CVE-2026-44435 in quicly
Summary
by MITRE • 07/17/2026
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.
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Analysis
by VulDB Data Team • 07/17/2026
The vulnerability in Quicly represents a critical denial of service weakness that affects the IETF QUIC protocol implementation within the H2O HTTP server environment. This flaw manifests as an assertion failure when the cumulative count of valid handshake messages received over a CRYPTO stream within a single packet number space surpasses the arbitrary threshold of 32KB. The issue stems from insufficient input validation and resource management during the QUIC handshake process, where the system fails to properly handle extended handshake message sequences that exceed predetermined limits.
The technical implementation flaw resides in how Quicly processes and tracks handshake messages within the QUIC protocol's cryptographic stream handling mechanism. When the cumulative size of valid CRYPTO stream data reaches 32KB, the assertion check triggers a failure that terminates the connection process or causes the server to become unresponsive. This represents a classic resource exhaustion vulnerability where the system's defensive mechanisms inadvertently create an attack vector rather than protecting against malicious input. The problem aligns with CWE-704 in the Common Weakness Enumeration catalog, which classifies improper handling of resource limits and excessive resource consumption as critical security flaws.
From an operational impact perspective, this vulnerability enables attackers to perform denial of service attacks against H2O servers implementing Quicly by carefully crafting handshake sequences that approach or exceed the 32KB threshold. The attack requires minimal resources and can be executed through standard network traffic manipulation, making it particularly dangerous for web services that rely on QUIC protocol implementations. The service disruption occurs at the protocol level during connection establishment, effectively blocking legitimate clients from establishing secure connections to affected servers.
The mitigation strategy involves implementing proper bounds checking and resource management within the handshake processing logic, ensuring that the system gracefully handles extended message sequences without triggering assertion failures. This fix aligns with ATT&CK technique T1499.004 which covers network denial of service attacks through resource exhaustion. The solution requires modifying the packet number space tracking mechanism to properly account for message accumulation limits while maintaining protocol compliance. Additionally, implementing rate limiting and connection monitoring can provide additional defense-in-depth measures against similar vulnerabilities in the QUIC implementation.