Summary
Socket.IO: Engine.IO WebTransport SID DoS
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
Impact
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
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-59724 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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The issue was fixed in:
- engine.io 6.6.7
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-2026-59724? CVE-2026-59724 is a high-severity improper input validation vulnerability in engine.io (npm), affecting versions >= 6.5.0, < 6.6.7. It is fixed in 6.6.7. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-59724? CVE-2026-59724 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-2026-59724? engine.io (npm) versions >= 6.5.0, < 6.6.7 is affected.
- Is there a fix for CVE-2026-59724? Yes. CVE-2026-59724 is fixed in 6.6.7. Upgrade to this version or later.
- Is CVE-2026-59724 exploitable, and should I be worried? Whether CVE-2026-59724 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-2026-59724 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-2026-59724? Upgrade
engine.ioto 6.6.7 or later.