mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')

mcp-handler versions prior to 1.1.0 accepted @modelcontextprotocol/sdk < 1.26.0 as a peer dependency. That SDK version contains a vulnerability [CVE-2026-25536] that causes concurrent requests from different clients to share server-side state including authentication context and tool execution results when a StreamableHTTPServerTransport instance is reused across requests.

Note: This is not a vulnerability in mcp-handler itself. The root cause is in the peer dependency @modelcontextprotocol/sdk.

Impact

A low-privileged attacker making concurrent requests to an mcp-handler endpoint can read another client's session data, including authentication information and tool execution state. This is a confidentiality breach with potential for limited integrity impact.

Root Cause: CVE-2026-25536 in @modelcontextprotocol/sdk < 1.26.0. The SDK did not prevent reuse of stateless transports across client connections.

Patches

Upgrade to [email protected]. This release raises the minimum peer dependency to @modelcontextprotocol/sdk@>=1.26.0, which contains the fix for CVE-2026-25536.

Workarounds

• Upgrade @modelcontextprotocol/sdk to >=1.26.0 (note: the SDK will throw on transport reuse, which will break mcp-handler < 1.1.0 which effectively forces the upgrade)
• Alternatively, manually create fresh McpServer and transport instances per request in your handler code