CVE-2026-109300 in Open WebUI
Summary
by MITRE • 10/12/2026
Open WebUI from 0.6.31 before 0.11.4 contains a stored cross-site scripting vulnerability in its Markdown renderer, which treats user-authored tool-call details blocks as tool output and mounts their embeds attribute in script-enabled iframes. Authenticated users can post crafted channel messages, chat messages, or citation content that runs JavaScript in viewers' browsers, enabling in-app phishing, or session hijacking where iframeSandboxAllowSameOrigin is enabled.
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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 significant security flaw within the application's Markdown rendering engine. This stored cross-site scripting (XSS) issue arises from an improper neutralization of special elements or values, specifically concerning how user-authored content is processed and displayed. The core technical failure lies in the renderer's logic for handling tool-call details blocks. Instead of treating these blocks strictly as metadata or descriptive text about a tool invocation, the system incorrectly interprets them as direct tool output. This misclassification allows an attacker to inject malicious payloads into fields that are subsequently rendered without adequate sanitization, particularly when specific attributes such as embeds are involved.
The operational mechanism of this exploit relies on the interaction between user input and iframe embedding capabilities. When a message containing crafted content is posted by an authenticated user, the Markdown renderer processes it and generates HTML output. Crucially, if the configuration includes settings that allow for sandboxed iframes with same-origin permissions enabled, such as iframeSandboxAllowSameOrigin being set to true, the malicious script embedded within the tool-call details block can execute in a context that retains significant privileges relative to the parent page. This execution environment allows the injected JavaScript to interact directly with the DOM of the viewing application, bypassing typical cross-site scripting protections that might otherwise restrict access to cookies or session tokens if stricter sandbox policies were enforced.
The impact of this vulnerability is severe for any instance where authenticated users can post messages, chat entries, or citation content. An attacker who has obtained valid credentials can craft a message containing the malicious payload and distribute it within channels or chats. When other users view this content, their browsers will execute the embedded JavaScript code. This capability enables sophisticated in-app phishing attacks, where the script could mimic legitimate UI elements to trick users into revealing sensitive information such as passwords or API keys. Furthermore, if session management is not sufficiently isolated from the iframe context due to the same-origin policy being allowed, attackers can perform session hijacking by stealing authentication tokens stored in cookies or local storage, thereby gaining unauthorized access to victim accounts without needing their credentials directly.
From a classification perspective, this vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. The specific technique of using iframes and sandbox attributes relates closely to CWE-1021, specifically regarding improper restriction of rendered UI elements or frames. In terms of the MITRE ATT&CK framework, this exploit maps to Tactic TA0005 Defense Evasion via Obfuscated Payloads and potentially TA0006 Credential Access if session tokens are harvested through techniques like Client-side Scripting (T1059). The stored nature of the XSS means that the payload persists on the server side until removed, increasing the blast radius as every user viewing the compromised content becomes a potential victim.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary and most effective mitigation is to upgrade Open WebUI to version 0.11.4 or later, where this rendering logic has been corrected to properly sanitize tool-call details blocks and prevent their execution as active script content. For environments that cannot immediately patch due to operational constraints, administrators should review the configuration settings related to iframe sandboxing. Specifically, disabling the iframeSandboxAllowSameOrigin option can significantly reduce the risk by ensuring that any embedded iframes operate under a strict same-origin policy that prevents access to parent window objects and cookies. Additionally, implementing Content Security Policy headers with restrictive directives for script sources can provide an additional layer of defense, although this is secondary to fixing the root cause in the application code. Regular security audits focusing on input validation and output encoding practices are recommended to prevent similar vulnerabilities from emerging in other parts of the Markdown processing pipeline.