CVE-2026-63718 in HTTP Server
Summary
by MITRE • 10/01/2026
Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') response smuggling vulnerability in Apache HTTP Server via mod_proxy_uwsgi and a crafted uwsgi response with Transfer-Encoding.
This issue affects Apache HTTP Server: from 2.4.30 through 2.4.68.
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Analysis
by VulDB Data Team • 10/01/2026
The identified vulnerability represents a critical security flaw within the Apache HTTP Server, specifically affecting versions ranging from 2.4.30 to 2.4.68 when configured with the mod_proxy_uwsgi module. This issue is classified as an Inconsistent Interpretation of HTTP Requests, commonly known as HTTP Request Smuggling or Response Smuggling depending on the direction of manipulation. The core technical flaw arises from a discrepancy in how Apache interprets the Transfer-Encoding header within uwsgi responses compared to the interpretation logic used by downstream clients or intermediate proxies. When mod_proxy_uwsgi processes an upstream response that includes specific Transfer-Encoding directives, it may fail to correctly parse or strip these headers before forwarding them to the client. This inconsistency creates a state desynchronization between the proxy and its peers, allowing an attacker to inject malicious HTTP requests into the stream of legitimate traffic by exploiting the differing parsing rules regarding chunked transfer encoding versus content-length based framing.
From a technical perspective, this vulnerability leverages the ambiguity in how different components handle message boundaries in HTTP/1.1 communications. The mod_proxy_uwsgi module acts as an intermediary between Apache and backend uwsgi application servers. If the upstream server sends a response with a Transfer-Encoding header that is not properly normalized or removed by Apache before transmission to the end-user, the client browser or proxy may interpret subsequent bytes in the connection stream as part of a new request rather than the body of the current response. This misinterpretation allows an attacker to smuggle arbitrary HTTP requests through the proxy. By carefully crafting the uwsgi response headers and payload, an adversary can trick Apache into treating data intended for one transaction as the start of another, thereby bypassing access controls or injecting unauthorized commands that are executed by backend services under the context of a different user session.
The operational impact of this vulnerability is severe, primarily enabling unauthorized access to sensitive resources and potential remote code execution depending on the application logic behind the uwsgi server. Attackers can perform HTTP request smuggling attacks which facilitate account hijacking, cross-site scripting (XSS), cache poisoning, and bypassing web application firewalls. Since the attack relies on inconsistent parsing at the proxy layer, it effectively neutralizes many perimeter security controls that rely on inspecting individual requests in isolation. The ability to inject arbitrary headers or body content means an attacker could manipulate backend applications into performing actions they were not intended to perform, such as accessing administrative panels, exfiltrating data from other users' sessions, or injecting malicious scripts that execute when the smuggled response is rendered by a victim's browser.
This vulnerability aligns with CWE-440, which describes HTTP Request Smuggling, and maps directly to MITRE ATT&CK technique T1592.003, specifically Client-side Proxy Discovery, although in this context it is more accurately mapped to the exploitation of proxy inconsistencies often seen in techniques like T1598 or general web application attacks under TA0001 Access. The lack of strict validation and normalization of Transfer-Encoding headers violates best practices for secure gateway design as outlined in OWASP guidelines regarding HTTP request parsing consistency. To mitigate this risk, organizations running affected versions must immediately upgrade Apache HTTP Server to a version later than 2.4.68 where the mod_proxy_uwsgi module has been patched to correctly handle and strip Transfer-Encoding headers from upstream responses before forwarding them downstream. Until an upgrade is feasible, administrators should consider disabling mod_proxy_uwsgi if it is not strictly required or placing a reverse proxy in front of Apache that enforces strict HTTP parsing rules and strips ambiguous headers, although this secondary mitigation may introduce its own latency and complexity. Regular monitoring for anomalous traffic patterns involving uwsgi endpoints can also help detect exploitation attempts while patching efforts are underway.