CVE-2026-59724 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in engine.io. A fix is available for engine.io — see the affected versions and patch details below.
Socket.IO: Engine.IO WebTransport SID DoS
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-59724.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-59724 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 390,400 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
engine.ionpmDescription
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
Patches
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.
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | engine.io | ≥ 6.5.0&&< 6.6.7 | 6.6.7npm install engine.io@6.6.7 |
Affected Products
engine.iosocketDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for engine.io, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update engine.io to 6.6.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59724 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw has been rated as Important. When Engine.IO servers have WebTransport enabled, a remote attacker can trigger a denial of service. This occurs by sending a crafted session ID that resolves an inherited property during WebTransport upgrade handling, leading to a TypeError and impacting service availability.
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Hardened Images | dotnet8-0-main-8.0.128-1.hum1 | RHSA-2026:26994 |
Frequently Asked Questions
Is CVE-2026-59724 in your dependencies?
Find it across npm, including transitive dependencies.