CVE-2025-55702 in Traffic Server
Summary
by MITRE • 10/21/2026
When a client sends a POST with Expect: 100-continue and the origin answers before reading the body, Apache Traffic Server parses the unread body left on the client connection as the next request. An attacker behind a front-end proxy that shares connections to Traffic Server can use this to smuggle requests past the front end and desynchronize responses for other users.
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 • 10/21/2026
The vulnerability described constitutes a critical HTTP request smuggling flaw within Apache Traffic Server, specifically affecting versions from 8.0.0 through 9.2.15 and 10.0.0 through 10.2.0. This security defect arises from an improper handling of the Expect: 100-continue header during POST requests when a front-end proxy is involved in the request path. The core technical flaw lies in how Traffic Server processes responses that arrive before the client has finished transmitting the message body. In standard HTTP/1.1 operations, a server may respond with a 100 Continue status to indicate it is ready to receive the request body. However, if Apache Traffic Server receives this interim response and proceeds to parse subsequent data on the connection without properly consuming or discarding the unread portion of the POST body, it misinterprets that leftover binary data as the start of a new HTTP request. This behavior creates a desynchronization between how the front-end proxy views the connection state and how Apache Traffic Server interprets the incoming byte stream.
The operational impact of this vulnerability is severe because it enables HTTP Request Smuggling attacks against systems protected by reverse proxies or load balancers. An attacker positioned behind a front-end proxy can craft malicious POST requests that include the Expect: 100-continue header and send them to Apache Traffic Server. By carefully controlling the timing and content of these requests, an adversary can force the server to treat part of one request as the beginning of another unrelated request. This allows the attacker to smuggle unauthorized requests past security controls implemented at the front-end layer, such as Web Application Firewalls or access control lists that might not inspect the smuggled payload correctly due to the parsing discrepancy. Furthermore, this desynchronization can lead to response mixing, where responses intended for one user are delivered to another, potentially exposing sensitive data like session cookies, personal identifiable information, or internal application states to unauthorized parties.
This vulnerability aligns with CWE-440, which describes HTTP Request Smuggling as a class of vulnerabilities resulting from inconsistent interpretation of HTTP requests by different components in the request path. It also maps to MITRE ATT&CK technique T1132, specifically Subtechnique T1132.005 for Data Encoding: Obfuscated Headers, although the primary mechanism here is protocol-level desynchronization rather than simple header obfuscation. The attack vector leverages the interaction between different HTTP parsers and their handling of chunked or content-length based message framing when interrupted by interim responses like 100 Continue. This misalignment in state management allows an attacker to bypass perimeter defenses that rely on a single, consistent view of the request stream.
To mitigate this risk, organizations running affected versions of Apache Traffic Server must upgrade immediately to version 9.2.16, 10.1.5, or 10.2.1, where these parsing logic errors have been corrected. Until an upgrade is feasible, administrators should consider disabling the Expect: 100-continue header processing if it is not strictly required by their application stack, although this may impact performance for large file uploads. Additionally, deploying strict input validation and ensuring that front-end proxies are configured to handle HTTP/1.1 continuations consistently with backend servers can reduce the attack surface. Regular security audits focusing on proxy configurations and request handling logic are recommended to prevent similar desynchronization issues in other components of the infrastructure.