CVE-2026-97629: 
NGINX vulnerability analysis and mitigation

CVE-2026-97629 is an SSL session ticket key management vulnerability in F5 NGINX Plus and NGINX Open Source that allows a remote unauthenticated attacker to forge client SSL sessions and bypass client certificate verification. The flaw is triggered when NGINX is configured with ssl_session_cache shared:name:size without explicit ssl_session_ticket_key file directives for automatic key rotation. Affected versions include NGINX Open Source 1.23.3, 1.30.0–1.30.5, and 1.31.0–1.31.6, as well as NGINX Plus R33–R36, 37.0.0–37.0.5, and 37.1.0. It was published on October 14, 2026, with a CVSS v3.1 score of 6.5 (Medium) and a CVSS v4.0 score of 8.3 (High) (GitHub Advisory, F5 Advisory).

The root cause is classified as CWE-908 (Use of Uninitialized Resource): when ssl_session_cache shared:name:size is configured without an explicit ssl_session_ticket_key directive, NGINX does not properly initialize or rotate SSL session ticket keys, leaving them in a predictable or uninitialized state. An attacker can exploit this to forge valid SSL session tickets in a non-deterministic manner, effectively impersonating a legitimately authenticated client and bypassing mutual TLS (mTLS) client certificate verification. The attack requires no privileges and no user interaction, but does require specific deployment conditions (the vulnerable configuration must be present) (GitHub Advisory, F5 Advisory).

Successful exploitation allows an unauthenticated remote attacker to bypass client certificate verification, gaining unauthorized access to resources and functionality that should be restricted to authenticated clients. The primary impact is high confidentiality loss, as attackers may access sensitive data or protected endpoints; integrity impact is low, and there is no availability impact. This is a data plane issue only — there is no control plane exposure — but it could enable unauthorized access to backend services or APIs protected by mTLS (GitHub Advisory, F5 Advisory).

As of the disclosure date (October 14, 2026), there is no public proof-of-concept exploit and no evidence of active in-the-wild exploitation (GitHub Advisory). The EPSS score is 0.0, indicating very low current exploitation probability. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported. Exploitation is non-deterministic and beyond the attacker's full control, which raises the practical bar for reliable abuse.

1. Reconnaissance: Identify NGINX Plus or NGINX Open Source instances running vulnerable versions (Open Source 1.23.3, 1.30.0–1.30.5, 1.31.0–1.31.6; Plus R33–R36, 37.0.0–37.0.5, 37.1.0) using network scanning tools such as Shodan, Censys, or nmap with TLS fingerprinting.
2. Confirm vulnerable configuration: Determine whether the target NGINX instance uses ssl_session_cache shared:name:size without an explicit ssl_session_ticket_key directive — this can sometimes be inferred from TLS handshake behavior or server banners.
3. Forge SSL session ticket: Craft or replay SSL session tickets that exploit the uninitialized/improperly rotated session ticket key state. Because the process is non-deterministic, an attacker may need to attempt multiple forged sessions to succeed.
4. Bypass client certificate verification: Present the forged SSL session ticket during the TLS handshake to resume a session that appears to have already completed client certificate authentication, bypassing the mTLS requirement.
5. Access protected resources: Once the forged session is accepted, interact with backend resources or APIs that were previously restricted to authenticated clients (GitHub Advisory, F5 Advisory).

• Network: Unusual TLS session resumption attempts (high volume of SessionTicket extension usage) from a single or small set of IP addresses; TLS handshakes that skip the client certificate exchange phase but still gain access to protected endpoints.
• Logs: NGINX access logs showing requests to mTLS-protected endpoints from clients that did not present a certificate in the initial handshake; repeated failed and then successful session resumption attempts from the same source IP.
• Process/Configuration: NGINX configuration files using ssl_session_cache shared:name:size without a corresponding ssl_session_ticket_key directive — this is a prerequisite indicator of a vulnerable deployment.

F5 has released patched versions: NGINX Open Source 1.30.6 and 1.31.7, and NGINX Plus 37.0.5.1 and 37.1.0.1. Upgrading to these versions is the primary recommended remediation (F5 Advisory, GitHub Advisory). As a configuration-based workaround, administrators should add explicit ssl_session_ticket_key file directives to their NGINX configuration to enable proper automatic SSL session ticket key rotation. Additionally, reviewing and enforcing strict client certificate verification policies can reduce exposure. NGINX Plus R33–R36 (without a patch path noted) should be upgraded to a supported release.

Coverage at the time of disclosure was primarily limited to automated vulnerability tracking services and Linux compatibility news outlets, with no notable independent researcher commentary or significant social media discussion identified (Linux Compatible). F5 published an official advisory (K000162606) and the vulnerability was catalogued in the GitHub Advisory Database and ENISA EUVD shortly after disclosure.