Summary
Mermaid allows CSS injection applying to sibling elements of the diagram
Mermaid does not fully restrict CSS to the rendered SVG subtree. Although selectors are prefixed with #mermaid-X, sibling (~ and +) combinators can still escape the Mermaid container and inject styles to DOM elements adjacent to the diagram <svg>.
Most users of mermaid would not be affected by this, as mermaid adds its <svg> as an only child of it's parent element. However, you may be affected if you manually insert the <svg> (or other elements) into the DOM yourself.
Details
Mermaid namespaces CSS through with a middleware intended to scope all rules to the diagram's SVG element. CSS nesting expands & ~ * { ... } to #svgId ~ *, which selects all sibling elements following the SVG in the DOM, outside the diagram boundary.
Workarounds
If you are inserting the <svg> into the DOM yourself, you can wrap it in an element with no other children, e.g. <div><svg>...</svg></div> or element.innerHTML = svg. Alternatively, you can use mermaid.run() or mermaid.initialize() which will do this for you.
Setting "securityLevel": "sandbox" will also prevent this, or setting the secure config value in the mermaid config to avoid allowing diagrams to modify fontFamily, themeCSS, altFontFamily, and themeVariables.
To test, you can try using a themeCSS with & + * { /* my CSS here */} and see if it's applied outside of your mermaid <svg>.
---
config:
themeCSS: |-
& + * { background:red !important; width:100vw !important; height:100vh !important; position:fixed !important; inset:0 !important; }
---
info
References
- GHSA-87f9-hvmw-gh4p/CVE-2026-41159 (related vulnerability)
- https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1
- https://github.com/mermaid-js/mermaid/commit/7e83f1533318b307764d961906a73377266f4c5e
- https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8
Impact
An attacker able to supply diagram source to a page (e.g., user-generated content rendered by Mermaid) could inject CSS rules affecting sibling elements to the diagram <svg> on the host page. This can be used for UI redressing, hiding content, conditional CSS-based probing, or phishing-style visual manipulation.
JavaScript execution is not possible via this vector.
Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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This has been patched in https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed and released in Mermaid v11.16.1.
A backport has been made for the v10 branch in 7e83f1533318b307764d961906a73377266f4c5e and was released in Mermaid v10.9.8
Frequently Asked Questions
- What is CVE-2026-50159? CVE-2026-50159 is a medium-severity code injection vulnerability in mermaid (npm), affecting versions >= 11.0.0-alpha.1, < 11.16.1. It is fixed in 11.16.1, 10.9.8. Untrusted input is evaluated as executable code within the application's runtime environment.
- Which versions of mermaid are affected by CVE-2026-50159? mermaid (npm) versions >= 11.0.0-alpha.1, < 11.16.1 is affected.
- Is there a fix for CVE-2026-50159? Yes. CVE-2026-50159 is fixed in 11.16.1, 10.9.8. Upgrade to this version or later.
- Is CVE-2026-50159 exploitable, and should I be worried? Whether CVE-2026-50159 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
- What actually determines whether CVE-2026-50159 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.
- How do I fix CVE-2026-50159?
- Upgrade
mermaidto 11.16.1 or later - Upgrade
mermaidto 10.9.8 or later
- Upgrade