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.
Scoring
- Severity
- HIGH
- CVSS base score
- 8.2
- CVSS vector
- CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- EPSS probability
- 0.30%
- CWE
- CWE-409
- Published
- 2026-08-26
- Last modified
- 2026-08-29
Affected products
- http4s http4s
- http4s http4s
- org.http4s http4s-ember-core_2.12
- org.http4s http4s-ember-core_2.13
- org.http4s http4s-ember-core_2.13
- org.http4s http4s-ember-core_3
- org.http4s http4s-ember-core_3
Weakness type
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