NGINX EUVD-2026-101113

Exploitation requires NGINX Plus or NGINX Open Source configured with 'ssl_session_cache shared:name:size' AND relying on NGINX's automatic SSL session ticket key rotation because no explicit 'ssl_session_ticket_key' file directive is set, AND performing client certificate verification (mutual TLS). Instances without mTLS, those using an explicit ssl_session_ticket_key file, or those not using a shared session cache are not exposed. Key limiting factors: the attack is non-deterministic and beyond the attacker's control (CVSS AC:H), affects only the data plane (no control plane exposure), and is irrelevant to the many NGINX deployments that do not enforce client certificate authentication.

This is a genuinely medium-severity, conditionally-exploitable issue rather than an urgent fire-drill. The CVSS 3.1 vector (6.5, AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N) is internally consistent with the description: AC:H correctly captures that the attacker forges sessions 'in a non-deterministic manner that is beyond the attacker's control,' meaning success is probabilistic rather than reliable. PR:N confirms unauthenticated reach, and the high-complexity + data-plane-only scope keep real-world risk moderate. No EPSS score, no CISA KEV listing, and no public POC were supplied, so there is no evidence of active exploitation or weaponization - all exploitation signals are absent rather than conflicting. The practical risk gate is deployment-specific: only NGINX instances performing client certificate verification (mTLS) with a shared ssl_session_cache and no explicit ssl_session_ticket_key rotation are affected, which is a narrower population than the (ubiquitous) nginx install base. Net: prioritize for mTLS-fronted environments, but it does not warrant emergency patching absent exploitation evidence.

An NGINX reverse proxy enforces mutual TLS (client certificate verification) and uses 'ssl_session_cache shared' while relying on NGINX's automatic SSL session ticket key rotation instead of an explicit ssl_session_ticket_key file. A remote unauthenticated attacker repeatedly initiates and resumes TLS sessions and, due to the mishandled ticket key state, eventually forges a resumed session that is accepted without valid client-certificate authentication, gaining access to resources reserved for certificate-authenticated clients. No public exploit code was identified, and the non-deterministic nature (AC:H) means success is opportunistic rather than reliable.