{"id":"GHSA-w2fm-25vw-vh7f","aliases":[],"status":"published","title":"mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')","description":"`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","severity":{"version":"3.1","score":7.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N","level":"high","from":"ghsa"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1395","CWE-362"],"published":"2026-04-01T23:58:50Z","modified":"2026-04-02T13:35:40Z","fixes":[{"ecosystem":"npm","package":"mcp-handler","fixed_in":["1.1.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"mcp-handler","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.1.0"}]}]},{"source":"osv","ecosystem":"npm","package":"mcp-handler","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.1.0"}]}]}],"references":[{"url":"https://github.com/vercel/mcp-handler/security/advisories/GHSA-w2fm-25vw-vh7f","tags":["web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-25536","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-345p-7cg4-v4c7","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-w2fm-25vw-vh7f"},{"url":"https://github.com/vercel/mcp-handler","tags":["package"]}],"sources":{"ghsa":[{"id":"GHSA-w2fm-25vw-vh7f","title":"mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')","description":"`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","status":"published","published":"2026-04-01T23:58:50Z","modified":"2026-04-01T23:58:50Z","cvss":[{"version":"3.1","score":7.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N","level":"high"}],"cwes":["CWE-362","CWE-1395"],"updated_at":"2026-11-02T14:39:47.945242Z"}],"osv":[{"id":"GHSA-w2fm-25vw-vh7f","title":"mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')","description":"`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","status":"published","published":"2026-04-01T23:58:50Z","modified":"2026-04-02T13:35:40.55329Z","cvss":[{"version":"3.1","score":7.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N","level":"high"}],"cwes":["CWE-1395","CWE-362"],"updated_at":"2026-11-02T15:46:58.04805Z"}]}}