Vulnerability-Lookup

CVE-2025-62800 (GCVE-0-2025-62800)

Vulnerability from cvelistv5 – Published: 2025-10-28 21:34 – Updated: 2025-10-29 15:52

Title

FastMCP vulnerable to reflected XSS in client's callback page

Summary

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 ?

5.3 (Medium)


                        
                          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

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M

References

URL

Tags

https://github.com/jlowin/fastmcp/security/adviso…

x_refsource_CONFIRM

Impacted products

	Vendor	Product	Version
	jlowin	fastmcp	Affected: < 2.13.0

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2025-62800

Vulnerability from fkie_nvd - Published: 2025-10-28 22:15 - Updated: 2025-11-07 01:49

Severity ?

6.1 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc	Exploit, Third Party Advisory
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/jlowin/fastmcp/security/advisories/GHSA-mxxr-jv3v-6pgc	Exploit, Third Party Advisory

Impacted products

	Vendor	Product	Version
	jlowin	fastmcp	*

JSON

To clipboard

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    }
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  "id": "CVE-2025-62800",
  "lastModified": "2025-11-07T01:49:53.133",
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GHSA-MXXR-JV3V-6PGC

Vulnerability from github – Published: 2025-10-29 15:38 – Updated: 2025-10-29 15:39

Summary

FastMCP vulnerable to reflected XSS in client's callback page

Details

Summary

While 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.

Details

The 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.

PoC

Setup a simple fastmcp client such as this one (the callback server's port was fixated for simplicity):

url="http://127.0.0.1:8000/mcp"
oauth = OAuth(mcp_url=url,callback_port=1337)

async def main():
    async with Client(url, auth=oauth) as client:
        await client.ping()

        # List available operations
        tools = await client.list_tools()

        print(f"tools: {tools}")

asyncio.run(main())

Ensure that the MCP server located at http://127.0.0.1:8000/mcp supports oauth.
Start the client.
As soon as the callback server has been started, access http://localhost:1337/callback?error=<img/src/onerror=alert(window.origin)>

Note 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.

Impact

The impact of this XSS vulnerability is the arbitrary JavaScript execution in the victim's browser in the callback server's origin.

Severity ?

5.3 (Medium)


                  
                    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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fastmcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.13.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-62800"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-10-29T15:38:29Z",
    "nvd_published_at": "2025-10-28T22:15:36Z",
    "severity": "MODERATE"
  },
  "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\u0027s 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\u0027s 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\u0027s browser in the callback server\u0027s origin.",
  "id": "GHSA-mxxr-jv3v-6pgc",
  "modified": "2025-10-29T15:39:21Z",
  "published": "2025-10-29T15:38:29Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "FastMCP vulnerable to reflected XSS in client\u0027s callback page"
}

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2025-62800 (GCVE-0-2025-62800)

FKIE_CVE-2025-62800

GHSA-MXXR-JV3V-6PGC

Summary

Details

PoC

Impact

Tags

Sightings

Nomenclature