---
id: CVE-2026-44434
title: >-
  Quicly is an IETF QUIC protocol implementation intended primarily for use
  within the H2O HTTP server
summary: >-
  Quicly is an IETF QUIC protocol implementation intended primarily for use
  within the H2O HTTP server. Prior to commit dccf5d4, Quicly was vulnerable to
  stateless reset injection through lack of packet entry validation. The QUIC
  protocol …
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-345
  - CWE-665
published: '2026-07-16'
updated: '2026-07-18'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-44434'
references:
  - url: >-
      https://github.com/h2o/quicly/commit/dccf5d4579c7ae9dd6e8f90c36d52e311bb60710
    label: security-advisories@github.com
  - url: 'https://github.com/h2o/quicly/security/advisories/GHSA-899f-49jq-pfh8'
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00209
epssPercentile: 0.09957
ingestedAt: '2026-07-18T20:24:54.794Z'
---

## Overview

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit dccf5d4, Quicly was vulnerable to stateless reset injection through lack of packet entry validation. The QUIC protocol is designed to withstand packet injection attacks, once the handshake is complete. Only packets that carry some secret patterns are considered as stateless resets. Quicly allows the peer to share up to 4 such patterns per connection. However, until now, it failed to determine which of the 4 slots that it uses to retain the secret patterns contains a valid entry. As the slots are zero-initialized, the failure meant that, unless the peer advertised 4 of such patterns, an all-zero pattern was treated as a stateless reset.In effect, this allowed an on-path attacker to reset QUIC connections governed by Quicly. This issue has been fixed by commit dccf5d4.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.