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.
Vulnerability Analysis
CVE-2026-56866 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality, with no impact on integrity and availability.
Weakness Type
What is a HTTP Request Smuggling Vulnerability?
When malformed or abnormal HTTP requests are interpreted by one or more entities in the data flow between the user and the web server, such as a proxy or firewall, they can be interpreted inconsistently, allowing the attacker to "smuggle" a request to one device without the other device being aware of it.
CVE-2026-56866 has been classified to as a HTTP Request Smuggling vulnerability or weakness.
Products Associated with CVE-2026-56866
Want to know whenever a new CVE is published for GoLang Go? stack.watch will email you.
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.