CVE-2026-44434 Quicly is vulnerable to stateless reset injection
An on-path attacker can terminate QUIC connections by injecting stateless resets due to improper validation of secret patterns in the Quicly library.
- 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-2026-44434?
CVE-2026-44434 is a medium-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 dccf5d4, Quicly was vulnerable to stateless reset injection through lack of packet entry validation. The QUIC protocol is designed to withstand packet injection attacks, once the handshake is complete. Only packets that carry some secret patterns are considered as stateless resets. Quicly allows the peer to share up to 4 such patterns per connection. However, until now, it failed to determine which of the 4 slots that it uses to retain the secret patterns contains a valid entry. As the slots are zero-initialized, the failure meant that, unless the peer advertised 4 of such patterns, an all-zero pattern was treated as a stateless reset.In effect, this allowed an on-path attacker to reset QUIC connections governed by Quicly. This issue has been fixed by commit dccf5d4.
Which versions are affected by CVE-2026-44434?
It affects 1 product.
Products include H2o Quicly.
How do I fix CVE-2026-44434?
No fix has been published for CVE-2026-44434 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-2026-44434 being exploited?
There is no known exploitation of CVE-2026-44434 in the wild and no public exploit code.
Other vulnerabilities in h2o quicly
Sources
Also known as GHSA-899f-49jq-pfh8.