CVE-2026-73257 in Mongoose
Summary
by MITRE • 08/20/2026
Mongoose is an embedded web server and network library. Priro to version 7.22, a remote unauthenticated attacker can send an HTTP request containing both Content-Length and Transfer-Encoding: chunked. The cl_count and te_count checks in the mg_http_parse() and http_cb() paths in src/http.c accept both headers and prioritize chunked encoding, while a Content-Length-preferring reverse proxy can use a different request boundary. This CL.TE desynchronization can inject requests that access or modify resources in another user context. This issue is fixed in version 7.22.
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Analysis
by VulDB Data Team • 08/21/2026
Mongoose serves as a widely adopted embedded web server and network library, frequently utilized in Internet of Things devices, microservices, and resource-constrained environments due to its lightweight architecture and ease of integration. The vulnerability identified prior to version 7.22 represents a critical HTTP request desynchronization flaw rooted in the inconsistent handling of conflicting transfer encoding headers. Specifically, when an attacker sends an HTTP request that includes both Content-Length and Transfer-Encoding: chunked headers, the internal parsing logic within mg_http_parse() and http_cb() functions exhibits ambiguous behavior by accepting both directives while prioritizing the chunked encoding mechanism for body consumption. This design choice creates a discrepancy between how Mongoose interprets the length of the incoming data stream versus how an upstream reverse proxy or load balancer might interpret it, particularly if that intermediary is configured to prioritize Content-Length over Transfer-Encoding as per certain RFC interpretations or legacy configurations.
The core technical flaw lies in this CL.TE desynchronization, a well-documented attack vector where the boundary between HTTP requests becomes blurred due to mismatched length calculations. Because Mongoose processes the request body based on chunked encoding boundaries while an upstream proxy might calculate the end of the request based on the Content-Length header value, the two entities effectively disagree on where one request ends and the next begins. This misalignment allows a remote unauthenticated attacker to craft malicious HTTP requests that appear as part of a legitimate ongoing transaction from the perspective of the reverse proxy, but are interpreted by Mongoose as distinct or additional commands. Consequently, this enables request smuggling attacks where an attacker can inject arbitrary HTTP requests into the backend server's processing queue.
The operational impact of this vulnerability is severe, primarily facilitating unauthorized access and data manipulation through privilege escalation within the application context. By exploiting the desynchronization, an attacker can bypass authentication checks that are applied to incoming traffic by performing actions in another user’s security context. This means sensitive resources belonging to other users could be accessed, modified, or deleted without proper authorization credentials. Furthermore, this vulnerability aligns with CWE-444, which describes inconsistent interpretation of HTTP requests, and maps directly to the Request Smuggling technique found in MITRE ATT&CK framework under T1190 (Exploit Public-Facing Application). The ability to inject requests effectively neutralizes security controls placed at network boundaries, such as Web Application Firewalls or access control lists, because those tools typically inspect traffic based on their own interpretation of request boundaries which may differ from the backend server’s parsing logic.
Mitigation for this vulnerability requires an immediate upgrade to Mongoose version 7.22 or later, where the developers have addressed the ambiguous handling of conflicting headers by enforcing stricter validation rules that reject requests containing both Content-Length and Transfer-Encoding: chunked unless explicitly allowed under specific, secure configurations. In environments where upgrading is not immediately feasible, administrators should configure upstream reverse proxies to strictly enforce a single transfer encoding method per request or ensure they align their parsing logic with the backend server’s behavior. Additionally, implementing strict input validation at the application layer to reject malformed HTTP headers can provide an additional layer of defense against such desynchronization attacks. Regular security audits and penetration testing focusing on HTTP protocol compliance are recommended to identify similar implementation flaws in other network components within the infrastructure.