# CVE-2026-44433: Quicly is vulnerable to memory exhaustion

> An adversarial peer can cause memory exhaustion in Quicly by sending crafted STREAM frames, leading to a Denial of Service.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **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:** GHSA-f7qr-4p37-9gx9

## What is CVE-2026-44433?

CVE-2026-44433 is a medium-severity denial of service vulnerability in h2o quicly, published on 16 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, an adversarial peer could send a STREAM frame carrying just one byte at the largest offset being permitted to obtain additional flow control credit, which under certain circumstances could lead to a Denial of Service. Assuming the application prepares a receive buffer for storing all data that arrive out-of-order, up to the largest offset being received, this behavior could lead to the application allocating large amount of memory with the peer sending only a handful of packets, resulting in memory exhaustion. In addition to the receive buffer allocation strategy, the severity of this vulnerability depends on how the application controls the stream concurrency. In case of the H2O HTTP server, under its default setting, this bug increases the maximum amount of memory allocated per connection by about 4 times. This issue has been fixed by commit 8b178e6.

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

## Which versions are affected by CVE-2026-44433?

It affects 1 product.


Products: H2o Quicly.

## How do I fix CVE-2026-44433?

No fix has been published for CVE-2026-44433 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-44433 being exploited?

There is no known exploitation of CVE-2026-44433 in the wild and no public exploit code.

## Other vulnerabilities in h2o quicly

- [CVE-2026-44435](https://insights.sbomify.com/CVE-2026-44435/): Quicly: Remote Denial of Service via assertion failure when CRYPTO stream handshake data exceeds 32KB (high 7.5)
- [CVE-2026-44436](https://insights.sbomify.com/CVE-2026-44436/): Quicly is vulnerable to connection state corruption (high 7.5)
- [CVE-2025-61684](https://insights.sbomify.com/CVE-2025-61684/): Quicly has assertion failures (high 7.5)
- [CVE-2026-44434](https://insights.sbomify.com/CVE-2026-44434/): Quicly is vulnerable to stateless reset injection (medium 5.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-44433 and fix it if it is affected.

What it is: CVE-2026-44433 is a medium-severity denial of service vulnerability in h2o quicly, 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-2026-44433 in the wild and no public exploit code.

Affected product: h2o quicly. 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-2026-44433/ (Markdown: https://insights.sbomify.com/CVE-2026-44433/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-44433
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-44433
- OSV: https://osv.dev/vulnerability/CVE-2026-44433

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-44433/
Summaries are generated and may contain mistakes; the official description above is the record.