CVE-2026-69218: Http4s Ember HTTP/2 Unbounded Continuation Frame Accumulation Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-cp4q-fqw9-4hf6",
  "modified" : "2026-09-15T20:01:08Z",
  "published" : "2026-09-15T20:01:08Z",
  "aliases" : [ "CVE-2026-69218" ],
  "summary" : "Http4s Ember HTTP/2: unbounded continuation frame accumulation",
  "details" : "When Ember receives an HTTP/2 `HEADERS` or `PUSH_PROMISE` frame without the `END_HEADERS` flag, it buffers the header block fragment and waits for subsequent `CONTINUATION` frames.  These accumulate unbounded until the connection closes. \n\n### Impact\n\nA remote, unauthenticated peer can exhaust the heap on any Ember endpoint that has HTTP/2 enabled:\n\n- **ember-server with `.withHttp2`**: any HTTP/2 client can trigger this against any reachable path (including paths that return 404).  No authentication is required because the attack completes before the request is decoded.\n- **ember-client with `.withHttp2`**:  a malicious or compromised origin server can trigger this via the response header block.  A single in-flight request is sufficient.\n\nMemory consumption is bounded only by the attacker's upload bandwidth and the connection lifetime.\n\n### Prerequisites\n\n- `EmberServerBuilder` configured with `.withHttp2`.\n    - For the server: the attacker can establish an HTTP/2 connection\n    - For the client: the application makes a request to an attacker-controlled origin.\nHTTP/2 incoming headers exceeding the configured size limit are buffer in order to generate an informative HTTP 413 response. If a client does not stop sending headers, this leads to memory exhaustion.\n\n### Patches\n\nThe fix bounds the accumulated header-block size at `SETTINGS_MAX_HEADER_LIST_SIZE` (derived from `EmberServerBuilder.maxHeaderSize` / `EmberClientBuilder.maxResponseHeaderSize`).  When a `CONTINUATION` frame would push the accumulated block over that limit, the connection is terminated with `GOAWAY`.  The `receiveHeadersTimeout` additionally bounds how long an incomplete header block may remain open.\n\n### Workarounds\n\n- Disable HTTP/2 (`.withHttp2`) until upgraded (default).  It is off by default.\n- If HTTP/2 must remain enabled, place ember behind a reverse proxy that terminates HTTP/2 and speaks HTTP/1.1 to ember.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4s:http4s-ember-core_2.12"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.23.35"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.23.34"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4s:http4s-ember-core_2.13"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.23.35"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.23.34"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4s:http4s-ember-core_3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.23.35"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.23.34"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4s:http4s-ember-core_2.13"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0-M1"
      }, {
        "fixed" : "1.0.0-M47"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.0.0-M46"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4s:http4s-ember-core_3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0-M1"
      }, {
        "fixed" : "1.0.0-M47"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.0.0-M46"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/http4s/http4s/security/advisories/GHSA-cp4q-fqw9-4hf6"
  }, {
    "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-15T20:01:08Z",
    "nvd_published_at" : null
  }
}