CVE-2025-62800: Fastmcp Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mxxr-jv3v-6pgc",
  "modified" : "2025-10-29T15:39:21Z",
  "published" : "2025-10-29T15:38:29Z",
  "aliases" : [ "CVE-2025-62800" ],
  "summary" : "FastMCP vulnerable to reflected XSS in client's callback page",
  "details" : "### Summary\nWhile setting up an oauth client, it was noticed that the callback page hosted by the client during the flow embeds user-controlled content without escaping or sanitizing it. This leads to a reflected Cross-Site-Scripting vulnerability.\n\n### Details\nThe affected code is located in *https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/client/oauth_callback.py*, which embeds all values passed to the `create_callback_html` function via the `message` parameter it into the callback page without escaping them. This can, for example, be abused by calling the callback server with an XSS payload inside the `error` GET parameter, the value of which will then be inserted into the callback page, causing the execution of attacker-controlled JavaScript code in the callback server's origin. Note that besides the `error` parameter, other parameters reaching this function are affected too.\n\n### PoC\n1. Setup a simple fastmcp client such as this one (the callback server's port was fixated for simplicity):\n\n```\nurl=\"http://127.0.0.1:8000/mcp\"\noauth = OAuth(mcp_url=url,callback_port=1337)\n\nasync def main():\n    async with Client(url, auth=oauth) as client:\n        await client.ping()\n        \n        # List available operations\n        tools = await client.list_tools()\n\n        print(f\"tools: {tools}\")\n       \nasyncio.run(main())\n```\n\n2. Ensure that the MCP server located at `http://127.0.0.1:8000/mcp` supports oauth.\n3. Start the client.\n4. As soon as the callback server has been started, access `http://localhost:1337/callback?error=<img/src/onerror=alert(window.origin)>`\n\nNote that the exploitation could also for example be initiated by a malicious authorization server by returning the exploitation URL mentioned before in the `authorization_endpoint` field. The client would then automatically open, causing the XSS to trigger immediatly.\n\n### Impact\nThe impact of this XSS vulnerability is the arbitrary JavaScript execution in the victim's browser in the callback server's origin.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "fastmcp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.13.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-62800"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jlowin/fastmcp/pull/2090"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jlowin/fastmcp/commit/2a20f54617a37213ed83894a8c2f0ac38a2e83a3"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jlowin/fastmcp"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-10-29T15:38:29Z",
    "nvd_published_at" : "2025-10-28T22:15:36Z"
  }
}