Apache Airflow FAB provider allows an attacker to forge an id_token that was issued for a different client application by a shared Authentik identity provider. The provider does not check the issuer or audience claims of the token it receives, so the token is accepted and the bearer is authenticated as the user specified in the token. This flaw permits unauthorized authentication to Airflow, enabling an attacker with a valid token for any other application served by the same Authentik instance to gain access to the Airflow deployment, potentially exposing or manipulating workflows and data.

Deployments that use the Apache Airflow FAB provider with the Authentik OAuth path are affected. The issue is present on installations where the same Authentik identity provider serves multiple applications, and an attacker holds a token for one of those other applications. Deployments that applied the CVE-2026-75156 fix but still use Authentik are still impacted and must upgrade to version 3.9.0 or later.

The vulnerability is exploitable through the standard OAuth flow; an attacker can submit a counterfeit id_token to the Airflow authentication endpoint. The CVSS score of 8.1 indicates a high severity, and the EPSS score of < 1% together with the fact that the vulnerability is not listed in CISA KEV point to a low probability of exploitation. However, the potential impact is significant because it allows arbitrary authentication. The flaw exists because the code path never checks the audience claim, meaning that clients can be impersonated across applications served by the same Authentik provider.