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EUVD-2026-337453

fkie_cve-2026-25640

Pydantic AI is a Python agent framework for building applications and workflows with Generative AI. From 1.34.0 to before 1.51.0, 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. 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. 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 and 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. This vulnerability is fixed in 1.51.0.

GHSA-wjp5-868j-wqv7 — Pydantic AI has Stored XSS via Path Traversal in Web UI CDN URL

Affected: PyPI: pydantic-ai, PyPI: pydantic-ai-slim

## Summary

A Path Traversal vulnerability in the [Pydantic AI web UI](https://ai.pydantic.dev/web/) 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.

2. The attacker gets the victim to visit this URL —…

PYSEC-2026-2979 — Pydantic AI has Stored XSS via Path Traversal in Web UI CDN URL

Affected: PyPI: pydantic-ai