---
title: "CVE-2026-78660: HTTP/2 Reverse Proxy - HTTP Request Smuggling"
description: "An HTTP/2 implementation in a reverse proxy forwards malformed framing-related headers to HTTP/1 clients, causing HTTP request smuggling when the HTTP/1 client is not strict, letting attackers smuggle requests."
url: https://panop.io/threat-heartbeat/CVE-2026-78660/
---

# CVE-2026-78660 HTTP/2 Reverse Proxy - HTTP Request Smuggling

An HTTP/2 implementation in a reverse proxy forwards malformed framing-related headers to HTTP/1 clients, causing HTTP request smuggling when the HTTP/1 client is not strict, letting attackers smuggle requests.

CVSS base score 7.5 /10

High

CISA KEV

Not listed

Public exploit

None known

Authentication

Not required

EPSS

0.30% chance of exploitation in 30 days

[View the NVD record ↗](https://nvd.nist.gov/vuln/detail/CVE-2026-78660)

## Details

Type

HTTP Request Smuggling

Impact

HTTP Request Smuggling

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/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 version with strict header validation and HTTP/2 to HTTP/1 proxy fixes.

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-2026-78660](https://nvd.nist.gov/vuln/detail/CVE-2026-78660)
- [https://go.dev/cl/835145](https://go.dev/cl/835145)
- [https://go.dev/cl/836385](https://go.dev/cl/836385)
- [https://go.dev/issue/81115](https://go.dev/issue/81115)
- [https://groups.google.com/g/golang-announce/c/U2fTuyDJznI](https://groups.google.com/g/golang-announce/c/U2fTuyDJznI)
- [https://pkg.go.dev/vuln/GO-2026-6610](https://pkg.go.dev/vuln/GO-2026-6610)

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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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