CVE-2026-54556
Fix: http4s/http4s@6e8eccdCVE-2026-54556 is a CWE-409 vulnerability in org.http4s:http4s-ember-core_2.12. O3 Security confirms whether CVE-2026-54556 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
http4s has HTTP/2 Denial of Service with Ember Backend
Real-World Exposure
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_2.13Real-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4s:http4s-ember-core_2.12 | all versions | 0.23.35 |
| ☕Maven | org.http4s:http4s-ember-core_2.13 | ≥ 1.0.0-M1&&< 1.0.0-M47 | 1.0.0-M47 |
| ☕Maven | org.http4s:http4s-ember-core_3 | all versions | 0.23.35 |
| ☕Maven | org.http4s:http4s-ember-core_3 | ≥ 1.0.0-M1&&< 1.0.0-M47 | 1.0.0-M47 |
| ☕Maven | org.http4s:http4s-ember-core_2.13 | all versions | 0.23.35 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4s:http4s-ember-core_2.12. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-54556 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-54556. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-54556 in your dependencies?
O3 detects CVE-2026-54556 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.