CVE-2026-109290 in Open WebUI
Summary
by MITRE • 10/12/2026
Open WebUI from 0.11.1 before 0.11.4 contains a stored cross-site scripting vulnerability in DocxPreview.svelte that allows authenticated users to execute script by uploading crafted DOCX attachments. Attackers can embed an OOXML altChunk HTML part or script-scheme hyperlink to steal a previewing administrator's session token from local storage and take over the instance.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified in Open WebUI versions prior to 0.11.4 represents a critical stored cross-site scripting flaw located within the DocxPreview.svelte component. This security defect allows authenticated users, including those with administrative privileges, to inject malicious script payloads into documents that are subsequently rendered by other users or system processes. The core of the issue lies in how the application handles and renders Office Open XML (OOXML) files during preview operations. When a user uploads a DOCX file containing specific crafted elements, such as an OOXML altChunk HTML part or a hyperlink utilizing the script scheme, the rendering engine fails to adequately sanitize these inputs before executing them within the browser context. This lack of proper input validation and output encoding enables the execution of arbitrary JavaScript code in the victim's browser session.
From a technical perspective, the exploitation vector relies on the interaction between the frontend Svelte component and the underlying document parsing logic. By embedding an altChunk element that contains HTML content or constructing a hyperlink with a javascript: URI scheme within the DOCX structure, an attacker can bypass standard security controls. When another user, particularly one with elevated privileges such as an administrator, opens this maliciously crafted file for preview, the browser executes the embedded scripts immediately. This execution occurs in the context of the Open WebUI application, granting the injected script full access to the page's Document Object Model and associated session data. The vulnerability is classified under CWE-79, which covers Improper Neutralization of Input During Web Page Generation known as Cross-site Scripting, specifically manifesting as a stored variant where the malicious payload persists on the server or in shared resources until accessed by victims.
The operational impact of this vulnerability is severe due to its potential for account takeover and data exfiltration. Attackers can leverage the executed scripts to access local storage mechanisms used by the application to maintain user sessions. By stealing session tokens stored locally, an attacker can hijack the active session of a privileged user without needing their password or multi-factor authentication credentials. This capability allows the adversary to assume the identity of the victim administrator, thereby gaining unrestricted control over the Open WebUI instance. Such access facilitates unauthorized interactions with connected AI models, potential exfiltration of sensitive conversation history and data processed through the platform, and further lateral movement within the organization's infrastructure if the web interface is integrated with other internal systems. This aligns with MITRE ATT&CK techniques related to Credential Access via Browser Session Hijacking and Defense Evasion by hiding malicious activity within legitimate document workflows.
Mitigation strategies must focus on immediate remediation through software updates and enhanced input validation practices. The primary defense is upgrading Open WebUI to version 0.11.4 or later, where the developers have addressed this specific rendering flaw in the DocxPreview.svelte component. In environments where an upgrade cannot be implemented immediately, administrators should restrict file upload permissions to trusted users only and disable document preview features for untrusted sources if possible. Additionally, implementing strict Content Security Policy headers can help mitigate the impact of any successful script injection by restricting the execution context and preventing unauthorized data exfiltration via local storage access. Regular security audits focusing on how third-party libraries handle OOXML parsing are also recommended to prevent similar vulnerabilities in other document processing components within the application stack.