CVE-2026-25640: Pydantic AI has Stored XSS via Path Traversal in Web UI CDN URL

Summary

A Path Traversal vulnerability in the Pydantic AI web UI allows an attacker to serve arbitrary JavaScript in the context of the application by crafting a malicious URL. If a victim clicks the link or visits it via an iframe, attacker-controlled code executes in their browser, enabling theft of chat history and other client-side data.

This vulnerability only affects applications that use:

• `Agent.to_web` to serve a chat interface
• `clai web` to serve a chat interface from the CLI

These are typically run locally (on localhost), but may also be deployed on a remote server.

Description

The web UI serves its frontend HTML by fetching it from a CDN. In affected versions, the CDN URL is constructed using a version query parameter from the request URL. This parameter is not validated, allowing path traversal sequences that cause the server to fetch and serve attacker-controlled HTML/JavaScript from an arbitrary source on the same CDN, instead of the legitimate chat UI package.

Who Is Affected

Projects are affected if your application uses Agent.to_web or clai web to serve the Pydantic AI chat interface.

Attack Scenario

1. An attacker crafts a URL pointing to the victim's Pydantic AI web UI instance (either localhost with the known port, or a remote server endpoint) with a malicious version query parameter containing path traversal sequences.

1. The attacker gets the victim to visit this URL — directly via a link, through a redirect, or by embedding it in an iframe.

1. When the victim's browser loads the page, the server fetches and serves attacker-controlled HTML/JavaScript instead of the legitimate chat UI.

1. The attacker's JavaScript executes in the victim's browser in the context of the Pydantic AI web application, with access to:
2. Chat history stored in localStorage (all user messages and AI responses)
3. Session cookies that are not set as HttpOnly, if authentication middleware is configured

Remediation

Upgrade to Patched Version

Upgrade to the patched version or later. The fix removes the user-controllable version parameter entirely. The CDN URL is now hardcoded at startup and cannot be influenced by request parameters.

A new html_source parameter is available on Agent.to_web and create_web_app for applications that need to customize the UI source (e.g., for enterprise environments, offline usage, or custom UI builds). This parameter is only settable in application code, not via query parameters.