CVE-2026-54011: Open WebUI: Stored XSS in Mermaid Markdown Preview
Summary
Open WebUI renders Mermaid blocks from Markdown files in the file preview panel and inserts the generated SVG into the DOM using innerHTML.
Because Mermaid is configured with securityLevel: 'loose', attacker-controlled Mermaid content can be rendered unsafely in this flow. A working payload was validated through the Markdown preview path, resulting in JavaScript execution in the victim’s browser under the application origin.
This is a confirmed stored XSS vulnerability reachable through normal product functionality.
Affected Version
- main - Reproduced on v0.8.12
Affected Code
Mermaid is initialized in permissive mode:
https://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/src/lib/utils/index.ts#L1698 The file preview path renders Mermaid output and injects the returned SVG into the DOM:
https://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/src/lib/components/chat/FileNav/FilePreview.svelte#L133
Impact
A successful exploit allows JavaScript execution in the victim’s browser under the Open WebUI origin when a malicious Markdown file is opened in the preview panel.
PoC
A malicious .md file containing the follwowing contents can be used to trigger the bug: mermaid flowchart LR A[click me] click A href "javascript:alert(document.domain)" "x" Steps to reproduce: 1- Create a new chat 2- Enable Code Interpreter and browse and upload the file with .md extension. <img width="331" height="258" alt="image" src="https://github.com/user-attachments/assets/bce2b754-56d1-4da1-90a9-22bcb93269f2" /> 3- Clicking on the file, and clicking click me should pop an alert <img width="1103" height="485" alt="image" src="https://github.com/user-attachments/assets/18754486-799b-434e-a2fc-dd7c09956a29" />
Remediation
Since mermaid has DOMPurify as a built-in, it is recommended to use the strict mode instead of loose.
Other sources
Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.6,Open WebUI renders Mermaid blocks from Markdown files in the file preview panel and inserts the generated SVG into the DOM using innerHTML. Because Mermaid is configured with securityLevel: 'loose', attacker-controlled Mermaid content can be rendered unsafely in this flow. A working payload was validated through the Markdown preview path, resulting in JavaScript execution in the victim’s browser under the application origin. This vulnerability is fixed in 0.9.6.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/open-webuito a version that resolves this vulnerability.Fixed in 0.9.6 - Upgrade
Upgrade
Open WebUIto a version that resolves this vulnerability.Fixed in 0.9.6 - Configuration
In Open WebUI’s Mermaid configuration (mermaid blocks rendered in the file preview panel), set `securityLevel` from `loose` to `strict` to avoid unsafe rendering; the material notes Open WebUI previously used `securityLevel: 'loose'` and this is recommended to be changed to `strict`.
Mermaid (Open WebUI Markdown preview rendering) securityLevel = strict
Event History
Frequently Asked Questions
What is the severity of CVE-2026-54011?
CVE-2026-54011 has a severity rating of 8.7, which is classified as high.
How does CVE-2026-54011 affect the Open WebUI software?
CVE-2026-54011 allows attacker-controlled Mermaid content to be rendered unsafely in the Open WebUI due to improper handling of SVG in the file preview panel.
What types of vulnerabilities are associated with CVE-2026-54011?
CVE-2026-54011 is associated with cross-site scripting (XSS) vulnerabilities.
How can I mitigate CVE-2026-54011?
Mitigation for CVE-2026-54011 involves adjusting the security configuration of Mermaid in Open WebUI to a stricter security level.
Is there an official advisory for CVE-2026-54011?
Yes, there is an official advisory regarding CVE-2026-54011 available on GitHub.