CVE-2026-41159

Workarounds

If you can't upgrade mermaid, you can set the secure config value in the mermaid config to avoid allowing diagrams to modify fontFamily, themeCSS, altFontFamily, and themeVariables.

Setting "securityLevel": "sandbox" will also prevent this.

Credits

Reported by @zsxsoft on behalf of @KeenSecurityLab

Mermaid's default configuration allows injecting CSS that applies outside of the Mermaid diagram via the fontFamily, themeCSS, and altFontFamily configuration options.

Live demo: mermaid.live

Example code:

%%{init: {"fontFamily": "x;a{b} :not(&){background:green !important} c{d}"}}%%
flowchart LR
    A --> B

The injected CSS exploits stylis's & (scope reference) handling. :not(&) escapes the #mermaid-xxx automatic scoping, applying styles to all page elements. Global at-rules (@font-face, @keyframes, @counter-style) are also injectable as stylis hoists them to top level.

This allows page defacement and DOM attribute exfiltration via CSS :has() selectors.

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

Kodem intelligence

• v11.15.0 (see 64769738d5b59211e1decb471ffbaca8afec51aa)
• v10.9.6 (see a9d9f0d8eb790349121508688cd338253fd80d76)

1. What is CVE-2026-41159? CVE-2026-41159 is a medium-severity code injection vulnerability in mermaid (npm), affecting versions >= 11.0.0-alpha.1, <= 11.14.0. It is fixed in 11.15.0, 10.9.6. Untrusted input is evaluated as executable code within the application's runtime environment.
2. Which versions of mermaid are affected by CVE-2026-41159? mermaid (npm) versions >= 11.0.0-alpha.1, <= 11.14.0 is affected.
3. Is there a fix for CVE-2026-41159? Yes. CVE-2026-41159 is fixed in 11.15.0, 10.9.6. Upgrade to this version or later.
4. Is CVE-2026-41159 exploitable, and should I be worried? Whether CVE-2026-41159 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-41159 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-41159?
   • Upgrade mermaid to 11.15.0 or later
   • Upgrade mermaid to 10.9.6 or later