CVE-2026-76893

The first client can monopolize tsParticles MCP sessions

tsParticles · @tsparticles/mcp-server HTTP session transport

First documented disclosure · Review checkpoint

Summary

One anonymous client can claim the server’s only HTTP session and block new clients.

Severity and exploitation evidence

Medium · 6.9/10 · CVSS 4.0. Severity source.

Exploitation not established: The researcher reproduced the second-client failure in three controlled trials. No malicious exploitation was reported.

Exploitation evidence source

Technical severity is not a measure of observed attacks or of risk to your deployment. Attempts, demonstrations, and successful compromise retain the qualifications in the source.

How the weakness works

The endpoint reused a single stateful MCP transport. Once initialized, that transport rejected independent initialization requests.

Conditions required

Network access to the HTTP endpoint and the ability to initialize the first session. The stdio transport does not use this shared HTTP session.

Potential impact

The first session continues working, while later clients cannot start their own session until the process restarts.

Affected versions and configurations

Versions before 4.3.2. Researchers tested 4.3.0 and identified the same pattern in 4.3.1.

Fix and mitigation status

4.3.2. Its tagged HTTP server allocates separate session entries. Package-registry metadata confirms the 4.3.2 package.

Practical next steps

Deploy a supported release containing the session fix. Test two clients independently, including cleanup when one disconnects.

Operating lesson

Give each client its own transport and bound the number and lifetime of active sessions.

Evidence and limits

The July 10 fix and disclosure predate the November 4 CVE. 4.3.1 addressed different HTTP denial-of-service paths and was still affected by this one.

Sources