CVE-2025-61684: Quicly, an IETF QUIC protocol implementation, is susceptible to a denial-of-service attack prior to commit d9d

Q: How do I fix CVE-2025-61684?

quicly: upgrade to 2026-01-18. Exact commands for each OS are on this page.

Q: Is CVE-2025-61684 being exploited?

CVE-2025-61684 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2025-61684 affect?

CVE-2025-61684 affects quicly. Severity is high (CVSS 7.5). 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 proce

Q: How urgent is CVE-2025-61684?

Priority 40/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2025-61684 is a high-severity flaw (CVSS 7.5) affecting quicly. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-20, CWE-617
Published	2026-01-19
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: h2o

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-01-19 16:15 UTCPublished CVE record published
2026-06-17 09:50 UTCUpdated Record last modified

h2o quicly UPGRADE

Affected: < 2026-01-18

Fixed in: 2026-01-18

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy quicly || rpm -q quicly || quicly --version

2. UPGRADE to 2026-01-18 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade quicly

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade quicly

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-61684?

quicly: upgrade to 2026-01-18. Exact commands for each OS are on this page.

Is CVE-2025-61684 being exploited?

CVE-2025-61684 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-61684 affect?

CVE-2025-61684 affects quicly. Severity is high (CVSS 7.5). 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 proce

How urgent is CVE-2025-61684?

Priority 40/100. High severity; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-44435 HIGH 7.5 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2026-44436 HIGH 7.5 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2026-44433 MEDIUM 5.3 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2026-44434 MEDIUM 5.3 UPGRADE Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server
CVE-2024-45396 HIGH 7.5 UPGRADE Quicly is an IETF QUIC protocol implementation

References

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