CVE-2026-1721
Published: February 13, 2026· Updated: Feb 13, 2026
Official Description
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 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.
Technical Analysis
CVE-2026-1721 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.
Exploitation does not require any privileges, though user interaction (A) is needed, which slightly reduces the risk of mass automated attacks.
A proof-of-concept (PoC) exploit exists for CVE-2026-1721. While not yet confirmed in active campaigns, the availability of PoC code increases exploitation risk substantially.
CVSS v3.1 Vector Breakdown
Affected Vendors & Products
Exploit & PoC Resources
All References (1)
Quick Facts
Recommended Actions
- →Apply vendor patches immediately
- →Monitor CVE-2026-1721 in threat intel feeds
- →Review IDS/IPS signatures for exploitation attempts