Vulnerability-Lookup

GHSA-VMM3-XGCX-67HM

Vulnerability from github – Published: 2026-08-26 14:10 – Updated: 2026-08-26 14:10

Summary

http4s has HTTP/2 Denial of Service with Ember Backend

Details

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

Severity

8.2 (High)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.12"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c 1.0.0-M46"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.13"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.0-M1"
            },
            {
              "fixed": "1.0.0-M47"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c 1.0.0-M46"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.0.0-M1"
            },
            {
              "fixed": "1.0.0-M47"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.23.34"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "org.http4s:http4s-ember-core_2.13"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.35"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-54556"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-409"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-26T14:10:59Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\nhttp4s 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.\n\n### Impact\n\nDenial of Service:\n- Affects any http4s server running the ember backend with HTTP/2 enabled that is exposed to untrusted traffic.\n- Affects any http4s client running the ember backend with HTTP/2 enabled that can be directed to an untrusted server.\n\n### Details\n\nThe issue occurs in the [Hpack wrapper](https://github.com/http4s/http4s/blob/c63fbff43dfe7e3bec25b789fd0a0027ec40ed62/ember-core/shared/src/main/scala/org/http4s/ember/core/h2/Hpack.scala#L54). 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:\n\n```\njava.lang.OutOfMemoryError: Java heap space\n        at scala.collection.mutable.ListBuffer.scala$collection$mutable$ListBuffer$$freshFrom(ListBuffer.scala:129)\n        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:147)\n        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:40)\n        at scala.collection.mutable.Growable.$plus$plus$eq(Growable.scala:69)\n        at scala.collection.mutable.Growable.$plus$plus$eq$(Growable.scala:69)\n        at scala.collection.mutable.AbstractBuffer.$plus$plus$eq(Buffer.scala:314)\n        at org.http4s.Header$.org$http4s$Header$ToRaw$$anon$10$$_$$lessinit$greater$$anonfun$2(Header.scala:192)\n        at org.http4s.Header$ToRaw$$anon$10$$Lambda/0x00001fe001239c50.apply(Unknown Source)\n        at scala.collection.immutable.List.foreach(List.scala:334)\n        at org.http4s.Header$ToRaw$$anon$10.\u003cinit\u003e(Header.scala:192)\n        at org.http4s.Header$ToRaw$.scalaCollectionSeqToRaw(Header.scala:195)\n        at org.http4s.Headers$.apply(Headers.scala:220)\n        at org.http4s.ember.core.h2.PseudoHeaders$.headersToRequestNoBody(PseudoHeaders.scala:95)\n        at org.http4s.ember.core.h2.H2Stream.receiveHeaders$$anonfun$1$$anonfun$2$$anonfun$3$$anonfun$2(H2Stream.scala:248)\n        at org.http4s.ember.core.h2.H2Stream$$Lambda/0x00001fe001511fd8.apply(Unknown Source)\n        at cats.effect.IOFiber.runLoop(IOFiber.scala:429)\n        at cats.effect.IOFiber.autoCedeR(IOFiber.scala:1460)\n        at cats.effect.IOFiber.run(IOFiber.scala:129)\n        at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:935)\n```\n\n### Fix\n  \nThe 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.\n\nA 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.\n\n### Workarounds\n\nIf you can\u0027t upgrade immediately:\n- Disable HTTP/2 in Ember backends",
  "id": "GHSA-vmm3-xgcx-67hm",
  "modified": "2026-08-26T14:10:59Z",
  "published": "2026-08-26T14:10:59Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/http4s/http4s"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/releases/tag/v0.23.35"
    },
    {
      "type": "WEB",
      "url": "https://github.com/http4s/http4s/releases/tag/v1.0.0-M47"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "http4s has HTTP/2 Denial of Service with Ember Backend"
}

CVE-2026-54556 (GCVE-0-2026-54556)

Vulnerability from cvelistv5 – Published: 2026-08-26 14:10 – Updated: 2026-08-26 14:10

Title

Http4s: HTTP/2 Denial of Service with Ember Backend

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/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.

Severity

8.2 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CWE

CWE-409 - Improper Handling of Highly Compressed Data (Data Amplification)

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/http4s/http4s/security/advisor…	x_refsource_CONFIRM
https://github.com/http4s/http4s/commit/6e8eccd64…	x_refsource_MISC
https://github.com/http4s/http4s/releases/tag/v0.23.35	x_refsource_MISC
https://github.com/http4s/http4s/releases/tag/v1.…	x_refsource_MISC

Impacted products

4 products

Vendor	Product	Version
http4s	http4s	Affected: < 0.23.35 Affected: >= 1.0.0-M1, < 1.0.0-M46
org.http4s	http4s-ember-core_2.12	Affected: < 0.23.35
org.http4s	http4s-ember-core_2.13	Affected: >= 1.0.0-M1, < 1.0.0-M46 Affected: < 0.23.35
org.http4s	http4s-ember-core_3	Affected: >= 1.0.0-M1, < 1.0.0-M46 Affected: < 0.23.35

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "cna": {
      "affected": [
        {
          "product": "http4s",
          "vendor": "http4s",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.23.35"
            },
            {
              "status": "affected",
              "version": "\u003e= 1.0.0-M1, \u003c 1.0.0-M46"
            }
          ]
        },
        {
          "product": "http4s-ember-core_2.12",
          "vendor": "org.http4s",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.23.35"
            }
          ]
        },
        {
          "product": "http4s-ember-core_2.13",
          "vendor": "org.http4s",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 1.0.0-M1, \u003c 1.0.0-M46"
            },
            {
              "status": "affected",
              "version": "\u003c 0.23.35"
            }
          ]
        },
        {
          "product": "http4s-ember-core_3",
          "vendor": "org.http4s",
          "versions": [
            {
              "status": "affected",
              "version": "\u003e= 1.0.0-M1, \u003c 1.0.0-M46"
            },
            {
              "status": "affected",
              "version": "\u003c 0.23.35"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "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."
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "attackComplexity": "LOW",
            "attackRequirements": "PRESENT",
            "attackVector": "NETWORK",
            "baseScore": 8.2,
            "baseSeverity": "HIGH",
            "privilegesRequired": "NONE",
            "subAvailabilityImpact": "NONE",
            "subConfidentialityImpact": "NONE",
            "subIntegrityImpact": "NONE",
            "userInteraction": "NONE",
            "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
            "version": "4.0",
            "vulnAvailabilityImpact": "HIGH",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "NONE"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-409",
              "description": "CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-26T14:10:22.349Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm"
        },
        {
          "name": "https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e"
        },
        {
          "name": "https://github.com/http4s/http4s/releases/tag/v0.23.35",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/http4s/http4s/releases/tag/v0.23.35"
        },
        {
          "name": "https://github.com/http4s/http4s/releases/tag/v1.0.0-M47",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/http4s/http4s/releases/tag/v1.0.0-M47"
        }
      ],
      "source": {
        "advisory": "GHSA-vmm3-xgcx-67hm",
        "discovery": "UNKNOWN"
      },
      "title": "Http4s: HTTP/2 Denial of Service with Ember Backend"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-54556",
    "datePublished": "2026-08-26T14:10:22.349Z",
    "dateReserved": "2026-06-15T19:04:14.456Z",
    "dateUpdated": "2026-08-26T14:10:22.349Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

GHSA-VMM3-XGCX-67HM

Summary

Impact

Details

Fix

Workarounds

CVE-2026-54556 (GCVE-0-2026-54556)

Tags

Sightings

Nomenclature