CVE-2026-71324 in Traefik
Summary
by MITRE • 08/07/2026
Traefik is an open source HTTP reverse proxy and load balancer. Prior to 2.11.53, 3.6.24, and 3.7.9, Traefik's default HTTP reverse proxy forwards a plain HTTP/2 or HTTP/3 CONNECT request and its body to an HTTP/1.1 upstream through a shared net/http.Transport. When the upstream answers the CONNECT with a keep-alive non-2xx response and does not drain the body, Traefik returns the desynchronized backend socket to its shared pool and reuses it for other clients. An unauthenticated attacker can use this behavior to make a different client read the attacker's smuggled response, which can include authenticated or private content from another request. The ForwardAuth middleware with forwardBody true and preserveRequestMethod true can re-issue a CONNECT with the buffered body attached, exposing the auth-client pool to the same desynchronization. This issue is fixed in 2.11.53, 3.6.24, and 3.7.9.
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Analysis
by VulDB Data Team • 08/07/2026
This vulnerability in Traefik represents a sophisticated desynchronization attack that exploits the interaction between HTTP/2 and HTTP/1.1 protocols within the reverse proxy's connection handling mechanism. The flaw manifests when Traefik processes HTTP/2 or HTTP/3 CONNECT requests that are forwarded to HTTP/1.1 upstream servers through a shared net/http.Transport instance. The vulnerability stems from improper handling of connection state management where the proxy fails to properly synchronize the underlying network socket state between concurrent requests. When an upstream server responds to a CONNECT request with a keep-alive non-2xx response without fully draining the request body, Traefik incorrectly returns the desynchronized socket to its shared connection pool. This creates a dangerous scenario where subsequent clients can inadvertently receive data from previous requests that were not properly cleaned up, leading to potential information leakage and cross-request contamination.
The technical exploitation occurs through a specific sequence of protocol interactions that violate fundamental connection management principles. When Traefik handles HTTP/2 or HTTP/3 CONNECT requests, it establishes a connection to upstream HTTP/1.1 servers using shared transport mechanisms. The vulnerability arises because the proxy does not properly account for the state of the underlying TCP socket when returning connections to the pool. This is particularly dangerous in scenarios involving authentication middleware like ForwardAuth where the body of requests may contain sensitive information such as authentication tokens or session data. The issue is exacerbated when forwardBody and preserveRequestMethod parameters are enabled, as these configurations can cause the proxy to re-issue CONNECT requests with buffered bodies, thereby exposing the connection pool to additional attack vectors.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable authenticated session hijacking and cross-client data leakage. An unauthenticated attacker can leverage this behavior to craft malicious requests that manipulate the shared connection pool state, allowing them to read responses from other clients that were processed through the same connection. This desynchronization attack can expose sensitive content including authentication tokens, private user data, or business-critical information from other concurrent sessions. The vulnerability affects all versions prior to 2.11.53, 3.6.24, and 3.7.9 and represents a critical security gap in the HTTP protocol handling logic of the reverse proxy implementation. Organizations using Traefik in production environments are particularly at risk as this flaw can be exploited without authentication credentials, making it an attractive target for attackers seeking to gain unauthorized access to sensitive data.
The fix implemented in the patched versions addresses the core issue by ensuring proper socket state management and connection pool hygiene when handling HTTP/2 and HTTP/3 CONNECT requests. The remediation involves stricter validation of upstream responses and ensures that connections are properly drained before being returned to the shared pool, preventing the reuse of desynchronized sockets. This vulnerability aligns with CWE-1295 which addresses improper handling of protocol state information and can be categorized under ATT&CK technique T1566.002 for credential access through exploitation of network protocols. Organizations should immediately upgrade to the patched versions and conduct thorough security reviews of their Traefik configurations, particularly focusing on authentication middleware usage and connection pooling settings. The vulnerability demonstrates the complexity inherent in modern reverse proxy implementations where protocol translation between different HTTP versions creates attack surfaces that require careful state management and validation.