Advisoriesby IbisOpen 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...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| open-webui PyPI | < 0.9.6 | 0.9.6 |
Details and references
## 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`.
- CVSS 3.1
- CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-79
- Also known as
- CVE-2026-54011, PYSEC-2026-2759
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