Summary
Socket.IO: Engine.IO Protocol Revision Mismatch DoS
Workarounds
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.
Impact
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, < 6.6.10
CVE-2026-102599 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 (6.6.10); 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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The issue was fixed in:
- engine.io 6.6.10
Users should upgrade to [email protected] 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.
Frequently Asked Questions
- What is CVE-2026-102599? CVE-2026-102599 is a high-severity security vulnerability in engine.io (npm), affecting versions >= 6.6.0, < 6.6.10. It is fixed in 6.6.10.
- How severe is CVE-2026-102599? CVE-2026-102599 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-2026-102599? engine.io (npm) versions >= 6.6.0, < 6.6.10 is affected.
- Is there a fix for CVE-2026-102599? Yes. CVE-2026-102599 is fixed in 6.6.10. Upgrade to this version or later.
- Is CVE-2026-102599 exploitable, and should I be worried? Whether CVE-2026-102599 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-2026-102599 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-2026-102599? Upgrade
engine.ioto 6.6.10 or later.