The vulnerability exists in the Model Context Protocol TypeScript SDK and allows a cross‑client response data leak. When a single McpServer/Server and transport instance is reused for multiple client connections, data returned for one client can be accessed by another client. This flaw manifests due to race conditions and improper isolation of concurrent requests, identified by CWE‑362 (Race Condition) and CWE‑367 (Concurrent Process Data Deletion). The primary impact is a confidentiality breach, exposing sensitive information from one client to others sharing the same server instance. No denial‑of‑service or privileged‑execution capability is granted by the flaw.

Vendor: Model Context Protocol – TypeScript SDK. Versions from 1.10.0 through 1.25.3 are affected. The fix was released in version 1.26.0 and later versions are not affected.

The CVSS base score of 7.1 indicates high severity, while the EPSS score of less than 1% suggests a very low probability of exploitation in the wild. The vulnerability is not listed in the CISA KEV catalog. Attackers can exploit the flaw by intentionally reusing the same server or transport instance across multiple client sessions, which is a common pattern in stateless StreamableHTTPServerTransport deployments. The attack likely requires control over the application code or the ability to influence the client connection lifecycle to orchestrate the reuse of the shared instance. No public exploit has been published, and the flaw is largely a design error rather than an externally triggerable exploit.