quicly
CVE-2026-44435
HIGH
Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
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AccessReach QUIC/HTTP-3 UDP listener
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DeliveryBegin QUIC handshake
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ExploitSend >32KB valid CRYPTO handshake data
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ExecutionTrigger reachable assertion
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PersistProcess aborts, connections dropped
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ImpactRepeat to sustain DoS
Vulnerability Assessment
| Weakness |
CWE
ASVS 5.0
V16 - Security Logging and Error Handling Reachable assertion handling
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| Exploitation Conditions | Exploitation requires the target to have QUIC/HTTP-3 enabled and reachable (the quicly-based listener active on UDP), and the attacker must drive the CRYPTO stream of a single packet number space past 32KB of valid handshake messages during connection establishment. Given the CVSS vector AV:N/AC:L/PR:N/UI:N, no authentication, user interaction, or non-default tuning is otherwise needed against a QUIC-enabled deployment. The principal limiting factor is that a server with QUIC/HTTP-3 disabled (TCP-only HTTP/2) is not exposed, and the attacker must be able to reach the UDP QUIC port; impact is confined to availability (process crash), with no path to confidentiality or integrity compromise. |
| Impact | The CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5 High) is internally consistent with the description: network-reachable, low complexity, no privileges or user interaction, and pure availability impact (assertion-driven crash, no data compromise). No EPSS score, KEV entry, or POC was provided, so real-world exploitation pressure cannot be quantified from the input - the signals are limited to CVSS plus a GHSA advisory and a fix commit. This is a credible availability priority for anyone exposing QUIC/HTTP-3 via quicly/H2O because triggering it only requires sending oversized-but-valid handshake data pre-authentication, but the impact ceiling is a process crash/restart, not code execution or data loss. Absence of an EPSS/KEV signal means this is best treated as a patch-soon availability bug rather than an actively-exploited emergency. |
| Exploit ScenarioDerived | An attacker initiates a QUIC/HTTP-3 connection to an internet-facing H2O/quicly endpoint and deliberately delivers a stream of valid handshake messages over the CRYPTO stream whose cumulative size exceeds 32KB in one packet number space. The assertion fires and the server process aborts, dropping all in-flight connections; the unauthenticated attacker can repeat this to sustain a denial of service. No public POC was provided in the input. |
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Alert meAnalysisAIv1 – Jul 16, 2026
Remote denial of service in quicly, the IETF QUIC implementation used by the H2O HTTP server, allows unauthenticated attackers to crash the process by sending more than 32KB of valid handshake messages over the CRYPTO stream within a single packet number space, tripping a reachable assertion (CWE-617). Affects all builds prior to commit 937d0e9. No public exploit identified at time of analysis and the flaw is not in CISA KEV, but the network-reachable, no-privilege nature makes it trivially triggerable once understood.
RemediationAI
Upstream fix available (PR/commit); released patched version not independently confirmed - update quicly to a build that includes commit 937d0e9e7c669fc2bee4920632ab6aaac60e4d81 (or later) and rebuild/redeploy any H2O or downstream binaries that statically embed it, as described in advisory GHSA-2cw9-5673-73gv (https://github.com/h2o/quicly/security/advisories/GHSA-2cw9-5673-73gv). If you cannot rebuild immediately, reduce exposure by disabling QUIC/HTTP-3 listeners and forcing clients onto HTTP/2 over TCP/TLS, which removes the vulnerable CRYPTO-stream path entirely at the cost of losing HTTP-3; alternatively, restrict UDP/443 QUIC reachability to trusted networks or place rate-limiting/anti-abuse controls in front of the QUIC endpoint, accepting that these only slow rather than eliminate a single crafted connection. Ensure process supervision (systemd/automatic restart) is configured so an assertion abort self-recovers, understanding this masks but does not prevent repeated DoS.
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Technical ContextAI · show details
quicly implements the QUIC transport protocol (RFC 9000/9001), where TLS 1.3 handshake data is carried inside QUIC CRYPTO frames reassembled per packet-number-space CRYPTO stream. The vulnerability is a CWE-617 reachable assertion: the code enforces an internal invariant assuming the cumulative volume of valid handshake messages in one packet number space will not exceed 32KB, and when a peer legitimately (or maliciously) supplies more, the assertion fires and aborts the process rather than gracefully rejecting the oversized input. Per the CPE (cpe:2.3:a:h2o:quicly), this affects the quicly library itself, and by extension any server such as H2O that embeds it to terminate QUIC/HTTP-3 connections.
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Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Remote, unauthenticated, no interaction over the QUIC handshake with low complexity; impact is solely an assertion-driven crash, so C/I:N and A:H.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
DescriptionCVE.org
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.
Executive ViewAI
H2O web servers using quicly for QUIC protocol handling are subject to unauthenticated remote denial-of-service attacks that can crash the server process, directly threatening service uptime. Any organization relying on H2O for web traffic faces potential availability disruptions until patches are applied.
No vendor-released patch identified at time of analysis. Interim mitigations: restrict QUIC protocol access via firewall to known, trusted clients; implement connection-rate limiting on initial TLS handshakes; deploy IDS signatures to detect abnormally large QUIC CRYPTO stream messages; consider disabling QUIC protocol in server configuration (falling back to HTTP/2) if operations permit.
Threat Intelligence Overview
NVD
CVE.org
EPSS
Vulnrichment
GitHub
VulDB
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.
Affected Products (CPE)CVE.org
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Exploit-DB