CVE-2026-111809 in Traffic Server
Summary
by MITRE • 10/21/2026
Apache Traffic Server allows request smuggling via HEAD requests that carry Content-Length or Transfer-Encoding.
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
Apache Traffic Server is a high-performance, scalable HTTP and reverse proxy server widely deployed in enterprise environments to manage web traffic, cache content, and balance loads across backend servers. The vulnerability identified involves a critical flaw in how the application processes specific HTTP request headers when combined with certain methods, specifically allowing for HTTP Request Smuggling attacks. This class of vulnerabilities arises from inconsistencies or ambiguities in how different network components interpret message boundaries, particularly regarding the Content-Length and Transfer-Encoding headers. In this instance, the issue is triggered by HEAD requests that include either a Content-Length header or a Transfer-Encoding header, creating an environment where the server's parsing logic can be manipulated to misinterpret the length of incoming data streams.
The technical root cause lies in the protocol handling logic within Apache Traffic Server versions 8.0.0 through 8.1.11, 9.0.0 through 9.2.15, and 10.0.0 through 10.1.4, as well as version 10.2.0. When a HEAD request is sent with these headers present, the server may fail to correctly determine where one HTTP message ends and another begins. This ambiguity allows an attacker to inject malicious requests that appear to be part of the current transaction but are actually interpreted by downstream servers or caches as separate, distinct requests. By carefully crafting the payload, an attacker can smuggle arbitrary HTTP commands past security controls such as Web Application Firewalls (WAFs) or access control lists that might not inspect the smuggled portion with equal rigor. This technique exploits the fundamental difference in how some proxies handle chunked transfer encoding versus fixed-length content headers, leading to a desynchronization between the front-end proxy and back-end servers.
The operational impact of this vulnerability is severe, as HTTP Request Smuggling can lead to a variety of malicious outcomes depending on the network architecture. Attackers may use this flaw to bypass authentication mechanisms by smuggling requests that appear to come from trusted sources or internal networks. It can also facilitate cache poisoning attacks, where an attacker injects responses into the proxy's cache for other users, potentially serving malware or phishing content. Furthermore, it enables session hijacking if the smuggled request allows access to another user’s authenticated state without proper authorization checks. The ability to bypass security filters makes this a high-risk vulnerability, as standard perimeter defenses may not detect the malicious payload due to its hidden nature within the smuggling technique.
This vulnerability is categorized under CWE-436, which refers to Interpretation Conflict, and aligns with MITRE ATT&CK techniques related to HTTP Request Smuggling, specifically those involving header manipulation to evade detection or access control mechanisms. The misinterpretation of message boundaries represents a classic interpretation conflict where the application layer logic does not strictly enforce RFC 7230 compliance regarding the mutual exclusivity of Content-Length and Transfer-Encoding headers in most contexts, particularly when combined with HEAD methods which typically do not return bodies but may still carry these metadata fields incorrectly.
To mitigate this risk, organizations running affected versions must upgrade Apache Traffic Server to version 9.2.16, 10.1.5, or 10.2.1 immediately. These releases contain the necessary patches to correct the header parsing logic and prevent the misinterpretation of message boundaries in HEAD requests with Content-Length or Transfer-Encoding headers. Until upgrades are applied, administrators should consider implementing strict input validation at a network level if possible, ensuring that any HTTP traffic containing conflicting headers is dropped before it reaches the proxy server. Additionally, monitoring logs for unusual patterns of HEAD requests with these specific header combinations can help detect potential exploitation attempts in real-time while mitigation strategies are deployed.