Pydantic AI Web chat UI (`Agent.to_web()`, `clai web`): the local chat endpoint does not validate the `Host` header

Pydantic AI is a Python agent framework for building applications and workflows with Generative AI. From 1.34.0 until 2.30.0, the Agent.to_web() and clai web development chat server does not validate the Host header, allowing a website visited by a developer to use DNS rebinding to reach a loopback-hosted agent as a same-origin service. The hostile page can read the served UI and submit chat requests that execute agent tools with the local process's privileges and credentials, causing data disclosure or unwanted side effects. Binding to localhost, Origin checks, and CSRF tokens do not prevent the same-origin DNS rebinding path. This issue is fixed in versions 1.107.5 and 2.30.0.

Source published

8 Oct 2026 16:26:48 UTC

Source modified

14 Oct 2026 01:01:07 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 6.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:H/A:L