# CVE-2025-61684: Quicly has assertion failures

> A remote attacker can trigger assertion failures in the Quicly library, causing a denial of service by crashing the application.

- **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:** 19 Jan 2026
- **Updated:** 20 Jan 2026
- **Also known as:** GHSA-wr3c-345m-43v9

## What is CVE-2025-61684?

CVE-2025-61684 is a high-severity denial of service vulnerability in h2o quicly, published on 19 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Quicly, an IETF QUIC protocol implementation, is susceptible to a denial-of-service attack prior to commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e. A remote attacker can exploit these bugs to trigger an assertion failure that crashes process using Quicly. Commit d9d3df6a8530a102b57d840e39b0311ce5c9e14e fixes the issue.

Weakness: CWE-617 (Reachable Assertion), CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2025-61684?

It affects 1 product.


Products: H2o Quicly.

## How do I fix CVE-2025-61684?

No fix has been published for CVE-2025-61684 yet. Limit exposure of the affected software until one is available.

## Is CVE-2025-61684 being exploited?

There is no known exploitation of CVE-2025-61684 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-2026-44433](https://insights.sbomify.com/CVE-2026-44433/): Quicly is vulnerable to memory exhaustion (medium 5.3)
- [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-2025-61684 and fix it if it is affected.

What it is: CVE-2025-61684 is a high-severity denial of service vulnerability in h2o quicly, published on 19 Jan 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-2025-61684 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-2025-61684/ (Markdown: https://insights.sbomify.com/CVE-2025-61684/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-61684
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-61684
- OSV: https://osv.dev/vulnerability/CVE-2025-61684

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-61684/
Summaries are generated and may contain mistakes; the official description above is the record.