PYSEC-2026-2744 
### Summary
Excel file attachments are previewed in an unsafe way. A crafted XLSX file payload can be used to cause the [sheetjs](https://git.sheetjs.com/sheetjs/sheetjs) function [sheet_to_html](https://git.sheetjs.com/sheetjs/sheetjs/src/commit/66cf8d2117d271f89e4f47b5fed35a3e1ea93f67/bits/79_html.js#L127) to embed an XSS payload into the generated HTML. This is subsequently added to the DOM unsanitized via [`@html`](https://svelte.dev/docs/svelte/@html) causing the payload to trigger.

### Details
The function used to convert XLSX documents to HTML for preview does not perform any input validation or sanitisation for the generated HTML
https://github.com/open-webui/open-webui/blob/a7271532f8a38da46785afcaa7e65f9a45e7d753/src/lib/components/common/FileItemModal.svelte#L120-L133
XLSX attachments are processed by this function, converted to HTML with `XLSX.utils.sheet_to_html` before ultimately being assigned to the variable `excelHtml`. Later there is logic that causes this to be assigned directly to the DOM when the preview tab is selected.
https://github.com/open-webui/open-webui/blob/a7271532f8a38da46785afcaa7e65f9a45e7d753/src/lib/components/common/FileItemModal.svelte#L358-L400

### PoC
A python script to generate a payload file is as follows:
```python
import xlsxwriter                                                                                                                
                                                                                                                                 
payload = '<img src=x onerror="alert(\'XSS Triggered by XLSX file\')">'                                                          
                                                                                                                             
workbook = xlsxwriter.Workbook('xss_payload.xlsx')                                                                           
worksheet = workbook.add_worksheet()                                                                                         
                                                                                                                             
payload_format = workbook.add_format()                                                                                       
                                                                                                                             
worksheet.write_rich_string('A1',                                                                                            
    'This cell contains a hidden payload: ',                                                                                 
    payload_format, payload                                                                                                  
)                                                                                                                            
                                                                                                                             
worksheet.write('A2', 'This is a safe cell.')                                                                                
worksheet.write('B1', 'Column B')                                                                                            
                                                                                                                             
workbook.close()
```

Upload the generated file as an attachment to a chat, open the file modal, and click preview. Observe the XSS triggers.
<img width="2444" height="1386" alt="image" src="https://github.com/user-attachments/assets/8400efb0-ea6f-4878-abdb-4c2fe529241f" />
This same process can be triggered in shared chats, allowing the payload to be distributed to victims.
<img width="2386" height="1646" alt="image" src="https://github.com/user-attachments/assets/d0eda49c-8fcf-4fc4-bbb0-c8951b0369c3" />


### Impact
Any user can create a weaponised chat that can be shared and subsequently used to target other users.

Low privilege users are at risk of having their session taken over by a payload that reads their token from local storage and exfiltrates it to an attacker controlled server.

Admins are at risk of exposing the server to RCE via same chain described in GHSA-w7xj-8fx7-wfch.

### Caveats
The file attachment in the shared chat must be opened and previewed to trigger the vulnerability.

### Recommendation
Sanitise the generated HTML with DOMPurify before assigning it to the DOM.
CVSS v3.1 score: 7.3