# CVE-2026-55213: h2o: musl libc stack overflow (QPACK)

> A remote attacker can crash the h2o HTTP/3 server by sending a crafted QPACK instruction that triggers a stack overflow via alloca.

- **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:** 10 Jul 2026
- **Updated:** 13 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-55213, GHSA-432c-8xmj-frmq, UBUNTU-CVE-2026-55213

## What is CVE-2026-55213?

CVE-2026-55213 is a high-severity denial of service vulnerability in h2o, published on 10 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 edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1, when h2o processes a QPACK instruction sent from the peer over HTTP/3, lib/http3/qpack.c might allocate an on-stack buffer as large as approximately 800 KB by calling alloca, which exceeds the default pthread stack size used by musl libc and causes the h2o server to crash with a segmentation fault while touching the guard page. This issue is fixed in commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1.

Weakness: CWE-789 (Memory Allocation with Excessive Size Value)

## Which versions are affected by CVE-2026-55213?


## How do I fix CVE-2026-55213?

No fix has been published for CVE-2026-55213 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-55213 being exploited?

There is no known exploitation of CVE-2026-55213 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55213 and fix it if it is affected.

What it is: CVE-2026-55213 is a high-severity denial of service vulnerability in h2o, published on 10 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-2026-55213 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-2026-55213/ (Markdown: https://insights.sbomify.com/CVE-2026-55213/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55213
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55213
- OSV: https://osv.dev/vulnerability/CVE-2026-55213

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55213/
Summaries are generated and may contain mistakes; the official description above is the record.