Http4s prior 0.23.35/1.0.0-M47 Ember Backend Hpack.scala denial of service

Summaryinfo

A vulnerability labeled as problematic has been found in Http4s. Affected is an unknown function of the file ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala of the component Ember Backend. The manipulation results in denial of service. This vulnerability is known as CVE-2026-54556. It is possible to launch the attack remotely. No exploit is available.

Detailsinfo

A vulnerability was found in Http4s. It has been classified as problematic. Affected is an unknown part of the file ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala of the component Ember Backend. The manipulation with an unknown input leads to a denial of service vulnerability. CWE is classifying the issue as CWE-404. The product does not release or incorrectly releases a resource before it is made available for re-use. This is going to have an impact on availability. CVE summarizes:

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, an unauthenticated HTTP/2 peer can cause an out-of-memory denial of service in the Ember backend with HTTP/2 enabled. The Hpack wrapper in ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala concatenates HEADERS and CONTINUATION frame fragments and decodes them into a single List, but maxHeaderSize accounting does not include indexed headers or HPACK per-header overhead. A small compressed header block can therefore expand into a much larger decoded representation that remains in memory for processing. Servers exposed to untrusted HTTP/2 traffic and clients directed to an untrusted HTTP/2 server are affected, and concurrent malicious connections can exhaust the process heap. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

The advisory is available at github.com. This vulnerability is traded as CVE-2026-54556 since 06/15/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. Technical details are known, but there is no available exploit. This vulnerability is assigned to T1499 by the MITRE ATT&CK project.

Upgrading to version 0.23.35 or 1.0.0-M47 eliminates this vulnerability.

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• Http4s

Website

• Product: https://github.com/http4s/http4s/

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