Vulnerability · Published Oct 8, 2026 · Assigned by Go
CVE-2026-56866HTTP/1 client connection desynchronization after CONNECT rejection in net/http
High 7.5
In plain English
A high-severity security flaw in net/http from Go standard library. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:8 | delve | 1.26.1-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:10 | go-toolset | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | go-toolset | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | go-toolset | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-bin | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-bin | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-bin | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-docs | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-docs | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-docs | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-misc | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-misc | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-misc | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-race | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-race | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-race | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-src | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-src | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-src | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| AlmaLinux:10 | golang-tests | 1.26.10-1.el10_2.alma.1 | ALSA-2026:86093 |
| AlmaLinux:8 | golang-tests | 1.26.10-1.module_el8.10.0+4307+6e4cbc60 | ALSA-2026:86203 |
| AlmaLinux:9 | golang-tests | 1.26.10-1.el9_8 | ALSA-2026:86196 |
| Alpine:v3.24 | go | 1.26.9-r0 | ALPINE-CVE-2026-56866 |
| Debian:12 | golang-1.19 | no fix listed | DEBIAN-CVE-2026-56866 |
| Debian:13 | golang-1.24 | no fix listed | DEBIAN-CVE-2026-56866 |
| Debian:14 | golang-1.26 | 1.26.9-1 | DEBIAN-CVE-2026-56866 |
| Debian:14 | golang-1.27 | 1.27.2-1 | DEBIAN-CVE-2026-56866 |
| Go | stdlib | 1.26.9, 1.27.2 | GO-2026-6605 |
| Ubuntu:22.04:LTS | golang-1.17 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.18 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.20 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.21 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.21 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.22 | 1.22.2-2~22.04.4 | USN-8935-1 |
| Ubuntu:22.04:LTS | golang-1.22 | 1.22.2-2~22.04.4 | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.22 | 1.22.2-2ubuntu0.5 | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.22 | 1.22.2-2ubuntu0.5 | USN-8935-1 |
| Ubuntu:22.04:LTS | golang-1.23 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.23 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.04:LTS | golang-1.23 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.24 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.24 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.04:LTS | golang-1.24 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.25 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.25 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.04:LTS | golang-1.25 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:22.04:LTS | golang-1.26 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:24.04:LTS | golang-1.26 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.04:LTS | golang-1.26 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.10 | golang-1.26 | no fix listed | UBUNTU-CVE-2026-56866 |
| Ubuntu:26.10 | golang-1.27 | no fix listed | UBUNTU-CVE-2026-56866 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Go standard library | net/http | 0 – before 1.26.91.27.0-0 – before 1.27.2 |
| Go standard library | net/http/httputil | 0 – before 1.26.91.27.0-0 – before 1.27.2 |
Weakness: CWE-444 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N