CVE-2026-25536 modelcontextprotocol CVE debrief

Who should care

Developers and administrators using the MCP TypeScript SDK in their applications should be aware of this vulnerability. Specifically, those who have implemented or are planning to implement the MCP TypeScript SDK in versions 1.10.0 to 1.25.3 should take immediate action to update to version 1.26.0 or later. Additionally, security teams responsible for monitoring and patching vulnerabilities in their organization's software stack should prioritize this CVE.

Technical summary

The MCP TypeScript SDK, used for Model Context Protocol servers and clients, has a vulnerability that allows cross-client response data leaks. This occurs when a single McpServer/Server and transport instance is reused across multiple client connections. The issue is particularly relevant in stateless StreamableHTTPServerTransport deployments. The vulnerability is caused by the improper handling of client connections, leading to potential data exposure. The CVSS vector for this vulnerability is CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N, indicating a high level of severity. The vulnerability has been addressed in version 1.26.0 of the MCP TypeScript SDK.

Defensive priority

This vulnerability should be prioritized for immediate attention due to its high severity and potential impact on data confidentiality. Affected users should update to version 1.26.0 or later as soon as possible.

Recommended defensive actions

• Update to MCP TypeScript SDK version 1.26.0 or later
• Review and update affected deployments to prevent reuse of McpServer/Server and transport instances across multiple client connections
• Monitor for any suspicious activity related to cross-client data leaks
• Implement additional security measures to protect sensitive data in transit
• Verify that all client connections are properly isolated and secured

Evidence notes

The CVE-2026-25536 vulnerability was publicly disclosed on February 4, 2026, and has since been modified on June 30, 2026. The vulnerability affects MCP TypeScript SDK versions 1.10.0 to 1.25.3. The issue has been patched in version 1.26.0. Multiple sources, including GitHub and Red Hat, have provided advisories and patches for this vulnerability.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.