# CVE-2026-56866: Go net/http HTTP/1 client connection desynchronization after CONNECT rejection

Source: https://junglewise.ai/threats/cve-2026-56866-go-2026-6605-http-1-client-connection-desynchronization-after  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-08. Updated: 2026-10-10  
License: CC BY 4.0 with a link back

CVE-2026-56866 · Severity: high · CVSS 7.5 · Published 2026-10-08

Go's HTTP client library can become desynchronized when a CONNECT request is rejected by a server. In reverse proxy scenarios, this allows an attacker to craft a CONNECT request with a malicious body that gets misinterpreted as pipelined HTTP requests, poisoning subsequent responses and exposing data from other users sharing the same pooled connection.

Technologies: [stdlib (Go)](https://junglewise.ai/threats/technologies/go-stdlib). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-56866 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56866 being exploited in the wild.

**How severe is CVE-2026-56866?** High severity, with a CVSS score of 7.5, published 8 October 2026.

**Which products and versions are affected?** stdlib (Go) SEMVER:introduced=0,fixed=1.26.9,introduced=1.27.0-0,fixed=1.27.2.

**Is there a fix for CVE-2026-56866?** Yes. stdlib (Go): upgrade to 1.27.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-56866 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-56866?** Check whether you run stdlib (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- stdlib (Go): upgrade to 1.27.2 or later

  ```sh
  go get stdlib@v1.27.2
  ```


## Technical details

HTTP/1 clients that send CONNECT requests with a non-empty Request.Body write the body without framing; if the server rejects the request with a non-2xx keep-alive response, the transport returns the connection to the idle pool. The server then interprets trailing body bytes as pipelined requests, desynchronizing the connection. In reverse proxies forwarding CONNECT through a shared Transport, this enables cross-user response poisoning. The fix closes connections after CONNECT requests regardless of response status, and ReverseProxy now rejects CONNECT with 405 Method Not Allowed.

## Affected products

- golang Go <unknown>

## Identifiers

- CVE-2026-56866
- GO-2026-6605

## Timeline

- 2026-10-08: disclosed

## References

- [https://go.dev/cl/847306](https://go.dev/cl/847306)
- [https://go.dev/issue/81740](https://go.dev/issue/81740)

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