Assertion Failure DoS in Quicly QUIC Protocol Implementation

Publication date: 2026-07-16

Last updated on: 2026-07-16

Assigner: GitHub, Inc.

Description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 937d0e9, an assertion failure is raised when the total number of valid handshake messages received over a CRYPTO stream of a single packet number space exceeds 32KB, causing a Denial of Service. This issue has been fixed by commit 937d0e9.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-07-16

Last Modified

2026-07-16

Generated

2026-07-17

AI Q&A

2026-07-17

EPSS Evaluated

N/A

NVD

CVE-2026-44435

Affected Vendors & Products

Currently, no data is known.

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-617	The 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.
CWE-400	The product does not properly control the allocation and maintenance of a limited resource.

Attack-Flow Graph

Executive Summary

This vulnerability is a Denial of Service issue in Quicly, an IETF QUIC protocol implementation. It occurs when the total number of valid handshake messages received over a CRYPTO stream in a single packet number space exceeds 32KB, triggering an assertion failure that crashes the system.

Impact Analysis

This vulnerability can cause a Denial of Service, meaning an attacker could send specially crafted handshake messages to crash the affected system, disrupting its normal operation and making it unavailable to legitimate users.

Mitigation Strategies

Update Quicly to commit 937d0e9 or later to fix the assertion failure issue causing Denial of Service.

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