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CVE-2025-62800 — FastMCP vulnerable to reflected XSS in client's callback page

Affected: jlowin fastmcp

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.

GHSA-mxxr-jv3v-6pgc — FastMCP vulnerable to reflected XSS in client's callback page

Affected: PyPI: fastmcp

### 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 1. 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()) ```