Open WebUI has stored XSS in Excel file preview

### 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

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-79

Also known as

CVE-2026-44549, PYSEC-2026-2744

• github.com/open-webui/open-webui/security/advisories/GHSA-jwf8-pv5p-vhmc
• nvd.nist.gov/vuln/detail/CVE-2026-44549
• github.com/open-webui/open-webui