{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "extracted_events": [
              {
                "introduced": "0"
              },
              {
                "fixed": "0.23.35"
              },
              {
                "introduced": "1.0.0-M1"
              },
              {
                "fixed": "1.0.0-M46"
              }
            ],
            "source": [
              "AFFECTED_FIELD",
              "REFERENCES"
            ]
          },
          "events": [
            {
              "introduced": "0"
            },
            {
              "introduced": "756ce63aee681c5b632bbeafe0b8fde2f3d237f4"
            },
            {
              "fixed": "a83f5f4f7e14c5b4d4a91dd71b105cbbbb391298"
            },
            {
              "fixed": "c63fbff43dfe7e3bec25b789fd0a0027ec40ed62"
            },
            {
              "fixed": "6e8eccd64a6a74ab4811897881e95e0e1b3a818e"
            },
            {
              "fixed": "6083780a3009bd7b26033b0ceb3d32c658b0377e"
            }
          ],
          "repo": "https://github.com/http4s/http4s",
          "type": "GIT"
        }
      ]
    }
  ],
  "aliases": [
    "GHSA-vmm3-xgcx-67hm"
  ],
  "database_specific": {
    "cna_assigner": "GitHub_M",
    "cwe_ids": [
      "CWE-409"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54556.json"
  },
  "details": "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.",
  "id": "CVE-2026-54556",
  "modified": "2026-08-30T03:30:29.878869238Z",
  "published": "2026-08-26T14:10:22.349Z",
  "references": [
    {
      "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"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54556.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/http4s/http4s/security/advisories/GHSA-vmm3-xgcx-67hm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-54556"
    },
    {
      "type": "FIX",
      "url": "https://github.com/http4s/http4s/commit/6e8eccd64a6a74ab4811897881e95e0e1b3a818e"
    }
  ],
  "schema_version": "1.9.0",
  "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: HTTP/2 Denial of Service with Ember Backend"
}