---
title: "CVE-2026-78659: HTTP/2 server memory exhaustion due to Trailer headers in net/http"
url: https://erroneo.us/security/cve/CVE-2026-78659/
description: "CVE-2026-78659 (Go standard library net/http): When \"Trailer\" headers are sent by a client, the HTTP server internally uses the header values to populate…"
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-2026-78659"
kev: false
cvss: 7.5
epss: 0.0057
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-78659: HTTP/2 server memory exhaustion due to Trailer headers in net/http

**Signal:** Normal patch cycle. No exploitation signal; fix it in your next routine update if you are affected. (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 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% 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

When "Trailer" headers are sent by a client, the HTTP server internally uses the header values to populate the Request.Trailer map passed to the server handler. Because Request.Trailer is a map, each entry incurs memory overhead. For HTTP/2 servers, a malicious client can exploit this by sending a "Trailer" header that declares a large number of fields, causing the server to allocate a disproportionate amount of memory while bypassing Server.MaxHeaderValueCount and Server.MaxHeaderBytes limits. This exploit is not applicable for HTTP/1 servers, which do not support multiplexing a large number of requests over one TCP connection, and whose Server.MaxHeaderBytes are calculated differently.

## 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: 7.5 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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 partial .
- FIRST EPSS (model of 2026-10-21): 0.57% probability of exploitation activity in the next 30 days; percentile 46.

## Sources

- [CVE.org record CVE-2026-78659](https://www.cve.org/CVERecord?id=CVE-2026-78659)
- [NVD entry CVE-2026-78659](https://nvd.nist.gov/vuln/detail/CVE-2026-78659)
- [go.dev](https://go.dev/cl/847185)
- [go.dev](https://go.dev/cl/847314)
- [go.dev](https://go.dev/issue/81857)
- [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-2026-6603)

---
Source page: https://erroneo.us/security/cve/CVE-2026-78659/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt