CVE-2026-54556
http4s has HTTP/2 Denial of Service with Ember Backend
Impact
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 packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| maven | org.http4s:http4s-ember-core_2.12 | <= 0.23.34 | 0.23.35 |
| maven | org.http4s:http4s-ember-core_2.13 | >= 1.0.0-M1, < 1.0.0-M46 | 1.0.0-M47 |
| maven | org.http4s:http4s-ember-core_3 | <= 0.23.34 | 0.23.35 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-vmm3-xgcx-67hm
- https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm
- https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e
- https://github.com/http4s/http4s/releases/tag/v0.23.35
- https://github.com/http4s/http4s/releases/tag/v1.0.0-M47