{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "mcp-handler"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.1.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-1395",
      "CWE-362"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-01T23:58:50Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "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\u0027s 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",
  "id": "GHSA-w2fm-25vw-vh7f",
  "modified": "2026-04-01T23:58:50Z",
  "published": "2026-04-01T23:58:50Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "mcp-handler has a tool response leak across concurrent client sessions (\u0027Race Condition\u0027)"
}