CVE-2026-59725: Engine.IO Denial of Service Vulnerability via Invalid Binary POST Requests
Summary
CVE-2026-59725: Socket.IO: Engine.IO Polling Transport Connection Exhaustion. Affects engine.io versions >= 4.1.0, < 6.6.7. Fixed in 6.6.7. How to fix ↓
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Background
Engine.IO is a transport layer for Socket.IO that enables real-time communication between clients and servers. It supports various transport methods, including WebSocket and polling, to ensure reliable message delivery. However, certain versions of Engine.IO are vulnerable to a denial of service (DoS) attack due to improper handling of invalid binary POST requests, which can exhaust server resources.
Vulnerability Detail
The vulnerability arises from an unauthenticated remote attacker being able to open multiple Engine.IO polling sessions and send malformed binary POST requests with the Content-Type set to application/octet-stream. The server fails to properly close the HTTP response associated with these requests, leading to open connections that consume server resources. This can result in denial of service, preventing legitimate clients from connecting as server resources become exhausted.
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
6.6.7
This is the first release that contains the fix.
npm install engine.io@6.6.7Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
If immediate upgrading is not feasible, consider implementing the following workarounds:
- Block or reject polling POST requests with Content-Type: application/octet-stream at a reverse proxy, load balancer, WAF, or application middleware.
- Disable HTTP long-polling if your deployment can use WebSocket-only transport.
- Enforce strict request/connection timeouts at the HTTP server, reverse proxy, or load balancer.
- Apply per-IP rate limits and connection limits for Engine.IO endpoints.
- Restrict access to the Socket.IO/Engine.IO endpoint where feasible.
Example Socket.IO configuration to disable polling:
const io = new Server(httpServer, {
transports: ["websocket"],
});
Conclusion
In summary, CVE-2026-59725 is a denial of service vulnerability in Engine.IO that can be exploited by sending malformed binary POST requests. Users are advised to upgrade to the patched version or implement workarounds to mitigate the risk. Monitoring tools like Vulert can assist in identifying vulnerabilities in your applications.
See every vulnerability recorded for engine.io →
FAQ
The CVSS score of this vulnerability is 7.5, indicating a high severity level.
You can use Vulert, which monitors and alerts for open-source vulnerabilities in real-time. You can check your application using the Vulert playground at Vulert Scanner without requiring signup.
If exploited, this vulnerability can lead to denial of service, preventing legitimate users from connecting to the server due to exhausted resources.
Yes, the issue has been fixed in engine.io version 6.6.7. Users should upgrade to this version or later.
Consider implementing workarounds such as blocking specific POST requests or enforcing strict timeouts and rate limits.
References
Related vulnerabilities
Recent vulnerabilities recorded for engine.io.
