---
title: "CVE-2026-56866: Go http.Transport - HTTP Request Smuggling"
description: "Go http.Transport contains a cross-user response poisoning vulnerability caused by improper handling of HTTP/1 CONNECT requests with non-empty bodies, letting attackers cause connection desynchronization and response injection, exploit requires use of shared Transport in reverse proxies."
url: https://panop.io/threat-heartbeat/CVE-2026-56866/
---

# CVE-2026-56866 Go http.Transport - HTTP Request Smuggling

Go http.Transport contains a cross-user response poisoning vulnerability caused by improper handling of HTTP/1 CONNECT requests with non-empty bodies, letting attackers cause connection desynchronization and response injection, exploit requires use of shared Transport in reverse proxies.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.31% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-56866)

## Details

Type

HTTP Request Smuggling

Impact

Data Tampering, Information Disclosure

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Published

8 October 2026

Updated

15 October 2026

Weakness

[CWE-444](https://cwe.mitre.org/data/definitions/444.html)

## Vendor fix

Update to the latest Go version with the fix for CONNECT request body handling.

Patching closes this CVE. It does not tell you which of your systems run the affected version, or what else is exposed alongside them.

## References

- [https://nvd.nist.gov/vuln/detail/CVE-2026-56866](https://nvd.nist.gov/vuln/detail/CVE-2026-56866)
- [https://go.dev/cl/847306](https://go.dev/cl/847306)
- [https://go.dev/issue/81740](https://go.dev/issue/81740)
- [https://groups.google.com/g/golang-announce/c/U2fTuyDJznI](https://groups.google.com/g/golang-announce/c/U2fTuyDJznI)
- [https://pkg.go.dev/vuln/GO-2026-6605](https://pkg.go.dev/vuln/GO-2026-6605)

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## What this record can't tell you

- Whether you run the affected software, and where
- Whether an attacker can actually reach and exploit it
- The misconfigurations that never get a CVE number
- What it would cost the business, ranked into what to fix first

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