---
title: "CVE-2026-78663: Double flow control refund on HTTP/2 server streams in net/http"
url: https://erroneo.us/security/cve/CVE-2026-78663/
description: "CVE-2026-78663 (Go standard library net/http): The HTTP/2 server can refund connection-level flow control twice for the same data: Once when a client resets…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-10-08
date_modified: 2026-10-21
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-78663"
kev: false
cvss: 9.1
epss: 0.00574
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-78663: Double flow control refund on HTTP/2 server streams in net/http

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.6% , percentile 47: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 47% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Editorial verdict: Patch this week. Go October 2026 Security Releases: 16 Stdlib Fixes — Rebuild and redeploy Go services with Go 1.27.3 or 1.26.10 this week, starting with internet-facing HTTP/2 servers and reverse proxies; none of these bugs is known to be exploited.

## What it is

The HTTP/2 server can refund connection-level flow control twice for the same data: Once when a client resets a stream (refunding data for any sent-but-unread portion of the stream), and again when a request handler reads the buffered data. A malicious client can exploit this to bypass the configured connection-level flow control limit (MaxReceiveBufferPerConnection). Total buffered data is still limited by the concurrent stream limit and stream-level flow control.

## Affected products (CVE record)

- Go standard library net/http: < 1.26.9
- Go standard library net/http/internal/http2: 1.27.0-0 < 1.27.2
- golang.org/x/net golang.org/x/net/http2: < 0.60.0

## Severity

- CISA-ADP CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.57% probability of exploitation activity in the next 30 days; percentile 47.

## Sources

- [CVE.org record CVE-2026-78663](https://www.cve.org/CVERecord?id=CVE-2026-78663)
- [NVD entry CVE-2026-78663](https://nvd.nist.gov/vuln/detail/CVE-2026-78663)
- [go.dev](https://go.dev/cl/847187)
- [go.dev](https://go.dev/cl/847310)
- [go.dev](https://go.dev/issue/81743)
- [groups.google.com](https://groups.google.com/g/golang-announce/c/U2fTuyDJznI)
- [groups.google.com](https://groups.google.com/g/golang-announce/c/ZPwCyRUuGBs)
- [pkg.go.dev](https://pkg.go.dev/vuln/GO-2026-6612)

---
Source page: https://erroneo.us/security/cve/CVE-2026-78663/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt