---
id: CVE-2026-54556
title: Http4s is a Scala interface for HTTP services
summary: >-
  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/s…
severity: high
cwe:
  - CWE-409
vendor: http4s
product: 'org.http4s:http4s-ember-core_2.12'
affected:
  - 'org.http4s:http4s-ember-core_2.12 <= 0.23.34'
  - 'org.http4s:http4s-ember-core_2.13 >= 1.0.0-M1, < 1.0.0-M46'
  - 'org.http4s:http4s-ember-core_3 <= 0.23.34'
  - 'org.http4s:http4s-ember-core_3 >= 1.0.0-M1, < 1.0.0-M46'
  - 'org.http4s:http4s-ember-core_2.13 <= 0.23.34'
patched:
  - 'org.http4s:http4s-ember-core_2.12 0.23.35'
  - 'org.http4s:http4s-ember-core_2.13 1.0.0-M47'
  - 'org.http4s:http4s-ember-core_3 0.23.35'
  - 'org.http4s:http4s-ember-core_3 1.0.0-M47'
  - 'org.http4s:http4s-ember-core_2.13 0.23.35'
published: '2026-08-26'
updated: '2026-09-09'
sourceUpdated: '2026-09-09T21:09:13.080'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-54556'
references:
  - url: >-
      https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/http4s/releases/tag/v0.23.35'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/http4s/releases/tag/v1.0.0-M47'
    label: security-advisories@github.com
  - url: 'https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm'
    label: security-advisories@github.com
  - url: 'https://github.com/advisories/GHSA-vmm3-xgcx-67hm'
tags:
  - nvd
  - ghsa
  - maven
epss: 0.00302
epssPercentile: 0.23096
aliases:
  - GHSA-vmm3-xgcx-67hm
ecosystem: maven
ingestedAt: '2026-08-26T14:45:08.505Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Package advisory (CVE-2026-54556)

Affected packages:

- `org.http4s:http4s-ember-core_2.12 <= 0.23.34`
- `org.http4s:http4s-ember-core_2.13 >= 1.0.0-M1, < 1.0.0-M46`
- `org.http4s:http4s-ember-core_3 <= 0.23.34`
- `org.http4s:http4s-ember-core_3 >= 1.0.0-M1, < 1.0.0-M46`
- `org.http4s:http4s-ember-core_2.13 <= 0.23.34`

Patched in:

- `org.http4s:http4s-ember-core_2.12 0.23.35`
- `org.http4s:http4s-ember-core_2.13 1.0.0-M47`
- `org.http4s:http4s-ember-core_3 0.23.35`
- `org.http4s:http4s-ember-core_3 1.0.0-M47`
- `org.http4s:http4s-ember-core_2.13 0.23.35`

Source: https://github.com/advisories/GHSA-vmm3-xgcx-67hm