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HTTP/1 client connection desynchronization after CONNECT rejection in net/http

CVE-2026-56866

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (stdlib 1.27.2, 1.26.9), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • Go stdlib

    Affected
    < 1.26.9>= 1.27.0-0, < 1.27.2
    Fixed in
    1.26.9, 1.27.2

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
not scored
EPSS
  • CVE-2026-56866: 0.47% estimated exploitation activity in the next 30 days; percentile 41.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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.

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from osv record GO-2026-6605

Agent brief

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# Patch Next advisory brief: CVE-2026-56866

Advisory: HTTP/1 client connection desynchronization after CONNECT rejection in net/http
IDs: CVE-2026-56866
Record: https://patchnext.com/advisories/cve-2026-56866 (revision 1, source checked 23 Oct 2026, 22:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Unrated
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: stdlib 1.27.2, 1.26.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go stdlib: affected < 1.26.9; >= 1.27.0-0, < 1.27.2; fixed in 1.26.9, 1.27.2

## Suggested inspection (read-only)
- Go stdlib: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on stdlib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GO-2026-6605 (OSV record GO-2026-6605 lists affected ranges: Go stdlib >=0 =1.27.0-0 <1.27.2 (fixed))

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