How severe is CVE-2026-54556?

No CVSS score has been assigned to CVE-2026-54556 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-54556?

CVE-2026-54556 affects the following packages: org.http4s:http4s-ember-core_2.12 (Maven), org.http4s:http4s-ember-core_2.13 (Maven), org.http4s:http4s-ember-core_3 (Maven), org.http4s:http4s-ember-core_3 (Maven), org.http4s:http4s-ember-core_2.13 (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-54556?

Update org.http4s:http4s-ember-core_2.12 to 0.23.35 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-54556 is resolved across your whole dependency graph.

How do I detect CVE-2026-54556 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-ember-core_2.12, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-54556 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-54556 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-54556 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-54556?

CVE-2026-54556 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 43th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-54556?

CVE-2026-54556 is classified as CWE-409 (CWE-409). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-54556 published, and has it been updated?

CVE-2026-54556 was published on August 26, 2026 and was last updated on September 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕ Maven

Not in CISA KEV

CVE-2026-54556 — http4s-ember-core_2.12

Fix: http4s/http4s@6e8eccd

CVE-2026-54556 is a CWE-409 vulnerability in org.http4s:http4s-ember-core_2.12. A fix is available for org.http4s:http4s-ember-core_2.12 — see the affected versions and patch details below.

Http4s: HTTP/2 Denial of Service with Ember Backend

Also known asGHSA-vmm3-xgcx-67hm

Published

Aug 26, 2026

Updated

Sep 11, 2026

Affected

5 pkgs

Patched

5 / 5

Exploits

None indexed

Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-54556.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs43th percentile — riskier than 43% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

5 pkgs affected

☕org.http4s:http4s-ember-core_2.12☕org.http4s:http4s-ember-core_2.13☕org.http4s:http4s-ember-core_3☕org.http4s:http4s-ember-core_3☕org.http4s:http4s-ember-core_2.13

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

Summary

http4s 0.23.x and 1.0 servers running ember with http2 enabled are vulnerable to a denial of service attack using a HPACK bomb vulnerability recently disclosed as affecting other http2 servers.

Impact

Denial of Service:

Affects any http4s server running the ember backend with HTTP/2 enabled that is exposed to untrusted traffic.
Affects any http4s client running the ember backend with HTTP/2 enabled that can be directed to an untrusted server.

Details

The issue occurs in the Hpack wrapper. Ember concatenates the header and continuation frames before decoding all headers at once. When the code decodes the attack payload, the relatively small packets decode to a significantly larger amount of data which is returned as a single List which is then held in memory for further processing. With enough concurrent connections (~5 with a 2GB heap in testing) this leads to an OOM, eg:

java.lang.OutOfMemoryError: Java heap space
        at scala.collection.mutable.ListBuffer.scala$collection$mutable$ListBuffer$$freshFrom(ListBuffer.scala:129)
        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:147)
        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:40)
        at scala.collection.mutable.Growable.$plus$plus$eq(Growable.scala:69)
        at scala.collection.mutable.Growable.$plus$plus$eq$(Growable.scala:69)
        at scala.collection.mutable.AbstractBuffer.$plus$plus$eq(Buffer.scala:314)
        at org.http4s.Header$.org$http4s$Header$ToRaw$$anon$10$$_$$lessinit$greater$$anonfun$2(Header.scala:192)
        at org.http4s.Header$ToRaw$$anon$10$$Lambda/0x00001fe001239c50.apply(Unknown Source)
        at scala.collection.immutable.List.foreach(List.scala:334)
        at org.http4s.Header$ToRaw$$anon$10.<init>(Header.scala:192)
        at org.http4s.Header$ToRaw$.scalaCollectionSeqToRaw(Header.scala:195)
        at org.http4s.Headers$.apply(Headers.scala:220)
        at org.http4s.ember.core.h2.PseudoHeaders$.headersToRequestNoBody(PseudoHeaders.scala:95)
        at org.http4s.ember.core.h2.H2Stream.receiveHeaders$$anonfun$1$$anonfun$2$$anonfun$3$$anonfun$2(H2Stream.scala:248)
        at org.http4s.ember.core.h2.H2Stream$$Lambda/0x00001fe001511fd8.apply(Unknown Source)
        at cats.effect.IOFiber.runLoop(IOFiber.scala:429)
        at cats.effect.IOFiber.autoCedeR(IOFiber.scala:1460)
        at cats.effect.IOFiber.run(IOFiber.scala:129)
        at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:935)

Fix

The Hpack handling code has a configurable max header size parameter, but it does not include indexed headers in that accounting, allowing the attack to bypass the limit. There is not any configuration available in current http4s versions that can be enabled to prevent this attack short of disabling http2.

A fix has been tested locally that threads the maxHeaderSize setting from the server builder into the Hpack code. Using this, connections can be prematurely terminated once the decoded data size exceeds the maxHeaderSize that users can already configure.

Workarounds

If you can't upgrade immediately:

Disable HTTP/2 in Ember backends

Affected Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.http4s:http4s-ember-core_2.12`	all versions	0.23.35`org.http4s:http4s-ember-core_2.12:0.23.35`
☕Maven	`org.http4s:http4s-ember-core_2.13`	≥ 1.0.0-M1&&< 1.0.0-M47	1.0.0-M47`org.http4s:http4s-ember-core_2.13:1.0.0-M47`
☕Maven	`org.http4s:http4s-ember-core_3`	all versions	0.23.35`org.http4s:http4s-ember-core_3:0.23.35`
☕Maven	`org.http4s:http4s-ember-core_3`	≥ 1.0.0-M1&&< 1.0.0-M47	1.0.0-M47`org.http4s:http4s-ember-core_3:1.0.0-M47`
☕Maven	`org.http4s:http4s-ember-core_2.13`	all versions	0.23.35`org.http4s:http4s-ember-core_2.13:0.23.35`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-ember-core_2.12, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.http4s:http4s-ember-core_2.12 to 0.23.35 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-54556 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Summary http4s 0.23.x and 1.0 servers running ember with http2 enabled are vulnerable to a denial of service attack using a HPACK bomb vulnerability recently disclosed as affecting other http2 servers. ### Impact Denial of Service: - Affects any http4s server running the ember backend with HTTP/2 enabled that is exposed to untrusted traffic. - Affects any http4s client running the ember backend with HTTP/2 enabled that can be directed to an untrusted server. ### Details The issue occurs in the [Hpack wrapper](https://github.com/http4s/http4s/blob/c63fbff43dfe7e3bec25b789fd0a0027ec40ed

O3 Security · Impact-Aware SCA

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