CVE-2026-54556

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.

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.

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

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.

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.

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