CVE-2026-25536 is a concurrency-related vulnerability classified under CWE-362 (Race Condition) found in the Model Context Protocol (MCP) TypeScript SDK, specifically versions from 1.10.0 up to but not including 1.26.0. The MCP TypeScript SDK is used to implement servers and clients for the Model Context Protocol, facilitating communication in distributed systems. The vulnerability occurs when a single McpServer/Server and transport instance is reused across multiple client connections, a common pattern in stateless StreamableHTTPServerTransport deployments. Due to improper synchronization, concurrent execution leads to cross-client response data leakage, where response data intended for one client can be accessed by another, violating confidentiality. The flaw does not require user interaction and can be exploited remotely with low privileges, as indicated by the CVSS vector (AV:N/AC:L/PR:L/UI:N). The impact on integrity is limited, and availability is unaffected. The vulnerability was publicly disclosed and assigned CVE-2026-25536 with a CVSS v3.1 score of 7.1 (high severity). No known exploits are currently reported in the wild. The issue has been addressed in MCP TypeScript SDK version 1.26.0 by implementing proper synchronization mechanisms to isolate client responses and prevent data leakage.

For European organizations, the primary impact of CVE-2026-25536 is the potential unauthorized disclosure of sensitive data across client boundaries in applications using the affected MCP TypeScript SDK versions. This can lead to breaches of confidentiality, regulatory non-compliance (e.g., GDPR), and erosion of customer trust. The vulnerability is particularly critical in multi-tenant environments or services handling sensitive or personal data where multiple clients share the same server and transport instances. Although the integrity and availability impacts are limited, the confidentiality breach alone can result in significant legal and financial consequences. Organizations relying on stateless StreamableHTTPServerTransport deployments or reusing server/transport instances without proper isolation are at higher risk. The lack of known exploits in the wild suggests limited immediate threat but does not preclude future exploitation, especially as the vulnerability is publicly known and patched. European sectors such as finance, healthcare, and government using MCP-based solutions should prioritize remediation to avoid data leaks.

1. Upgrade all MCP TypeScript SDK instances to version 1.26.0 or later, where the race condition has been fixed. 2. Audit existing deployments to identify any reuse of McpServer/Server and transport instances across multiple client connections, especially in stateless StreamableHTTPServerTransport setups. 3. Refactor server implementations to ensure that each client connection uses isolated server and transport instances or that proper synchronization mechanisms are in place to prevent concurrent data leakage. 4. Implement strict access controls and monitoring to detect anomalous cross-client data access patterns. 5. Conduct thorough testing in staging environments to verify that no cross-client data leakage occurs post-mitigation. 6. Educate development and operations teams about concurrency risks and secure coding practices related to shared resource management in TypeScript SDKs. 7. Review and update incident response plans to include scenarios involving data leakage due to race conditions.