---
id: "CVE-2026-78660"
kind: "vulnerability"
aliases: ["CVE-2026-78660", "GO-2026-6610"]
title: "HTTP/2 transport accepts malformed framing-related headers in net/http"
severity: "high"
cvss: 7.5
epss: 0.00303
exploited: false
status: "open"
category: "runtime"
products: [{"name": "stdlib", "fixed": ["1.26.9"], "affected": "< 1.26.9", "ecosystem": "go"}, {"name": "stdlib", "fixed": ["1.27.2"], "affected": ">= 1.27.0-0, < 1.27.2", "ecosystem": "go"}, {"name": "golang.org/x/net", "fixed": ["0.60.0"], "affected": "< 0.60.0", "ecosystem": "go"}]
ecosystems: ["go"]
tags: ["go", "http2", "net/http", "reverse-proxy", "runtime"]
not_affected_if: ["go-not-used", "no-http2-reverse-proxy"]
published_at: "2026-10-08T22:31:09+00:00"
first_seen_at: "2026-10-23T19:13:08+00:00"
updated_at: "2026-10-23T19:56:03+00:00"
sources: [{"source": "govuln", "url": "https://pkg.go.dev/vuln/GO-2026-6610", "first_seen_at": "2026-10-23T19:13:08+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-78660", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78660", "first_seen_at": "2026-10-23T19:35:23+00:00"}]
---

# HTTP/2 transport accepts malformed framing-related headers in net/http

Go's net/http HTTP/2 implementation accepts malformed framing-related headers and can forward them to HTTP/1 clients when acting as a reverse proxy. A downstream HTTP/1 client that parses them loosely can be tricked into response smuggling, letting an attacker desync responses.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- go:stdlib affected < 1.26.9 fixed 1.26.9
- go:stdlib affected >= 1.27.0-0, < 1.27.2 fixed 1.27.2
- go:golang.org/x/net affected < 0.60.0 fixed 0.60.0

## How to check

Check whether the Go binary's stdlib is below 1.26.9 or in 1.27.0-0 to 1.27.1 (go version -m <binary>), and whether golang.org/x/net is below 0.60.0 in go.mod or go.sum.

## Fix

Upgrade the Go toolchain to 1.26.9 or 1.27.2, and golang.org/x/net to 0.60.0, then rebuild.

## Timeline

- 2026-10-23T19:13:08+00:00 govuln https://pkg.go.dev/vuln/GO-2026-6610
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-78660
- 2026-10-23T19:35:23+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-78660

## From the sources

Historically, we have been rather lax about malformed framing-related headers in our HTTP/2 implementation, as they cannot interfere with HTTP/2 framing. However, this makes it possible for our HTTP/2 implementation to forward responses containing such headers to an HTTP/1 client when acting as a reverse proxy. If the HTTP/1 client also does not behave strictly enough, this can result in response smuggling.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_