# CVE-2026-59724: Socket.IO Engine.IO denial of service in WebTransport upgrade handling

Source: https://junglewise.ai/threats/cve-2026-59724-socket-io-engine-io-denial-of-service-in-webtransport-upgrade  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-08. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-59724 · Severity: high · CVSS 7.5 · Published 2026-07-08

Engine.IO is a real-time communication library used by Socket.IO applications to power WebSocket and other transport protocols. When WebTransport is enabled (an opt-in feature), an unauthenticated attacker can crash the server by sending a specially crafted request using a prototype pollution technique. Repeated exploitation may cause the service to enter a crash loop, resulting in service unavailability and operational disruption.

Technologies: [engine.io (npm)](https://junglewise.ai/threats/technologies/engine-io). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-59724 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59724 being exploited in the wild.

**How severe is CVE-2026-59724?** High severity, with a CVSS score of 7.5, published 8 July 2026.

**Which products and versions are affected?** engine.io (npm) >= 6.5.0, < 6.6.7, SEMVER:introduced=6.5.0,fixed=6.6.7.

**Is there a fix for CVE-2026-59724?** Yes. engine.io (npm): upgrade to 6.6.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59724 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59724?** Check whether you run engine.io (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- engine.io (npm): upgrade to 6.6.7 or later

  ```sh
  npm install engine.io@6.6.7
  ```


## Technical details

The vulnerability is a prototype pollution issue (CWE-20: Improper Input Validation) in Engine.IO's WebTransport upgrade handler. When processing a WebTransport upgrade request, the code performs a session ID lookup in the clients object without properly verifying that the key is an own property, allowing resolution to inherited prototype properties such as __proto__. This causes a TypeError during the asynchronous upgrade handling that, depending on Node.js configuration, may remain unhandled and terminate the process. The attack requires network access but no authentication or user interaction. The vulnerability affects Engine.IO versions 6.5.0 through 6.6.6; it is fixed in version 6.6.7 and later. WebTransport is disabled by default, limiting scope to deployments that explicitly enable it.

## Affected products

- Socket.IO Engine.IO 6.5.0 through 6.6.6

## Identifiers

- CVE-2026-59724
- GHSA-gr94-w7qr-f4j3

## Timeline

- 2026-06-25: disclosed: GitHub Advisory Database publication
- 2026-04-27: patched: Fix released in engine.io 6.6.7
- 2026-08-31: advisory: GHSA-gr94-w7qr-f4j3 published

## References

- [https://api.github.com/users/ni8walk3r](https://api.github.com/users/ni8walk3r)
- [https://github.com/ni8walk3r](https://github.com/ni8walk3r)
- [https://api.github.com/users/ni8walk3r/gists%7B/gist_id%7D](https://api.github.com/users/ni8walk3r/gists%7B/gist_id%7D)
- [https://api.github.com/users/ni8walk3r/repos](https://api.github.com/users/ni8walk3r/repos)
- [https://avatars.githubusercontent.com/u/50462098?v=4](https://avatars.githubusercontent.com/u/50462098?v=4)
- [https://api.github.com/users/ni8walk3r/events%7B/privacy%7D](https://api.github.com/users/ni8walk3r/events%7B/privacy%7D)

## Related threats

- [Socket.IO Engine.IO protocol revision mismatch DoS](https://junglewise.ai/threats/cve-2026-102599-socket-io-engine-io-protocol-revision-mismatch-dos)
- [Socket.IO Engine.IO connection exhaustion in polling transport](https://junglewise.ai/threats/cve-2026-59725-socket-io-engine-io-connection-exhaustion-in-polling-transport)
- [Socket.IO engine.io uncaught exception denial of service](https://junglewise.ai/threats/cve-2023-31125-socket-io-engine-io-uncaught-exception-denial-of-service)
- [Engine.IO uncaught exception denial of service](https://junglewise.ai/threats/cve-2022-41940-engine-io-uncaught-exception-denial-of-service)
- [Socket.io engine.io resource exhaustion via POST](https://junglewise.ai/threats/cve-2020-36048-socket-io-engine-io-resource-exhaustion-via-post)