CVE-2025-61684: Quicly has assertion failures
Vulnerability summary
CVE-2025-61684 is a high (CVSS 7.5) Improper Input Validation vulnerability in h2o quicly. EPSS estimates a 0.40% chance of exploitation in the next 30 days, higher than 33.7% of scored vulnerabilities.
Severe, CVSS 7.5 (CVE.org); no exploitation reported by the 5 checked sources; EPSS 0% (FIRST, 03-11-2026)
Source freshness unavailable
How is this decided?Timeline
- 19-01-2026PublishedVulnerability first disclosed.
- 20-01-2026Last modifiedVulnerability information updated.
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.
CVSS metrics
2 scores · 2 sources · 1 version- NVDNIST7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.40%
Chance of exploitation in the next 30 days
Percentile
34%
Versus all other CVEs
30-day change
Increase +0.01 pp
From 0.39% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root cause- CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
- CWE-617Reachable AssertionThe product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.
Affected systems
Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.
H2o1 product
- quicly< d9d3df6 | < 2026-01-18
1 vendor · 1 product