id: PYSEC-2026-1364
published: "2026-07-07T16:03:08.608105Z"
modified: "2026-07-07T17:24:07.802569Z"
aliases:
- CVE-2025-62800
- GHSA-mxxr-jv3v-6pgc
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."
affected:
- package:
    name: fastmcp
    ecosystem: PyPI
    purl: pkg:pypi/fastmcp
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 2.13.0
  versions:
  - 0.1.0
  - 0.2.0
  - 0.3.0
  - 0.3.1
  - 0.3.2
  - 0.3.3
  - 0.3.4
  - 0.3.5
  - 0.4.0
  - 0.4.1
  - "1.0"
  - 2.0.0
  - 2.1.0
  - 2.1.1
  - 2.1.2
  - 2.10.0
  - 2.10.1
  - 2.10.2
  - 2.10.3
  - 2.10.4
  - 2.10.5
  - 2.10.6
  - 2.11.0
  - 2.11.1
  - 2.11.2
  - 2.11.3
  - 2.12.0
  - 2.12.0rc1
  - 2.12.1
  - 2.12.2
  - 2.12.3
  - 2.12.4
  - 2.12.5
  - 2.13.0rc1
  - 2.13.0rc2
  - 2.13.0rc3
  - 2.2.0
  - 2.2.1
  - 2.2.10
  - 2.2.2
  - 2.2.3
  - 2.2.4
  - 2.2.5
  - 2.2.6
  - 2.2.7
  - 2.2.8
  - 2.2.9
  - 2.3.0
  - 2.3.0rc1
  - 2.3.1
  - 2.3.2
  - 2.3.3
  - 2.3.4
  - 2.3.5
  - 2.4.0
  - 2.5.0
  - 2.5.1
  - 2.5.2
  - 2.6.0
  - 2.6.1
  - 2.7.0
  - 2.7.1
  - 2.8.0
  - 2.8.1
  - 2.9.0
  - 2.9.1
  - 2.9.2
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
- type: PACKAGE
  url: https://pypi.org/project/fastmcp
- type: ADVISORY
  url: https://github.com/advisories/GHSA-mxxr-jv3v-6pgc
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