CVE-2025-61684: Quicly has assertion failures

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-19 · updated 2026-06-17
Weakness: CWE-20

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.

Affected products

h2o quicly: before 2026-01-18 — fixed in 2026-01-18

How to fix

Upgrade to a fixed version: h2o quicly 2026-01-18. Check the vendor advisory for mitigations.

References

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

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