Is CVE-2026-44435 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-44435 being exploited in the wild.

How severe is CVE-2026-44435?

High severity, with a CVSS score of 7.5, published 16 July 2026.

Which products and versions are affected?

H2o Quicly Prior to commit 937d0e9.

Is there a fix for CVE-2026-44435?

No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

How is CVE-2026-44435 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-44435?

Check whether you run H2o Quicly at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-44435: H2O Quicly assertion failure in CRYPTO stream handling

CVE-2026-44435 · Severity: high · CVSS 7.5 · Published 2026-07-16

Quicly is a software library used by the H2O web server to handle modern internet traffic. A flaw in how it processes encrypted handshake messages allows a remote attacker to crash the server by sending a specific sequence of data. This results in a denial-of-service, making the website or service unavailable to legitimate users.

Technologies: H2o Quicly. Vendors: H2o.

Quick answers

Is CVE-2026-44435 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-44435 being exploited in the wild.
How severe is CVE-2026-44435?: High severity, with a CVSS score of 7.5, published 16 July 2026.
Which products and versions are affected?: H2o Quicly Prior to commit 937d0e9.
Is there a fix for CVE-2026-44435?: No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-44435 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-44435?: Check whether you run H2o Quicly at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A reachable assertion vulnerability exists in Quicly's handling of CRYPTO streams. The software triggers an assertion failure (CWE-617) when it receives more than 32KB of valid handshake messages within a single packet number space. This occurs due to uncontrolled resource consumption (CWE-400) during the QUIC handshake process. An unauthenticated remote attacker can exploit this by sending a large volume of handshake data, leading to an immediate process crash and denial of service. The issue is resolved in commit 937d0e9.

Affected products

h2o quicly Prior to commit 937d0e9

Timeline

2026-05-29: advisory: GitHub Security Advisory published
2026-07-16: disclosed: CVE published to NVD

References

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