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GHSA-q2xc-rrxj-58x9
pydantic-aipydantic-ai-slimpydantic-aiSummary
The Pydantic AI development web chat UI (Agent.to_web(), clai web) does not validate the Host header of incoming requests. A website a developer visits can use DNS rebinding to make requests to a chat UI running on that developer's machine appear same-origin to the browser, causing the served agent to run and to execute its tools with the privileges and credentials of the local process.
Details
Once a name the attacker controls resolves to the loopback address, the browser treats the request as same-origin, so neither an Origin check nor a CSRF token constrains it — a same-origin page can read the served UI and any token in it.
Binding the web UI to localhost — the default — does not prevent this.
Impact
Applications and developers serving an agent through Agent.to_web() or clai web. The consequences depend on the tools the served agent exposes, and can include data disclosure as well as unwanted tool side effects.
Current browser protections reduce but do not remove this exposure: Chromium's Local Network Access gates loopback subresource requests, but does not cover top-level navigations, and Safari does not implement it.
Mitigation
Upgrade to pydantic-ai/pydantic-ai-slim >= 2.30.0, or >= 1.107.5 on the v1 maintenance line.
The fix validates the Host header and rejects anything other than localhost, a loopback/LAN IP address, or an explicitly allowed host, responding 421 Misdirected Request otherwise. If you serve the web chat UI under a real hostname — behind a reverse proxy, tunnel, or similar — name it explicitly:
app = agent.to_web(allowed_hosts=['ui.example.com'])
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The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The impact is confined to the system where the vulnerability exists. There is a low impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a low impact on the availability of the system.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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