Cloudflare Public Bug Bounty · Report #3211031

`use-mcp`'s oauth2 process uses a window.open call with untrusted mcp server provided data allowing for code execution under the page using it

Validstatus

mediumseverity

$550bounty

75upvotes

30 Sept 2025disclosed

Summary

The authorizeEndpoint parameter from use-mcp version was susceptible to XSS. Sanitization of that parameter has been added in version 0.0.10 of use-mcp. A skilled attacker was able to turn this XSS into code execution on the client (https://verialabs.com/blog/from-mcp-to-shell/#xss---rce-abusing-mcps-stdio-transport).

The researcher was rewarded 550$ for this finding.

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