CVE-2026-73051 in actix-http
Summary
by MITRE • 08/14/2026
actix-http versions before 3.12.1 contain an HTTP request smuggling vulnerability in the HTTP/1.1 parser that accepts requests with both Content-Length and Transfer-Encoding: chunked headers. Unauthenticated remote attackers can exploit this through a front-end intermediary to desynchronize backend requests and smuggle malicious HTTP requests to the Actix service.
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Analysis
by VulDB Data Team • 08/14/2026
This vulnerability exists within the actix-http library's HTTP/1.1 parser implementation prior to version 3.12.1, representing a critical security flaw that enables HTTP request smuggling attacks. The technical root cause stems from the parser's failure to properly validate and reject conflicting HTTP headers, specifically allowing both Content-Length and Transfer-Encoding: chunked headers to coexist in the same request. This violates fundamental HTTP protocol specifications where these two headers are mutually exclusive and should trigger a 400 Bad Request response when present together.
The operational impact of this vulnerability is severe as it enables unauthenticated remote attackers to exploit front-end intermediaries such as load balancers, proxies, or API gateways that forward requests to Actix services. Through careful crafting of malicious HTTP requests, attackers can manipulate the request parsing behavior to desynchronize requests between frontend and backend systems, creating opportunities for request smuggling attacks. This allows malicious actors to inject arbitrary HTTP requests that bypass normal security controls and potentially access protected resources or perform unauthorized operations against the vulnerable service.
According to CWE classification, this vulnerability maps to CWE-16: Improper Operation on Resource in Wrong Phase of Lifecycle, specifically involving improper handling of HTTP headers during request parsing. The attack pattern aligns with ATT&CK technique T1190: Proxying, where adversaries manipulate intermediaries to redirect or modify traffic. The vulnerability also relates to CWE-444: Inconsistent Interpretation of HTTP Requests, which describes how different systems interpret the same HTTP request differently due to implementation flaws in header processing.
The exploitation requires minimal privileges as attackers need only send carefully constructed HTTP requests through any front-end intermediary that forwards traffic to the vulnerable Actix service. The attack vector is particularly dangerous because it can be executed without authentication and can potentially lead to data breaches, privilege escalation, or further attacks on backend systems. Organizations using Actix-http versions prior to 3.12.1 should immediately implement the patch to address this vulnerability.
Mitigation strategies include upgrading to actix-http version 3.12.1 or later, which properly validates HTTP headers and rejects requests containing conflicting Content-Length and Transfer-Encoding headers. Additionally, organizations should configure front-end intermediaries to reject requests with both headers or implement additional validation layers. Network monitoring should be enhanced to detect anomalous request patterns that may indicate exploitation attempts. Security teams should also review their existing HTTP header validation policies and ensure proper enforcement of HTTP protocol compliance across all web applications and services.
The vulnerability demonstrates the critical importance of strict HTTP protocol adherence in web application frameworks, particularly when handling request parsing and validation. It highlights how seemingly minor implementation flaws in core HTTP processing components can lead to severe security consequences that affect entire application ecosystems. Organizations should conduct thorough security assessments of their web infrastructure to identify other potential instances of similar header validation issues that could enable similar attacks.
This vulnerability type represents a classic example of how protocol violations in web frameworks can create opportunities for sophisticated attack techniques, emphasizing the need for rigorous security testing and proper adherence to established HTTP standards during framework development. The remediation process requires not only updating the affected components but also implementing comprehensive monitoring and validation strategies to prevent similar issues from emerging in other parts of the infrastructure stack.
Security researchers should be aware that this vulnerability could potentially be leveraged in combination with other HTTP-related flaws to create more complex attack scenarios, particularly when multiple vulnerable systems exist within the same network environment. The incident underscores the importance of continuous security monitoring and rapid patch deployment for widely used web frameworks and libraries that form the foundation of modern internet applications.