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

> Attackers can cause denial of service in h2o HTTP server by exploiting HTTP/2 state amplification via HPACK decompression and stream stalling.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 16 Jul 2026
- **Updated:** 17 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-54340, GHSA-qcrr-wrhc-pgq9, UBUNTU-CVE-2026-54340

## What is CVE-2026-54340?

CVE-2026-54340 is a high-severity denial of service vulnerability in h2o, published on 16 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2026-54340?

It affects 1 product.


Products: H2o H2o.

## How do I fix CVE-2026-54340?

No fix has been published for CVE-2026-54340 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-54340 being exploited?

There is no known exploitation of CVE-2026-54340 in the wild and no public exploit code.

## Other vulnerabilities in h2o

- [CVE-2023-6016](https://insights.sbomify.com/CVE-2023-6016/): H2O Remote Code Execution via POJO Model Import (critical 10.0)
- [CVE-2025-6544](https://insights.sbomify.com/CVE-2025-6544/): Deserialization Vulnerability in h2oai/h2o-3 (critical 9.8)
- [CVE-2024-10553](https://insights.sbomify.com/CVE-2024-10553/): Jdbc Deserialization in h2oai/h2o-3 (critical 9.8)
- [CVE-2023-6569](https://insights.sbomify.com/CVE-2023-6569/): External Control of File Name or Path in h2oai/h2o-3 (critical 9.3)
- [CVE-2023-6013](https://insights.sbomify.com/CVE-2023-6013/): H2O Local File Include (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54340 and fix it if it is affected.

What it is: CVE-2026-54340 is a high-severity denial of service vulnerability in h2o, published on 16 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-54340 in the wild and no public exploit code.

Affected product: h2o. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54340/ (Markdown: https://insights.sbomify.com/CVE-2026-54340/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54340
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54340
- OSV: https://osv.dev/vulnerability/CVE-2026-54340

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54340/
Summaries are generated and may contain mistakes; the official description above is the record.