Cloudflare Agents is Vulnerable to Reflected Cross-Site Scripting in the AI Playground's OAuth callback handler
Affected releases · agents
- 0.3.x0.3.0 – 0.3.9Fixed in 0.3.10
- 0.2.x0.2.0 – 0.2.35No fix on this line yet
- 0.1.x and older0.0.1 – 0.1.6No fix, no longer updated
On 0.3.x, the newest line:
npm install agents@0.3.10Affected
0.0.1 – 0.3.9
130 releases
Fixed
0.3.10 – 0.29.0
75 releases, latest
Is your project affected?
Paste your package.json, or type the range you declare for agents. It is read in your browser and sent nowhere.
What happens
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: