CVE-2026-44433 h2o CVE debrief

Who should care

Users of the Quicly IETF QUIC protocol implementation, particularly those using the H2O HTTP server, should be aware of this vulnerability and take steps to defend against it. This includes reviewing and applying patches, monitoring for suspicious activity, and implementing compensating controls to limit the impact of the vulnerability. System administrators and security teams responsible for the H2O HTTP server and Quicly implementation should prioritize assessment and remediation efforts.

Technical summary

The vulnerability occurs when an adversarial peer sends a STREAM frame carrying just one byte at the largest offset being permitted to obtain additional flow control credit. This could lead to a Denial of Service under certain circumstances. The severity of this vulnerability depends on how the application controls the stream concurrency. In case of the H2O HTTP server, under its default setting, this bug increases the maximum amount of memory allocated per connection by about 4 times. The application prepares a receive buffer for storing all data that arrive out-of-order, up to the largest offset being received.

Defensive priority

Medium

Recommended defensive actions

• Review and apply the patch provided by the vendor
• Monitor for suspicious activity
• Implement compensating controls to limit the impact of the vulnerability
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed

Evidence notes

The CVE record was published on 2026-07-16T23:16:16.727Z and has not been modified since then. The NVD entry is currently 5.3 MEDIUM. The vulnerability details indicate that an adversarial peer could send a STREAM frame carrying just one byte at the largest offset being permitted to obtain additional flow control credit, which under certain circumstances could lead to a Denial of Service. The application prepares a receive buffer for storing all data that arrive out-of-order, up to the largest offset being received. This behavior could lead to the application allocating large amounts of memory with the peer sending only a handful of packets, resulting in memory exhaustion. The severity of this vulnerability depends on how the application controls the stream concurrency.

Official resources