CVE-2026-102599: Engine.io Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2gc4-cqfq-p2gv",
  "modified" : "2026-09-29T23:44:33Z",
  "published" : "2026-09-29T23:44:33Z",
  "aliases" : [ "CVE-2026-102599" ],
  "summary" : "Socket.IO: Engine.IO Protocol Revision Mismatch DoS",
  "details" : "### Impact\n\nA denial-of-service vulnerability exists in Engine.IO / Socket.IO servers that allow transport upgrades.\n\nThe Engine.IO protocol revision is negotiated during the initial handshake and stored on the session, but a newly-created transport, including a WebSocket upgrade transport, could independently derive a different protocol revision from the upgrade request query parameters. The server did not verify that the protocol revision of an upgrade request matched the protocol revision of the existing session.\n\nA malicious client could exploit this mismatch by establishing a valid Engine.IO session and then sending an upgrade request with a different, or omitted, `EIO` query parameter. This could cause the server to attach a transport using a parser and heartbeat behavior inconsistent with the session. Under some conditions, a crafted heartbeat packet could trigger an uncaught exception and terminate the Node.js process.\n\nServers using the default Engine.IO v4 protocol are impacted. The issue can be triggered even when Engine.IO v3 compatibility is disabled, because an omitted `EIO` parameter is interpreted as protocol v3 on the affected transport path.\n\nThe impact is **denial of service** through process crash.\n\nAffected versions:\n\n- `engine.io >= 6.6.0, < 6.6.10`\n\n## Patches\n\nThe issue was fixed in:\n\n- **engine.io 6.6.10**\n\nUsers should upgrade to `engine.io@6.6.10` or later.\n\nIf using Socket.IO packages that depend on Engine.IO, users should update to a Socket.IO release that includes the patched Engine.IO version.\n\n### Workarounds\n\nIf upgrading is not immediately possible, users can reduce exposure by disabling transport upgrades:\n\n```javascript\nconst io = new Server(httpServer, {\n  allowUpgrades: false\n});\n```\n\nor by allowing only a single transport, for example WebSocket only:\n\n```javascript\nconst io = new Server(httpServer, {\n  transports: [\"websocket\"]\n});\n```\n\nThese mitigations avoid the vulnerable upgrade path, but they may affect client compatibility and connection behavior.\n\nAs an additional temporary mitigation, deployments may reject Engine.IO requests for an existing `sid` when the `EIO` query parameter is missing or does not match the protocol revision used during the initial handshake. This is best done at the application proxy or middleware layer only if the deployment can reliably track the session’s negotiated protocol.\n\nUpgrading remains the recommended fix.",
  "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" : "npm",
      "name" : "engine.io"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.6.0"
      }, {
        "fixed" : "6.6.10"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/socketio/socket.io/security/advisories/GHSA-2gc4-cqfq-p2gv"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/socketio/socket.io/commit/86db1fc3db2cd315a065d64c4e48918ccf9b4729"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/socketio/socket.io"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/socketio/socket.io/releases/tag/engine.io@6.6.10"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-248" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-29T23:44:33Z",
    "nvd_published_at" : null
  }
}