Summary
Socket.IO: Engine.IO Polling Transport Connection Exhaustion
Workarounds
If upgrading immediately is not possible, possible mitigations include:
- Block or reject polling
POSTrequests withContent-Type: application/octet-streamfor Engine.IO protocol v4 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, if compatible with your clients:
const io = new Server(httpServer, {
transports: ["websocket"],
});
References
- Fix commit: https://github.com/socketio/socket.io/commit/fc11285e14964c2132d122164bf130c355f60671
- 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 - socket.io repository: https://github.com/socketio/socket.io
- engine.io package: https://www.npmjs.com/package/engine.io
Impact
An unauthenticated remote attacker can cause a denial of service in affected versions of engine.io by opening Engine.IO polling sessions and sending an invalid binary POST request with:
Content-Type: application/octet-stream
against an Engine.IO protocol v4 polling transport.
In the vulnerable code path, the server reports a transport error but does not properly close the HTTP response associated with the malformed request. As a result, the underlying HTTP connection may remain open, consuming one server-side socket/resource per crafted request.
An attacker can repeat this with many sessions to exhaust available HTTP connections, sockets, file descriptors, or related server resources, potentially preventing legitimate clients from connecting.
CVE-2026-59725 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.6.7); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
The issue was fixed in:
- engine.io
6.6.7
The fix ensures that invalid binary polling POST requests are explicitly rejected with an HTTP response and closed properly.
Users should upgrade to:
npm install engine.io@^6.6.7
or a later fixed version.
If using Socket.IO through the monorepo/packages, update to a Socket.IO release that depends on a fixed engine.io version.
Frequently Asked Questions
- What is CVE-2026-59725? CVE-2026-59725 is a high-severity security vulnerability in engine.io (npm), affecting versions >= 4.1.0, < 6.6.7. It is fixed in 6.6.7.
- How severe is CVE-2026-59725? CVE-2026-59725 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of engine.io are affected by CVE-2026-59725? engine.io (npm) versions >= 4.1.0, < 6.6.7 is affected.
- Is there a fix for CVE-2026-59725? Yes. CVE-2026-59725 is fixed in 6.6.7. Upgrade to this version or later.
- Is CVE-2026-59725 exploitable, and should I be worried? Whether CVE-2026-59725 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-59725 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-59725? Upgrade
engine.ioto 6.6.7 or later.