CVE-2026-44435Network attack vector

Remote service crash via CRYPTO stream assertion

Published Jul 16, 2026 · Updated Jul 16, 2026

Reachable assertion in H2O quicly before commit 937d0e9 allows remote attackers to crash a QUIC endpoint with oversized handshake data. The receive path applies stream flow-control update logic to an internal negative CRYPTO stream ID after valid handshake data crosses the threshold, reaching an assertion. No authentication or user interaction is required; the result is service interruption without reported data disclosure or modification.

CVSS severity7.5
High
EPSS probability0.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Reachable assertion in H2O quicly before commit 937d0e9 allows remote attackers to crash a QUIC endpoint with oversized handshake data. The receive path applies stream flow-control update logic to an internal negative CRYPTO stream ID after valid handshake data crosses the threshold, reaching an assertion. No authentication or user interaction is required; the result is service interruption without reported data disclosure or modification.

The record

CVE
CVE-2026-44435
Published
Jul 16, 2026
Updated
Jul 16, 2026
Vendor
H2O Project
Product
quicly
Classifications
CWE-400, CWE-617, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 16, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 937d0e9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

H2O Project · quiclyversion=< 937d0e9

Published CVSS scores

7.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-617Reachable Assertion
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo