Denial of Service Vulnerability in Quicly Implementation for H2O HTTP Server
CVE-2026-44436

7.5HIGH

Key Information:

Vendor

H2o

Status
Quicly
Vendor
CVE Published:
16 July 2026

What is CVE-2026-44436?

Quicly, a QUIC protocol implementation used within the H2O HTTP server, is susceptible to a Denial of Service attack due to connection state corruption. This vulnerability arises from a mismatch in the maximum length of Connection IDs handled by the packet decoder and the actual buffer size within the library. Specifically, while QUIC version 1 limits Connection IDs to 20 bytes, Quicly accepts IDs up to 255 bytes, leading to potential inconsistencies and assertion failures when applications lack proper enforcement mechanisms. This issue has been addressed in commit 8b178e6.

Affected Version(s)

quicly < 8b178e6

References

https://github.com/h2o/quicly/security/advisories/GHSA-v5...
x_refsource_CONFIRM
https://github.com/h2o/quicly/commit/8b178e692c51a3b10316...
x_refsource_MISC

CVSS V3.1

Score:
7.5
Severity:
HIGH
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.