CVE-2026-111815 in Traffic Server
Summary
by MITRE • 10/22/2026
Apache Traffic Server can resume a TLS session across SNI virtual hosts without re-verifying a client certificate required by the resumed host's verify_client STRICT policy.
This issue affects Apache Traffic Server: from 8.0.0 through 8.1.11, from 9.0.0 through 9.2.15, from 10.0.0 through 10.1.4, 10.2.0.
Users are recommended to upgrade to version 9.2.16, 10.1.5, or 10.2.1, which fix the issue.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability in Apache Traffic Server represents a critical flaw in the implementation of Transport Layer Security session resumption mechanisms when combined with Server Name Indication and strict client certificate verification policies. In standard TLS operations, session resumption is designed to optimize performance by allowing clients to reuse previously established cryptographic contexts without performing a full handshake. However, this optimization introduces a significant security gap when virtual hosts are configured with differing authentication requirements. Specifically, the flaw allows an attacker to initiate a connection to one virtual host that does not require client certificate verification or has a less stringent policy, thereby obtaining a valid TLS session ID. The attacker can then present this same session ID while connecting to a different SNI-based virtual host that mandates strict mutual authentication via verify_client STRICT. Because the server incorrectly assumes that the resumed session is inherently secure and associated with the current request context, it fails to re-verify the client certificate against the policies of the target virtual host. This effectively bypasses mandatory access controls designed to restrict entry to sensitive resources based on identity verification.
From a technical perspective, this issue stems from how Apache Traffic Server manages TLS session cache entries in relation to SNI routing logic. When a connection is resumed, the server retrieves the cached session parameters and applies them directly without re-evaluating the specific security requirements of the virtual host identified by the new SNI value. This architectural oversight means that the stateful nature of the resumed session overrides the stateless requirement for certificate validation inherent in the STRICT policy configuration. The vulnerability is particularly dangerous because it exploits a common deployment pattern where multiple domains are hosted on a single IP address, each with distinct security postures. An attacker does not need to possess valid credentials or certificates; they only need access to any service running on the same server that permits unauthenticated TLS connections to harvest a session token for later misuse against authenticated services.
The operational impact of this vulnerability is severe, as it can lead to unauthorized access to protected APIs, administrative interfaces, or internal services that rely exclusively on client certificate authentication for identity assurance. In environments where mutual TLS is the primary defense mechanism against impersonation and man-in-the-middle attacks, bypassing this check undermines the entire trust model. Attackers could potentially exfiltrate sensitive data, perform actions with elevated privileges, or pivot further into a network by masquerading as an authorized client. The risk is amplified in multi-tenant environments where different customers or departments are served through distinct virtual hosts on the same infrastructure, allowing cross-tenant privilege escalation if one tenant's host has weaker TLS settings than another.
This flaw aligns with CWE-287 Improper Authentication and CWE-345 Insufficient Verification of Data Authenticity within the Common Weakness Enumeration framework. It also maps to MITRE ATT&CK technique T1078 Valid Accounts, as it allows an attacker to leverage valid session tokens obtained from less secure endpoints to authenticate against more secured ones without possessing actual credentials. To mitigate this risk, organizations must upgrade Apache Traffic Server to version 9.2.16, 10.1.5, or 10.2.1 immediately, as these releases contain the necessary patches to enforce certificate verification during session resumption regardless of SNI context. Until an upgrade is performed, administrators should consider isolating services that require strict mutual authentication on separate IP addresses or disabling TLS session resumption for those specific virtual hosts if performance impact is acceptable. Regular auditing of TLS configurations across all virtual hosts ensures consistency in security policies and reduces the attack surface available to adversaries seeking to exploit session management flaws.