CVE-2026-102599: Engine.IO Denial-of-Service Vulnerability in Socket.IO
Summary
CVE-2026-102599: Socket.IO: Engine.IO Protocol Revision Mismatch DoS. Affects engine.io versions >= 6.6.0, < 6.6.10. Fixed in 6.6.10. How to fix ↓
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Background
Engine.IO is a transport layer for Socket.IO, a popular library used for real-time web applications. It enables communication between clients and servers using various transport mechanisms, including WebSocket. The library is widely used for its ability to provide real-time bidirectional communication. However, a vulnerability exists in Engine.IO that allows for denial-of-service attacks due to improper handling of protocol revisions during transport upgrades.
Vulnerability Detail
A denial-of-service vulnerability exists in Engine.IO and Socket.IO servers that allow transport upgrades. During the initial handshake, the Engine.IO protocol revision is negotiated and stored in the session. However, a newly-created transport, such as a WebSocket upgrade, could derive a different protocol revision from the upgrade request query parameters. The server does not verify that the protocol revision of an upgrade request matches the existing session. A malicious client can exploit this by establishing a valid Engine.IO session and sending an upgrade request with a different or omitted 'EIO' query parameter. This can lead to the server attaching a transport with inconsistent parser and heartbeat behavior, potentially causing an uncaught exception that crashes the Node.js process. The vulnerability affects Engine.IO versions from 6.6.0 to 6.6.9, leading to a denial of service through process crash.
How To Fix
Latest published version: 6.6.11 as of 23 Oct 2026
npm install engine.io@6.6.11Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
6.6.10
This is the first release that contains the fix.
npm install engine.io@6.6.10Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
If immediate upgrading is not feasible, you can reduce exposure by disabling transport upgrades with the following code:
const io = new Server(httpServer, { allowUpgrades: false });
Alternatively, you can restrict to a single transport, such as WebSocket, using:
const io = new Server(httpServer, { transports: ['websocket'] });
These mitigations may affect client compatibility and connection behavior.
Conclusion
CVE-2026-102599 is a significant denial-of-service vulnerability in Engine.IO that can lead to application crashes. Users are strongly advised to upgrade to the patched version 6.6.10 or later to mitigate the risk. Workarounds are available for those unable to upgrade immediately, but upgrading remains the best solution.
See every vulnerability recorded for engine.io →
FAQ
The impact is a denial of service through process crash, which can disrupt the availability of applications using affected versions of Engine.IO.
Affected versions are Engine.IO versions >= 6.6.0 and < 6.6.10.
You can use Vulert, which monitors and alerts for open-source vulnerabilities in real-time. Check your application using the Vulert playground at Vulert Scanner.
You can disable transport upgrades or restrict to a single transport to mitigate the risk until you can upgrade.
The CVSS score for this vulnerability is 7.5, indicating a high severity level.
References
Related vulnerabilities
Recent vulnerabilities recorded for engine.io.
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