CVE Catalog

CVE-2026-54556

HighCVSS 8.2
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.30%

22th percentile - higher than 22% of all known CVEs

Summary

Http4s before versions 0.23.35 and 1.0.0-M47 has a DoS vulnerability via HTTP/2 in the Ember backend. The Hpack wrapper does not include indexed headers or per-header overhead in the header size limit, allowing a small block to expand into a large in-memory representation. Multiple malicious connections can exhaust process memory.

Risk Assessment

An unauthenticated attacker can cause memory exhaustion and service unavailability, impacting application availability.

Recommendation

Update Http4s to version 0.23.35 or 1.0.0-M47. Limit the number of concurrent HTTP/2 connections to reduce risk.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS