Is CVE-2021-29456 actively exploited?

No active exploitation of CVE-2021-29456 has been confirmed so far. Continue monitoring, as exploit status can change.

Can CVE-2021-29456 be exploited remotely?

Yes. CVE-2021-29456 is exploitable remotely over the network (CVSS attack vector: Network).

What is the CVSS score of CVE-2021-29456?

CVE-2021-29456 has a CVSS score of 5.4 (MEDIUM).

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MEDIUM

CVE-2021-29456

CWE-601Published: April 21, 2021· Updated: Jun 17, 2026

5.4

CVSS v3.1

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.

NVD Source

Technical Analysis

CVE-2021-29456 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

Exploitation requires low privileges, which limits the exposure to scenarios where an attacker has already gained initial access.

The vulnerability has a "Changed" scope, meaning successful exploitation can impact components beyond the vulnerable component itself — such as the host operating system or adjacent services.