CVE-2026-25536 is a concurrency vulnerability classified under CWE-362 (Race Condition) affecting the Model Context Protocol (MCP) TypeScript SDK versions from 1.10.0 up to but not including 1.26.0. The issue occurs when a single McpServer/Server instance and its transport layer are reused across multiple client connections without proper synchronization mechanisms. This is particularly prevalent in stateless StreamableHTTPServerTransport deployments where server and transport instances are shared to handle multiple clients concurrently. Due to improper locking or synchronization, concurrent requests can interfere with each other, leading to cross-client response data leakage. This means that data intended for one client could be inadvertently sent to another, violating confidentiality guarantees. The vulnerability can be exploited remotely over the network with low privileges and does not require user interaction, increasing the risk profile. The flaw was identified and patched in version 1.26.0 of the SDK. No known exploits have been reported in the wild yet, but the nature of the vulnerability makes it a significant concern for multi-tenant server environments or any deployment where server instances are reused across clients. The CVSS v3.1 base score is 7.1, reflecting a high severity due to the high confidentiality impact and ease of exploitation. The integrity impact is low since the vulnerability primarily leaks data rather than modifying it, and availability is unaffected. The vulnerability highlights the importance of proper concurrency control and synchronization in server software handling multiple clients.

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 violations), reputational damage, and potential legal consequences. Organizations deploying multi-tenant services, cloud-based platforms, or microservices architectures that reuse server and transport instances are particularly vulnerable. The data leak could expose personal data, intellectual property, or other sensitive information to unintended parties. Although no integrity or availability impacts are noted, the confidentiality breach alone is significant, especially in sectors handling sensitive data such as finance, healthcare, and government. The ease of remote exploitation without user interaction increases the urgency for European entities to address this vulnerability promptly. Additionally, the lack of known exploits in the wild suggests a window of opportunity for proactive mitigation before attackers potentially develop exploit code.

1. Upgrade all MCP TypeScript SDK instances to version 1.26.0 or later, where the race condition vulnerability has been patched. 2. Audit server and transport instance usage to avoid sharing a single McpServer/Server and transport instance across multiple client connections, especially in stateless StreamableHTTPServerTransport deployments. 3. Implement strict synchronization mechanisms such as mutexes or locks around shared resources to prevent concurrent access issues if upgrading is not immediately feasible. 4. Conduct thorough code reviews and concurrency testing to detect similar race conditions in custom implementations or integrations. 5. Monitor network traffic and logs for unusual cross-client data patterns that might indicate exploitation attempts. 6. Enforce least privilege principles for accounts interacting with the MCP SDK to limit potential damage. 7. Educate developers and DevOps teams about concurrency risks and secure coding practices related to shared resource management. 8. Consider deploying runtime application self-protection (RASP) or Web Application Firewalls (WAF) that can detect anomalous request patterns indicative of exploitation attempts.