---
id: CVE-2026-56866
title: >-
  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
summary: >-
  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-ali…
severity: none
published: '2026-10-08'
updated: '2026-10-08'
sourceUpdated: '2026-10-08T23:17:01.620'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-56866'
references:
  - url: 'https://go.dev/cl/847306'
    label: security@golang.org
  - url: 'https://go.dev/issue/81740'
    label: security@golang.org
  - url: 'https://groups.google.com/g/golang-announce/c/U2fTuyDJznI'
    label: security@golang.org
  - url: 'https://pkg.go.dev/vuln/GO-2026-6605'
    label: security@golang.org
tags:
  - nvd
ingestedAt: '2026-10-09T00:19:50.974Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.