High 8.2Published Aug 26, 2026

CVE-2026-54556Http4s: HTTP/2 Denial of Service with Ember Backend

Updated under an hour ago (2026-10-19 08:50 UTC)

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It affects http4s and 3 other products. Add what you run to find out whether that includes you.

No exploitation reported and no EPSS estimate yet.

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Every CVE for http4s

Exploited in the last 30 days

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.

CVSS: 8.2 HIGH (v4.0, CNA)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
Weakness (CWE): CWE-409
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: GitHub_M
NVD analysis: Deferred
Record last updated: Aug 29, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

http4s http4s http4s:http4s (from the name)	before 0.23.35; from 1.0.0-M1, < 1.0.0-M46	CNA record
org.http4s http4s-ember-core_2.12 org.http4s:http4s_ember_core_2.12 (from the name)	before 0.23.35	CNA record
org.http4s http4s-ember-core_2.13 org.http4s:http4s_ember_core_2.13 (from the name)	from 1.0.0-M1, < 1.0.0-M46; before 0.23.35	CNA record
org.http4s http4s-ember-core_3 org.http4s:http4s_ember_core_3 (from the name)	from 1.0.0-M1, < 1.0.0-M46; before 0.23.35	CNA record

References

https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hmx-refsource-confirm
https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818ex-refsource-misc
https://github.com/http4s/http4s/releases/tag/v0.23.35x-refsource-misc
https://github.com/http4s/http4s/releases/tag/v1.0.0-M47x-refsource-misc

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.