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GHSA-gr94-w7qr-f4j3
engine.ioImpact
Engine.IO servers with WebTransport enabled are vulnerable to a remotely triggerable denial of service.
A malicious unauthenticated client can send a crafted WebTransport upgrade request containing a specially chosen session ID, such as __proto__. Because the session ID lookup did not properly verify that the key was an own property of the clients object, the lookup could resolve to an inherited prototype property instead of a valid Engine.IO client.
This can cause a TypeError during WebTransport upgrade handling. Since the failure occurs in an asynchronous context, it may result in an unhandled Promise rejection and terminate the Node.js process on affected Node.js versions/configurations.
Successful exploitation can allow an unauthenticated remote attacker to crash the server process and cause denial of service. Under a process supervisor, repeated exploitation may cause crash loops.
Affected configurations are limited to deployments where WebTransport support is enabled. WebTransport is not enabled by default.
Affected versions:
engine.io >= 6.5.0 < 6.6.7
Patches
The issue was fixed in:
- engine.io 6.6.7
Users should upgrade to engine.io@6.6.7 or later.
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.
Workarounds
If upgrading immediately is not possible, affected users can mitigate the issue by disabling WebTransport support.
WebTransport is opt-in, so servers should ensure that webtransport is not included in the enabled transports list.
For example, use only the default HTTP long-polling and WebSocket transports:
const engine = new Server({
transports: ["polling", "websocket"],
});
Additional operational mitigations may include:
- Restricting access to WebTransport endpoints at the reverse proxy or HTTP/3 layer.
- Disabling HTTP/3/WebTransport support until the patched version can be deployed.
- Running the service under a supervisor as a partial availability mitigation, though this does not prevent repeated crash attempts.
References
- Fix commit: https://github.com/socketio/socket.io/commit/1fa1f46cd420ac5b57bb4c04c959b58f3c79158c
- Engine.IO changelog entry for
6.6.7: https://github.com/socketio/socket.io/blob/main/packages/engine.io/CHANGELOG.md#667-2026-04-27 - Engine.IO package: https://www.npmjs.com/package/engine.io
- Socket.IO repository: https://github.com/socketio/socket.io
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The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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