Go http.Transport CONNECT Body Desync Causing Cross-User Response Poisoning
CVE-2026-56866 Published on October 8, 2026
HTTP/1 client connection desynchronization after CONNECT rejection in net/http
When http.Transport sends an HTTP/1 CONNECT request with a non-empty Request.Body, it writes the body directly to the connection without framing after the request headers. If the server rejects the CONNECT request with a non-2xx keep-alive response, Transport returns the connection to the idle pool. Because CONNECT requests do not have a request body, the server may interpret the trailing body bytes as a subsequent pipelined HTTP/1.1 request on the connection, leaving the pooled connection desynchronized and causing the next caller that reuses it to read the response to the injected request. In reverse proxies (including httputil.ReverseProxy) that forward CONNECT requests through a shared Transport, this can lead to cross-user response poisoning.
Products Associated with CVE-2026-56866
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Affected Versions
Go standard library net/http:- Before 1.26.9 is affected.
- Version 1.27.0-0 and below 1.27.2 is affected.
- Before 1.26.9 is affected.
- Version 1.27.0-0 and below 1.27.2 is affected.