CVE-2026-91184: CWE-76 in F5 NGINX Gateway Fabric

The vulnerability arises because NGINX Gateway Fabric renders the spec.basic.realm and spec.jwt.realm fields from a namespaced AuthenticationFilter CRD verbatim, without escaping or validation, into the shared nginx.conf as auth_basic "<realm>";. An authenticated attacker with namespace privileges can craft a realm value that breaks out of the quoted directive and enclosing location block, injecting arbitrary NGINX directives into the shared Gateway's data-plane configuration. This enables the attacker to potentially read secrets of other tenants, perform SSRF via proxy_pass, access pod ServiceAccount tokens, or hijack routes belonging to other tenants. The vulnerability affects versions >=2.4.0 and <2.8.0 of F5 NGINX Gateway Fabric. It is a control plane vulnerability with no direct data plane exposure. The CVSS v3.1 score is 8.8, indicating high severity.

An authenticated attacker with namespace-level access can exploit this vulnerability to inject arbitrary NGINX directives into the shared Gateway's data-plane configuration. This can lead to unauthorized disclosure of co-located tenants' secrets, server-side request forgery (SSRF) attacks, reading of pod ServiceAccount tokens, and hijacking of other tenants' routes. The impact is high confidentiality, integrity, and availability impact on the control plane. There is no direct data plane exposure.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, restrict namespace-level access to trusted users only and monitor for suspicious AuthenticationFilter CRD configurations. Avoid deploying untrusted or unauthenticated configurations that modify the AuthenticationFilter spec.basic.realm or spec.jwt.realm fields.