The vulnerability is a stored cross‑site scripting flaw located in the DocxPreview.svelte file, which is triggered when an authenticated user uploads a specially crafted DOCX attachment. The script can be embedded using an OOXML altChunk HTML part or a script‑scheme hyperlink. Once a privileged administrator previews the file, the malicious script runs in the administrator’s browser context, capturing the session token stored in local storage and allowing the attacker to hijack the administrator session and fully control the Open WebUI instance.

This issue affects the Open WebUI product from the open-webui vendor, versions 0.11.1 up to, but not including, 0.11.4. Any installation running one of those releases is impacted.

The vulnerability carries a CVSS score of 8.4, placing it in the High severity range. The EPSS score is < 1%, indicating a very low but nonzero exploitation probability, and the flaw is not currently listed in the CISA KEV catalog. Exploitation requires that the attacker is logged in with upload privileges and that a privileged administrator later previews the malicious attachment. If these conditions are met, the attacker can achieve full control of the instance through session hijacking.