{"id":"GHSA-w2fm-25vw-vh7f","summary":"mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')","details":"`mcp-handler` versions prior to 1.1.0 accepted `@modelcontextprotocol/sdk` \u003c 1.26.0 as a peer dependency. That SDK version contains a vulnerability [[CVE-2026-25536](https://nvd.nist.gov/vuln/detail/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.\n\n**Note:** This is _not_ a vulnerability in `mcp-handler` itself. The root cause is in the peer dependency `@modelcontextprotocol/sdk`. \n\n### Impact\n\nA 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.\n\n**Root Cause:** [CVE-2026-25536](https://nvd.nist.gov/vuln/detail/CVE-2026-25536) in `@modelcontextprotocol/sdk` \u003c 1.26.0. The SDK did not prevent reuse of stateless transports across client connections.\n\n### Patches\n\nUpgrade to `mcp-handler@1.1.0`. This release raises the minimum peer dependency to `@modelcontextprotocol/sdk@\u003e=1.26.0`, which contains the fix for CVE-2026-25536. \n\n### Workarounds\n\n- Upgrade `@modelcontextprotocol/sdk` to `\u003e=1.26.0` (note: the SDK will throw on transport reuse, which will break `mcp-handler` \u003c 1.1.0 which effectively forces the upgrade)\n- Alternatively, manually create fresh `McpServer` and transport instances per request in your handler code","modified":"2026-04-02T13:35:40.553290Z","published":"2026-04-01T23:58:50Z","database_specific":{"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-04-01T23:58:50Z","nvd_published_at":null,"cwe_ids":["CWE-1395","CWE-362"]},"references":[{"type":"WEB","url":"https://github.com/vercel/mcp-handler/security/advisories/GHSA-w2fm-25vw-vh7f"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-25536"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-345p-7cg4-v4c7"},{"type":"PACKAGE","url":"https://github.com/vercel/mcp-handler"}],"affected":[{"package":{"name":"mcp-handler","ecosystem":"npm","purl":"pkg:npm/mcp-handler"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.1.0"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-w2fm-25vw-vh7f/GHSA-w2fm-25vw-vh7f.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N"}]}