The Model Context Protocol TypeScript SDK is vulnerable to a cross-client data leak that allows authenticated users to access information belonging to other client sessions.

This vulnerability is a race condition (CWE-362) occurring when a single McpServer or transport instance is reused across multiple client connections. An attacker with low-level privileges can exploit this improper synchronization to intercept response data intended for other clients, particularly in stateless StreamableHTTPServerTransport deployments.

The potential for unauthorized disclosure of sensitive cross-client data poses a significant risk to data confidentiality and regulatory compliance. Given the CVSS score of 7.1, this vulnerability is classified as High severity, as it allows for the leakage of potentially proprietary or sensitive information between independent system users.

Immediate Action: Update the @modelcontextprotocol/sdk package to version 1.26.0 or later to resolve the underlying synchronization flaw.

Proactive Monitoring: Review server logs for unusual patterns in client connection handling or anomalous response data delivery that may indicate session state leakage.

Compensating Controls: If immediate patching is not feasible, restrict the reuse of McpServer and transport instances across different client connections, or deploy in a stateful manner that isolates client contexts.

Public Exploit Available: Unknown

The risk of data leakage between clients necessitates prompt action, particularly for organizations deploying Model Context Protocol servers in multi-tenant or shared-instance environments. Security teams should prioritize updating the SDK to version 1.26.0 immediately to eliminate the possibility of unauthorized cross-client data exposure.