Cloudflare Public Bug Bounty · Report #3424998

AI Playground XSS to steal user-chat messages and access to connected MCP Server

Validstatus

lowseverity

62upvotes

26 Feb 2026disclosed

Summary

Reflected XSS was found in the AI Playground OAuth handler (CVE-2026-1721) due to unescaped interpolation of the error_description parameter into a <script> tag by the researcher. A targeted attacker could execute JavaScript to access session-based chat history or interact with connected MCP servers on the victim's behalf during the user's session following a user interaction to click a phishing link. This issue is patched now and users of the open-source Agents SDK should upgrade to v0.3.10

Disclosed responsibly by Nishant Kumawat via Cloudflare Bug Bounty program on Hackerone

Full Writeup: https://kazama.in/ai-playground-xss-to-mcp-takeover

How it was found

The write-up for this report has not been downloaded yet, or the program disclosed it without one. Read it on HackerOne.

This report was disclosed publicly on HackerOne and belongs to its author and the program. See the original