CVE-2026-44435 Quicly: Remote Denial of Service via assertion failure when CRYPTO stream handshake data exceeds 32KB
Remote attackers can cause a denial of service in Quicly by sending handshake data exceeding 32KB, triggering an assertion failure.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-44435?
CVE-2026-44435 is a high-severity denial of service vulnerability in h2o quicly, published on 16 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-44435?
It affects 1 product.
Products include H2o Quicly.
How do I fix CVE-2026-44435?
No fix has been published for CVE-2026-44435 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-44435 being exploited?
There is no known exploitation of CVE-2026-44435 in the wild and no public exploit code.
Other vulnerabilities in h2o quicly
Sources
Also known as GHSA-2cw9-5673-73gv.