---
id: "CVE-2026-78669"
kind: "vulnerability"
aliases: ["CVE-2026-78669", "GO-2026-6611"]
title: "Excessive CPU consumption from repeated initial window changes in net/http"
severity: "high"
cvss: 7.5
epss: 0.00568
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", "kubernetes", "net-http", "runtime", "self-hosted"]
not_affected_if: ["behind-proxy-terminating-http2", "http1-only", "managed-hosting", "no-go-http-server"]
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-2026-6611", "first_seen_at": "2026-10-23T19:13:08+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-78669", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-78669", "first_seen_at": "2026-10-23T19:35:23+00:00"}]
---

# Excessive CPU consumption from repeated initial window changes in net/http

Go's net/http HTTP/2 implementation burns excessive CPU when a malicious peer opens many streams and then sends many small SETTINGS frames that change SETTINGS_INITIAL_WINDOW_SIZE. An attacker can tie up CPU on a Go HTTP/2 client or server with a modest stream of frames.

_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 the Go version with 'go version' on the built binary or 'go list -m golang.org/x/net' for the x/net module; affected stdlib is below 1.26.9 or 1.27.0 through 1.27.1, and x/net is below 0.60.0.
Run 'govulncheck ./...' in the project to confirm whether the vulnerable code is reachable.

## Fix

Upgrade the Go toolchain to 1.26.9 or 1.27.2, and upgrade golang.org/x/net to 0.60.0, then rebuild and redeploy.
Rebuild all Go binaries after upgrading, since the stdlib fix is compiled in.

## Timeline

- 2026-10-23T19:13:08+00:00 govuln https://pkg.go.dev/vuln/GO-2026-6611
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-78669
- 2026-10-23T19:35:23+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-78669

## From the sources

A malicious HTTP/2 peer can cause excessive CPU consumption in the client or server by opening a large number of streams and then sending many small SETTINGS frames containing SETTINGS_INITIAL_WINDOW_SIZE values.

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