CVE-2026-67181 in rouille
Summary
by MITRE • 07/28/2026
Rouille 0.3.3 through 3.6.2 contains an HTTP request smuggling vulnerability that allows remote attackers to desynchronize HTTP message boundaries by exploiting improper header forwarding in the proxy implementation. The proxy in src/proxy.rs forwards the client's Transfer-Encoding header to upstream backends unchanged while transmitting a body already de-chunked by tiny_http, enabling CL.TE desynchronization attacks where attackers control where the backend believes the request body ends.
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Analysis
by VulDB Data Team • 07/28/2026
The vulnerability under discussion affects Rouille versions 0.3.3 through 3.6.2 and represents a critical HTTP request smuggling flaw that exploits improper header handling within the proxy component. This issue stems from the proxy implementation in src/proxy.rs which fails to properly normalize HTTP message boundaries when forwarding requests to upstream backends. The flaw specifically manifests when the proxy forwards the client's Transfer-Encoding header without accounting for the fact that tiny_http has already processed and de-chunked the request body, creating a fundamental mismatch between what the backend expects versus what it actually receives.
The technical execution of this vulnerability relies on CL.TE (Content-Length/Transfer-Encoding) desynchronization attacks where an attacker manipulates HTTP headers to cause a discrepancy in how the proxy and backend interpret request boundaries. When the proxy receives a request with Transfer-Encoding header, it forwards this unchanged to the backend while having already processed the body through tiny_http's chunked decoding mechanism. This creates a scenario where the backend believes the request body ends at a different location than where the actual data stream terminates, allowing attackers to inject additional requests or manipulate response handling in ways that can lead to various security consequences.
From an operational impact perspective, this vulnerability enables remote attackers to perform HTTP request smuggling attacks that can potentially bypass security controls, access unauthorized resources, or manipulate application behavior. The desynchronization allows attackers to craft malicious requests that appear legitimate to the proxy but contain unexpected content for the backend system. This can result in data leakage, authentication bypasses, or even full system compromise depending on the target application's architecture and the specific manipulation techniques employed by threat actors.
The vulnerability aligns with CWE-444 and CWE-1287 categories related to HTTP request smuggling and improper handling of HTTP headers, respectively. It also maps to ATT&CK technique T1190 - Exploit Public-Facing Application which covers the exploitation of web applications through various HTTP-based attack vectors. Organizations using affected Rouille versions should immediately implement mitigations including proper header normalization, disabling Transfer-Encoding forwarding when body processing occurs, or upgrading to patched versions that correctly handle HTTP message boundaries.
The root cause analysis reveals that this is fundamentally a protocol boundary violation where the proxy layer fails to maintain consistency between client and backend communication channels. The implementation does not account for the fact that tiny_http's request processing creates a state where the body has been transformed from chunked format to a continuous stream, yet headers are forwarded without consideration of this transformation. This gap in logic creates the exact conditions necessary for CL.TE attacks to succeed, as the backend receives conflicting information about request boundaries while the actual data flow has already been partially processed by the intermediate proxy layer.
Mitigation strategies should focus on implementing proper HTTP header validation and normalization within the proxy component, ensuring that when body content has been pre-processed, corresponding headers are either removed or adjusted appropriately to maintain consistency across the entire request chain. The recommended approach includes either disabling Transfer-Encoding header forwarding when body processing occurs, implementing strict header validation rules, or upgrading to patched versions that properly handle HTTP message boundary synchronization. Organizations should also consider network-level protections such as web application firewalls that can detect and block suspicious HTTP smuggling patterns while monitoring for unusual header combinations that may indicate exploitation attempts.
This vulnerability demonstrates the critical importance of maintaining consistent HTTP protocol handling across all layers of proxy implementations, particularly when dealing with body processing transformations that alter the fundamental nature of request data flow. The flaw serves as a reminder that seemingly minor implementation details in HTTP processing can create significant security implications when not properly accounted for in the context of multi-layered network architectures where proxy components act as intermediaries between clients and backend services.