h2o quicly Flow Control memory allocation

Summaryinfo

A vulnerability, which was classified as problematic, has been found in h2o quicly. Impacted is an unknown function of the component Flow Control. This manipulation causes memory allocation. This vulnerability is handled as CVE-2026-44433. The attack can be initiated remotely. There is not any exploit available.

Detailsinfo

A vulnerability was found in h2o quicly (affected version unknown). It has been declared as problematic. Affected by this vulnerability is an unknown functionality of the component Flow Control. The manipulation with an unknown input leads to a memory allocation vulnerability. The CWE definition for the vulnerability is CWE-789. The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated. As an impact it is known to affect availability. The summary by CVE is:

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 8b178e6, 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. Assuming 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 amount of memory with the peer sending only a handful of packets, resulting in memory exhaustion. In addition to the receive buffer allocation strategy, 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. This issue has been fixed by commit 8b178e6.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-44433 since 05/06/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Neither technical details nor an exploit are publicly available.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• h2o

Name

• quicly

Website

• Product: https://github.com/h2o/quicly/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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