CVE-2026-28755: 
NGINX vulnerability analysis and mitigation

CVE-2026-28755 is an improper certificate validation vulnerability in NGINX Plus and NGINX Open Source affecting the ngx_stream_ssl_module module. When configured with both ssl_verify_client on and ssl_ocsp on directives, the module improperly handles revoked certificates, allowing TLS handshakes to succeed even after an OCSP check identifies the certificate as revoked. The vulnerability was published on March 24, 2026, and carries a CVSS v3.1 base score of 5.4 (Medium) and a CVSS v4.0 base score of 5.3 (Medium) (F5 Advisory, Red Hat). Affected versions include NGINX Open Source 1.27.2–1.28.2 and 1.29.0–1.29.6, as well as NGINX Plus R33 through R36 P2 (F5 Advisory).

The root cause is classified under CWE-295 (Improper Certificate Validation) and CWE-863 (Incorrect Authorization). The flaw resides in the ngx_stream_ssl_module, which fails to properly enforce OCSP revocation status during TLS handshake processing — specifically when both ssl_verify_client on and ssl_ocsp on are configured together. An attacker with a revoked client certificate can successfully complete a TLS handshake because the OCSP check result is not correctly acted upon, bypassing the intended certificate revocation enforcement. Exploitation requires low privileges (possession of a revoked certificate) and no user interaction, and is achievable over the network with low attack complexity (F5 Advisory, Red Hat).

Successful exploitation allows an attacker holding a revoked client certificate to bypass certificate revocation controls and gain unauthorized access to services protected by NGINX's mutual TLS authentication. The impact is limited to low confidentiality and low integrity compromise, with no availability impact, as the attacker can access resources they should no longer be permitted to reach following certificate revocation. This is particularly significant in environments relying on OCSP-based certificate revocation as a primary access control mechanism for stream-based services (F5 Advisory).

1. Identify target: Locate an NGINX Plus or NGINX Open Source instance (versions 1.27.2–1.28.2, 1.29.0–1.29.6, or NGINX Plus R33–R36 P2) configured with ssl_verify_client on and ssl_ocsp on in the stream block, which enforces mutual TLS with OCSP revocation checking.
2. Obtain a revoked certificate: Acquire a client certificate that was previously issued by a trusted CA for the target service but has since been revoked (e.g., a certificate the attacker previously held legitimately, or one obtained through other means).
3. Initiate TLS handshake: Attempt a TLS connection to the vulnerable NGINX stream endpoint using the revoked client certificate.
4. Bypass revocation check: Due to the improper handling of the OCSP response in the ngx_stream_ssl_module, the TLS handshake completes successfully despite the certificate's revoked status.
5. Gain unauthorized access: The attacker is now authenticated to the backend service as if holding a valid certificate, enabling access to resources or data that should have been denied following revocation (F5 Advisory).

• Network: TLS connections from clients presenting certificates with serial numbers that appear in the CA's Certificate Revocation List (CRL) or have been reported as revoked via OCSP; successful TLS handshakes from IP addresses associated with previously revoked certificate holders.
• Logs: NGINX access logs showing successful stream connections from clients whose certificate serial numbers match known revoked certificates; absence of OCSP-related rejection log entries despite revoked certificate use.
• Configuration Audit: Presence of both ssl_verify_client on and ssl_ocsp on directives in the NGINX stream block on unpatched versions, indicating exposure to the vulnerability.

F5 has released patches addressing this vulnerability. Users should upgrade to: NGINX Open Source 1.28.3 or later (for the 1.27.x/1.28.x branch), NGINX Open Source 1.29.7 or later (for the 1.29.x branch), NGINX Plus R35 P2 or later, or NGINX Plus R36 P3 or later (F5 Advisory). As a temporary workaround until patching is possible, consider disabling OCSP on-demand checking (ssl_ocsp off) and implementing alternative certificate revocation controls, or restricting access to the affected stream endpoints. Organizations should also audit all NGINX configurations for the combined use of ssl_verify_client on and ssl_ocsp on in stream contexts.

The vulnerability was disclosed by F5/NGINX and picked up by multiple Linux distribution security teams, including Red Hat, openSUSE, Amazon Linux, and Ubuntu, all of which issued advisories or updated packages (Red Hat, openSUSE). A blog post on undercodetesting.com noted the vulnerability in the context of NGINX bug bounty findings, referencing small open models discovering NGINX CVEs (undercodetesting.com). Community reaction has been measured, consistent with the medium severity rating and the limited attack surface (requires specific NGINX stream SSL configuration).