Http4s Ember HTTP/2: unbounded outbound frame queue

published 2026-09-15 54 d ago modified 2026-09-17 52 d ago also GHSA-8f3q-3jmv-7prw , EUVD-2026-79183

cvss: 7.5; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.6%; EPSS 0.6% chance of exploitation in 30 days, percentile 50%
public exploits: 0; none known

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember HTTP/2 serializes outbound frames through one unbounded queue consumed by writeLoop. When the peer stops reading, an unauthenticated HTTP/2 client can continue sending PING, SETTINGS, or DATA frames that cause Ember to enqueue acknowledgments or WINDOW_UPDATE frames faster than the writer drains them, exhausting heap memory on a server built with withHttp2. The shared behavior also affects an ember-client connected to a hostile HTTP/2 server, and the patch replaces the unbounded path with bounded, backpressured outbound queues. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Patch / fix

fixed in 3 links 3

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

github.com/http4s/http4s@13fe24d commit
github.com/http4s/http4s/releases/tag/v0.23.35 release
github.com/http4s/http4s/releases/tag/v1.0.0-M47 release

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 52 d ago (2026-09-17)

Exploit prediction

0.63% chance of exploitation in the next 30 days, higher than 50% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem	package	affected	fixed in
maven	org.http4s:http4s-ember-core_2.12	<0.23.35	0.23.35
maven	org.http4s:http4s-ember-core_2.13	<0.23.35 >=1.0.0-M1, <1.0.0-M47	0.23.35 1.0.0-M47
maven	org.http4s:http4s-ember-core_3	<0.23.35 >=1.0.0-M1, <1.0.0-M47	0.23.35 1.0.0-M47

Affected products

http4s http4s

Classification

Weakness

CWE-400 Uncontrolled Resource Consumption
CWE-770 Allocation of Resources Without Limits or Throttling

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary