CVE-2026-111815: 
Apache Traffic Server vulnerability analysis and mitigation

CVE-2026-111815 is a TLS session resumption authentication bypass vulnerability in Apache Traffic Server (ATS) that allows an unauthenticated attacker to resume a TLS session across SNI virtual hosts without re-verifying a client certificate required by the resumed host's verify_client STRICT policy. It affects Apache Traffic Server versions 8.0.0 through 8.1.11, 9.0.0 through 9.2.15, 10.0.0 through 10.1.4, and 10.2.0. The vulnerability was published on October 21, 2026, and is classified as CWE-295 (Improper Certificate Validation). It carries a CVSS v3.1 base score of 10.0 (Critical) and a CVSS v4.0 base score of 7.8 (High) (GitHub Advisory).

The root cause is improper certificate validation (CWE-295): when a TLS session is resumed via session tickets or session IDs, Apache Traffic Server fails to re-verify the client certificate against the verify_client STRICT policy of the target SNI virtual host. This means a client that established a valid TLS session on one virtual host can resume that session on a different virtual host that enforces strict mutual TLS (mTLS) authentication, effectively bypassing the certificate check entirely. The attack requires no privileges, no user interaction, and is exploitable remotely over the network with low complexity, making it trivially automatable (GitHub Advisory, Apache Mailing List).

Successful exploitation allows an unauthenticated network attacker to access resources on SNI virtual hosts that are protected by strict client certificate authentication, bypassing the intended mTLS access controls. The primary impacts are high confidentiality and integrity loss on subsequent systems (i.e., backend services or virtual hosts protected by ATS), as an attacker can read sensitive data and potentially modify requests to protected resources. Availability is not directly impacted, but unauthorized access to certificate-protected APIs or internal services could facilitate lateral movement or data exfiltration (GitHub Advisory).

As of the time of disclosure, there is no public proof-of-concept exploit and no evidence of in-the-wild exploitation (Feedly). The EPSS score is 0.0, indicating a currently low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. However, the low attack complexity and lack of required privileges make it a candidate for future weaponization, particularly against organizations using ATS as a TLS-terminating reverse proxy with mTLS-enforced virtual hosts.

1. Reconnaissance: Identify Apache Traffic Server instances acting as TLS-terminating reverse proxies, particularly those hosting multiple SNI virtual hosts where at least one enforces verify_client STRICT (mutual TLS). Tools like Shodan or Censys can identify ATS deployments by banner or TLS certificate patterns.
2. Establish a legitimate TLS session: Connect to an SNI virtual host on the target ATS instance that does not require client certificate authentication (or one for which the attacker possesses a valid client certificate), completing a full TLS handshake and obtaining a resumable session ticket or session ID.
3. Craft a session resumption request: Initiate a new TLS connection to the target ATS instance, but specify a different SNI hostname — one that enforces verify_client STRICT — while presenting the previously obtained session ticket or session ID for resumption.
4. Bypass client certificate verification: Due to the flaw, ATS resumes the session without re-verifying the client certificate against the strict policy of the new SNI host, granting the attacker an authenticated TLS session to the protected virtual host.
5. Access protected resources: Use the resumed session to send HTTP requests to backend resources or APIs that are intended to be accessible only to clients with valid certificates, achieving unauthorized data access or manipulation (GitHub Advisory, Apache Mailing List).

• Network: TLS session resumption attempts (ClientHello with session ticket or session ID) targeting SNI hostnames that differ from the original session establishment; connections to mTLS-protected virtual hosts from clients that did not present a client certificate during the initial handshake.
• Logs: ATS access logs showing requests to verify_client STRICT virtual hosts from clients with no associated client certificate DN; TLS handshake logs indicating session resumption (resumed session) for connections to protected SNI hosts without a corresponding certificate verification log entry.
• Anomalous Behavior: Unexpected access to certificate-protected endpoints from IP addresses or clients that have not previously completed a full mTLS handshake with a valid certificate; unusual cross-virtual-host session patterns in TLS session cache logs.

Apache has released patched versions: 9.2.16, 10.1.5, and 10.2.1, which fix the session resumption bypass. Users on the 8.x branch should upgrade to a supported release, as no patch is listed for 8.1.x. As a temporary workaround, administrators can disable TLS session resumption on ATS instances, or ensure that SNI virtual hosts enforcing verify_client STRICT do not share TLS session caches with less-restrictive virtual hosts. Upgrading to a patched version is the recommended long-term solution (GitHub Advisory, Apache Mailing List).