High, CVSS 7.1npmAmendedPublished
mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')
GHSA-w2fm-25vw-vh7f
- Affected
- npm mcp-handler < 1.1.0
- Fixed in
- 1.1.0· Vendor fix available
- Issue
- 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 mcp-handler@1.1.0. This release raises the minimum peer…
Recommended action: Upgrade to 1.1.0 or later, then redeploy and verify.A vendor fix does not resolve your finding until you verify it in your own systems.