{"id":"CVE-2025-62800","aliases":["EUVD-2025-36568","GHSA-mxxr-jv3v-6pgc","PYSEC-2026-1364","ROOT-APP-PYPI-CVE-2025-62800"],"status":"published","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.","severity":{"version":"4.0","score":5.3,"vector":"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","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00276,"percentile":0.20281,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-79"],"published":"2025-10-28T21:34:40Z","modified":"2026-10-28T02:30:53Z","fixes":[{"ecosystem":"PyPI","package":"fastmcp","fixed_in":["2.13.0"],"sources":["ghsa","osv"]},{"ecosystem":"Root:PyPI","package":"rootio-fastmcp","fixed_in":["2.11.3+root.io.4","2.11.1+root.io.2","2.12.4+root.io.4","2.11.3+root.io.5","2.11.1+root.io.3","2.12.4+root.io.5"],"sources":["osv"]},{"ecosystem":"Root:PyPI","package":"fastmcp","fixed_in":["2.11.3+aikido.5","2.11.1+aikido.3","2.12.4+aikido.5"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"fastmcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.13.0"}]}]},{"source":"osv","ecosystem":"PyPI","package":"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"]},{"source":"osv","ecosystem":"PyPI","package":"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"]},{"source":"osv","ecosystem":"Root:PyPI","package":"rootio-fastmcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.11.3+root.io.4"},{"fixed":"2.11.1+root.io.2"},{"fixed":"2.12.4+root.io.4"},{"fixed":"2.11.3+root.io.5"},{"fixed":"2.11.1+root.io.3"},{"fixed":"2.12.4+root.io.5"}]}]},{"source":"osv","ecosystem":"Root:PyPI","package":"fastmcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.11.3+aikido.5"},{"fixed":"2.11.1+aikido.3"},{"fixed":"2.12.4+aikido.5"}]}]}],"products":[{"vendor":"jlowin","product":"fastmcp"}],"references":[{"url":"https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc","tags":["x_refsource_CONFIRM","exploit","Exploit","Third Party Advisory","advisory","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2025-62800","tags":["advisory"]},{"url":"https://github.com/jlowin/fastmcp/pull/2090","tags":["web"]},{"url":"https://github.com/jlowin/fastmcp/commit/2a20f54617a37213ed83894a8c2f0ac38a2e83a3","tags":["web"]},{"url":"https://github.com/advisories/GHSA-mxxr-jv3v-6pgc","tags":["advisory"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/62xxx/CVE-2025-62800.json","tags":["advisory"]},{"url":"https://github.com/jlowin/fastmcp","tags":["package"]},{"url":"https://pypi.org/project/fastmcp","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2025-62800","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.","status":"published","published":"2025-10-28T21:34:40.392Z","modified":"2025-10-29T15:52:44.433Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium","assessor":"cna"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:32:54.87586Z"}],"euvd":[{"id":"EUVD-2025-36568","description":"FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.","status":"published","published":"2025-10-28T21:34:40Z","modified":"2025-10-29T15:52:44Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium"}],"updated_at":"2026-11-02T19:49:36.236088Z"}],"ghsa":[{"id":"GHSA-mxxr-jv3v-6pgc","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"### 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=\u003cimg/src/onerror=alert(window.origin)\u003e`\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.","status":"published","published":"2025-10-29T15:38:29Z","modified":"2025-10-29T15:39:21Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:39:04.350802Z"}],"nvd":[{"id":"CVE-2025-62800","description":"FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.","status":"published","published":"2025-10-28T22:15:36.983Z","modified":"2026-10-08T11:10:00.25Z","cvss":[{"version":"4.0","score":5.3,"vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium","assessor":"secondary"},{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","level":"medium","assessor":"primary"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T14:11:40.754193Z"}],"osv":[{"id":"CVE-2025-62800","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"FastMCP is the standard framework for building MCP applications. Versions prior to 2.13.0 have a reflected cross-site scripting vulnerability in the OAuth client callback page (oauth_callback.py) where unescaped user-controlled values are inserted into the generated HTML, allowing arbitrary JavaScript execution in the callback server origin. The issue is fixed in version 2.13.0.","status":"published","published":"2025-10-28T21:34:40.392Z","modified":"2026-10-28T02:30:53.577174768Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T15:44:51.851511Z"},{"id":"GHSA-mxxr-jv3v-6pgc","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"### 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=\u003cimg/src/onerror=alert(window.origin)\u003e`\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.","status":"published","published":"2025-10-29T15:38:29Z","modified":"2026-07-07T17:56:05.411756133Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium"}],"cwes":["CWE-79"],"updated_at":"2026-11-02T15:46:52.534897Z"},{"id":"PYSEC-2026-1364","title":"FastMCP vulnerable to reflected XSS in client's callback page","description":"### 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=\u003cimg/src/onerror=alert(window.origin)\u003e`\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.","status":"published","published":"2026-07-07T16:03:08.608105Z","modified":"2026-07-07T17:47:12.914598102Z","cvss":[{"version":"4.0","score":5.3,"vector":"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","level":"medium"}],"updated_at":"2026-11-02T15:52:54.622769Z"},{"id":"ROOT-APP-PYPI-CVE-2025-62800","title":"CVE-2025-62800 in rootio-fastmcp - Patched by Root","description":"Root has patched CVE-2025-62800 in the rootio-fastmcp package for Root:PyPI. Multiple fixed versions available.","status":"published","published":"2026-06-03T14:52:53Z","modified":"2026-06-03T19:30:04.763188177Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","level":"medium"}],"updated_at":"2026-11-02T15:53:27.940619Z"}]}}