions | 4.28.0 |
| `github.com/authelia/authelia/v4` | Go | All versions | 4.28.0 |

Products: Authelia Authelia.

## How do I fix CVE-2021-29456?

Upgrade github.com/authelia/authelia/v4 to 4.28.0 or later.

## Is CVE-2021-29456 being exploited?

There is no known exploitation of CVE-2021-29456 in the wild and no public exploit code.

## Other vulnerabilities in authelia

- [CVE-2021-32637](https://insights.sbomify.com/CVE-2021-32637/): Authentication bypassed with malformed request URI (critical 10.0)
- [CVE-2026-33525](https://insights.sbomify.com/CVE-2026-33525/): Authelia: Improper Neutralization of Input During Web Page Generation Leads to Potential Cross-site Scripting (low 0.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-29456 and fix it if it is affected.

What it is: CVE-2021-29456 is a medium-severity vulnerability in authelia, published on 21 Apr 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-29456 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/authelia/authelia/v4 (Go): affected all versions; fixed in 4.28.0
- github.com/authelia/authelia/v4 (Go): affected all versions; fixed in 4.28.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-29456/ (Markdown: https://insights.sbomify.com/CVE-2021-29456/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-29456
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-29456
- OSV: https://osv.dev/vulnerability/CVE-2021-29456

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From sbomify Insights: https://insights.sbomify.com/CVE-2021-29456/
Summaries are generated and may contain mistakes; the official description above is the record.