Http4s: HTTP/2 Denial of Service with Ember Backend

Summary

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.

Timeline

1. Aug 26, 2026, 2:10 PM UTCCVE published
2. Aug 29, 2026, 2:38 AM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Aug 29, 2026, 2:38 AM UTCRecord last updated

Affected versions

• http4s http4s

  fixed in 0.23.35; From 1.0.0-M1 · fixed in 1.0.0-M46

• org.http4s http4s-ember-core_2.12

  fixed in 0.23.35

• org.http4s http4s-ember-core_2.13

  From 1.0.0-M1 · fixed in 1.0.0-M46; fixed in 0.23.35

• org.http4s http4s-ember-core_3

  From 1.0.0-M1 · fixed in 1.0.0-M46; fixed in 0.23.35

• org.http4s:http4s-ember-core_2.12Maven

  fixed in 0.23.35

• org.http4s:http4s-ember-core_2.13Maven

  From 1.0.0-M1 · fixed in 1.0.0-M47

• org.http4s:http4s-ember-core_3Maven

  fixed in 0.23.35

• org.http4s:http4s-ember-core_3Maven

  From 1.0.0-M1 · fixed in 1.0.0-M47

• org.http4s:http4s-ember-core_2.13Maven

  fixed in 0.23.35

References 4