CVE-2026-1721 Reflected Cross-Site Scripting (XSS) vulnerability in AI Playground site
Attackers can execute arbitrary JavaScript via the OAuth callback error parameter, stealing chat history and accessing connected MCP servers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-1721?
CVE-2026-1721 is a medium-severity cross-site scripting vulnerability in agents, published on 13 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
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.
Weakness: CWE-79
Which versions are affected by CVE-2026-1721?
2 open source packages ship an affected version, including agents.
| Package | Affected | Fixed in |
|---|---|---|
agents npm | All versions | 0.3.10 |
agents npm | All versions | 0.3.10 |
How do I fix CVE-2026-1721?
Upgrade agents to 0.3.10 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-1721 being exploited?
There is no known exploitation of CVE-2026-1721 in the wild and no public exploit code.
Sources
Also known as GHSA-cvhv-6xm6-c3v4.