Vulnerability in Http4s
CVE-2026-54556
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.
Published · last modified .
EPSS exploit prediction
EPSS: 0.005 (42.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 42.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.005 · change: +0.002
Affected products
- Http4s — versions < 0.23.35, >= 1.0.0-M1, < 1.0.0-M46
- Org.http4s Http4s-Ember-Core 2.12 — versions < 0.23.35
- Org.http4s Http4s-Ember-Core 2.13 — versions < 0.23.35, >= 1.0.0-M1, < 1.0.0-M46
- Org.http4s Http4s-Ember-Core 3 — versions < 0.23.35, >= 1.0.0-M1, < 1.0.0-M46