CVE-2026-54556: Http4s: HTTP/2 Denial of Service with Ember Backend

Severity: high (CVSS 8.2 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-26 · updated 2026-09-09
Weakness: CWE-409

Description

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.

Affected products

org.http4s http4s-ember-core_2.12: before 0.23.35
org.http4s http4s-ember-core_2.13: from 1.0.0-M1, < 1.0.0-M46; before 0.23.35
org.http4s http4s-ember-core_3: from 1.0.0-M1, < 1.0.0-M46; before 0.23.35
http4s: before 0.23.35; from 1.0.0-M1, < 1.0.0-M46

References

Get alerted when CVE-2026-54556 or the next vulnerability affects your stack