Cloudflare Agents has a Reflected Cross-Site Scripting (XSS) vulnerability in AI Playground site

Summary

## Summary A Reflected Cross-Site Scripting (XSS) vulnerability was discovered in the AI Playground's OAuth callback handler. The error_description query parameter was directly interpolated into an HTML script tag without proper escaping, allowing attackers to execute arbitrary JavaScript in the context of the victim's session. ### Root cause The OAuth callback handler in `site/ai-playground/src/server.ts` directly interpolated the `authError` value, sourced from the `error_description` query parameter, into an inline `<script>` tag. ### Impact An attacker could craft a malicious link that, when clicked by a victim, would: - Steal user chat message history - Access all LLM interactions stored in the user's session. - Access connected MCP Servers - Interact with any MCP servers connected to the victim's session (public or authenticated/private), potentially allowing the attacker to perform actions on the victim's behalf ### Mitigation: - PR: https://github.com/cloudflare/agents/pull/841 - Agents-sdk users should upgrade to `[email protected]` - Developers using `configureOAuthCallback` with custom error handling in their own applications should ensure all user-controlled input is escaped before interpolation. ### Credits Disclosed responsibly by Nishant Kumawat