CVE-2026-56866 HTTP/1 client connection desynchronization after CONNECT rejection in net/http
Attackers can poison cached connections in Go HTTP clients, causing cross-user response leakage in reverse proxies.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-56866?
CVE-2026-56866 is a high-severity vulnerability in Go standard library net/http, published on 8 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-444
Which versions are affected by CVE-2026-56866?
2 open source packages ship an affected version, including golang and stdlib.
| Package | Affected | Fixed in |
|---|---|---|
golang Bitnami | All versions | 1.26.9, 1.27.2 |
stdlib Go | All versions | 1.26.9, 1.27.2 |
How do I fix CVE-2026-56866?
Upgrade golang to a fixed version for your release line: 1.26.9 or 1.27.2.
Is CVE-2026-56866 being exploited?
There is no known exploitation of CVE-2026-56866 in the wild and no public exploit code.
Sources
Also known as AZL-107399, AZL-107460, AZL-107492, AZL-107853, AZL-107928, BELL-CVE-2026-56866 and 2207 more.