ENGINE.IO · CVE-2026-102599 · Package
FIX → 6.6.10
Engine.IO and Socket.IO servers do not check that an upgrade request uses the same protocol revision as the existing session. A client that opens a valid session and then sends an upgrade with a different or missing EIO parameter can make the server crash its Node.js process.
first public Sep 14, 2026 · 3 sources
Could this affect your app?
Free first check from your website. Connect your repo for the full answer.
Servers that run Engine.IO or Socket.IO on Node.js with the default Engine.IO v4 protocol and transport upgrades enabled are exposed. The advisory says the bug triggers even when v3 compatibility is disabled. Applications that do not use Engine.IO or Socket.IO, or that only accept a fixed transport and never upgrade, can skip it.
Not affected if allowupgrades-false, no-socket.io, websocket-only-transport
Who cares engine.io, node.js, socket.io, websocket
npm ls engine.io in the project and check whether any copy resolves to a version from 6.6.0 up to but not including 6.6.10.engine.io entries in that same range, since Socket.IO packages pull it in as a dependency.Upgrade to 6.6.10
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, < 6.6.10The issue was fixed in:
Users should upgrade to engine.io@6.6.10 or later.
Check it with your coding agent
It reads this page and tells you if your code is affected.
Are you affected?
Ask from here. InfraStack checks the affected versions against your stack and answers in your inbox.
InfraStack checks the affected versions against your stack and answers in Slack, with the evidence and the fix.
engine.io to 6.6.10 or later, per the advisory.allowUpgrades: false or restrict transports to ["websocket"], which may affect client compatibility.If using Socket.IO packages that depend on Engine.IO, users should update to a Socket.IO release that includes the patched Engine.IO version.
If upgrading is not immediately possible, users can reduce exposure by disabling transport upgrades:
const io = new Server(httpServer, {
allowUpgrades: false
});
or by allowing only a single transport, for example WebSocket only:
const io = new Server(httpServer, {
transports: ["websocket"]
});
These mitigations avoid the vulnerable upgrade path, but they may affect client compatibility and connection behavior.
As 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.
Upgrading remains the recommended fix.
This page as .md · .json · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub Advisory Database, CC-BY 4.0 · NVD data is provided by NIST and is public domain. Posts quoted belong to their authors.