Description

the Undertow AJP listener honours forged ssl_cert and is_ssl AJP attributes without requiring any shared-secret authentication. This enables an unauthenticated attacker with direct TCP access to port 8009 to bypass CLIENT-CERT authentication by injecting a forged X.509 certificate via the AJP protocol.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.4",
  "pubDate": "2026-08-11T09:17:12.400Z",
  "pubdate": "2026-08-11T09:17:12.400Z",
  "executiveSummary": "An authentication bypass vulnerability has been identified in the Undertow AJP listener, specifically affecting how it processes incoming AJP attribute parameters. The vulnerability stems from the AJP listener accepting and honoring forged ssl_cert and is_ssl AJP attributes directly from network streams without enforcing any underlying shared-secret authentication or cryptographic verification.\nThis security flaw enables an unauthenticated remote attacker who possesses direct network access to the AJP port 8009 to completely bypass robust CLIENT-CERT authentication mechanisms. By injecting a maliciously crafted or forged X.509 certificate directly via the AJP protocol payload, the attacker can spoof trusted client identities and improperly access protected application resources without possessing valid cryptographic credentials.\nThe impact of this vulnerability is critical, as it compromises the confidentiality and integrity of web applications relying on AJP-based reverse proxy architectures for client certificate validation. The risk is elevated by the lack of required authentication prerequisites at the listener level, requiring only network reachability to the vulnerable service port. Remediation necessitates enforcing strict validation mechanisms, enabling shared-secret communication, or restricting network access to the AJP listener interface.",
  "technicalDetails": "The root cause of this vulnerability lies in the trust relationship handling within the Undertow AJP listener component when parsing attribute-value pairs sent across the Apache JServ Protocol (AJP) channel. Specifically, the component processes attributes such as ssl_cert and is_ssl from the incoming AJP request without validating whether the connection originates from a trusted reverse proxy or verifying these attributes against a pre-shared secret or cryptographic signature.\nThe vulnerable component is the Undertow AJP listener handling network traffic on TCP port 8009. The affected systems are configurations where the AJP connector is exposed to untrusted networks or unverified upstream intermediaries. Exploitation requires neither prior authentication nor high privileges, but strictly mandates direct network reachability via TCP to the AJP port.\nThe step-by-step attack flow proceeds as follows: First, the unauthenticated attacker establishes a direct TCP connection to the AJP listener running on port 8009. Second, the attacker formulates a malicious AJP request packet. Within this packet payload, the attacker injects arbitrary values into the request attributes, explicitly populating the is_ssl attribute to indicate a secure connection and supplying a forged X.509 certificate within the ssl_cert attribute.\nThird, the Undertow AJP listener parses the incoming stream and improperly accepts the supplied ssl_cert and is_ssl attributes as authentic data passed from a trusted front-end web server. Fourth, the application container trusts the injected certificate details, bypassing the standard cryptographic validation checks required for genuine CLIENT-CERT authentication.\nFinally, the backend application grants the attacker unauthorized access based on the spoofed client identity, resulting in a complete authentication bypass. The post-exploitation impact includes unauthorized data access, privilege escalation within the application context, and potential execution of administrative functions depending on the privileges mapped to the forged certificate."
}