GHSA-w2fm-25vw-vh7f: mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')

April 1, 2026

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.

References

github.com/advisories/GHSA-345p-7cg4-v4c7
github.com/advisories/GHSA-w2fm-25vw-vh7f
github.com/vercel/mcp-handler
github.com/vercel/mcp-handler/security/advisories/GHSA-w2fm-25vw-vh7f
nvd.nist.gov/vuln/detail/CVE-2026-25536

Code Behaviors & Features

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