vulnerablemcp.infoadvisory

MCP TypeScript SDK Cross-Client Data Leak (CVE-2026-25536)

CVE-2026-25536 is a cross-client data leak in the official MCP TypeScript SDK (v1.10.0-1.25.3, CVSS 7.1): when a single McpServer instance reuses a StreamableHTTPServerTransport across multiple concurrent client connections, a race condition in the response multiplexing logic can route one client's tool results, resource content, or errors to a different client. This breaks tenant isolation in multi-user MCP server deployments, a common production pattern, potentially exposing sensitive API responses, file contents, or database query results across security boundaries. Fixed in SDK v1.26.0; recommended mitigations include upgrading, using per-client McpServer instances, and adding client-side response correlation checks.