CVE-2026-44549: open-webui: XSS via XLSX preview enables session hijack

### 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.

An attacker with a standard Open WebUI user account uses the public Python PoC to generate a weaponized XLSX file containing an `<img src=x onerror=...>` payload that reads the JWT from localStorage and POSTs it to an attacker-controlled server. The attacker creates a shared chat, attaches the file, and distributes the link to high-value targets including admins via internal Slack or email. When an admin clicks Preview in the file modal, the XSS fires silently. The attacker receives the admin session token, replays it against the Open WebUI API, and chains the admin access with GHSA-w7xj-8fx7-wfch to achieve RCE on the server, from which they can exfiltrate API keys for connected LLMs, access RAG database contents, and pivot to internal infrastructure.