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Socket.IO: Engine.IO Protocol Revision Mismatch DoS

Type: advisorySource: OSV1 source record

### Impact A denial-of-service vulnerability exists in Engine.IO / Socket.IO servers that allow transport upgrades. The 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. A 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. Servers 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. The impact is **denial of service** through process crash. Affected versions: - `engine.io >= 6.6.0, < 6.6.10` ## Patches The issue was fixed in: - **engine.io 6.6.10** Users should upgrade to `engine.io@6.6.10` or later. If using Socket.IO packages that depend on Engine.IO, users should update to a Socket.IO release that in

Announced Source revision 1
Original source ↗

GHSA-2gc4-cqfq-p2gv

Primary source · Revision 1

Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.

Security context

Source reference: osv:GHSA-2gc4-cqfq-p2gv

engine.io

Source reference: /affected/0/package/name

Affected versions
Not stated in this record
Fixed versions
6.6.10

Source reference: /affected/0/ranges

Prerequisites
Not stated in this record

Structured affected-version data

Original package identities and version rules from this source.

[
  {
    "ranges": [
      {
        "type": "SEMVER",
        "events": [
          {
            "introduced": "6.6.0"
          },
          {
            "fixed": "6.6.10"
          }
        ]
      }
    ],
    "package": {
      "name": "engine.io",
      "purl": "pkg:npm/engine.io",
      "ecosystem": "npm"
    },
    "database_specific": {
      "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-2gc4-cqfq-p2gv/GHSA-2gc4-cqfq-p2gv.json"
    }
  }
]
Related CVEs
CVE-2026-102599

Source reference: /aliases

Publisher severity

high

Source reference: /database_specific/severity

Linked CVE context

These ratings describe the referenced vulnerabilities and products. They are not a severity rating for every application using this dependency.

CVE-2026-102599 · High

socket.io

Event
#33266 · Revision 1
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Publisher date
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