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

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 agents@0.3.10
• 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