GHSA-6X64-9X62-F2GX
Vulnerability from github – Published: 2026-08-06 19:55 – Updated: 2026-08-06 19:55Summary
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.
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.
Patches
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
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
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "mermaid"
},
"ranges": [
{
"events": [
{
"introduced": "11.0.0-alpha.1"
},
{
"fixed": "11.16.1"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "mermaid"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "10.9.8"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-50159"
],
"database_specific": {
"cwe_ids": [
"CWE-94"
],
"github_reviewed": true,
"github_reviewed_at": "2026-08-06T19:55:31Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\n\nMermaid 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 `\u003csvg\u003e`.\n\n**Most users of mermaid would not be affected by this**, as mermaid adds its `\u003csvg\u003e` as an only child of it\u0027s parent element. However, you may be affected if you manually insert the `\u003csvg\u003e` (or other elements) into the DOM yourself.\n\n### Details\n\nMermaid namespaces CSS through with a middleware intended to scope all rules to the diagram\u0027s SVG element. CSS nesting expands `\u0026 ~ * { ... }` to `#svgId ~ *`, which selects all sibling elements following the SVG in the DOM, outside the diagram boundary.\n\n### Impact\n\nAn 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 `\u003csvg\u003e` on the host page. This can be used for UI redressing, hiding content, conditional CSS-based probing, or phishing-style visual manipulation.\n\nJavaScript execution is not possible via this vector.\n\n### Patches\n\nThis has been patched in https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed and released in [Mermaid v11.16.1](https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1).\n\nA backport has been made for the v10 branch in 7e83f1533318b307764d961906a73377266f4c5e and was released in [Mermaid v10.9.8](https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8)\n\n### Workarounds\n\nIf you are inserting the `\u003csvg\u003e` into the DOM yourself, you can wrap it in an element with no other children, e.g. `\u003cdiv\u003e\u003csvg\u003e...\u003c/svg\u003e\u003c/div\u003e` or `element.innerHTML = svg`. Alternatively, you can use `mermaid.run()` or `mermaid.initialize()` which will do this for you. \n\nSetting [\"securityLevel\": \"sandbox\"](https://mermaid.js.org/config/schema-docs/config.html#securitylevel) will also prevent this, or setting the [`secure`](https://mermaid.js.org/config/schema-docs/config.html#secure) config value in the mermaid config to avoid allowing diagrams to modify `fontFamily`, `themeCSS`, `altFontFamily`, and `themeVariables`.\n\nTo test, you can try using a `themeCSS` with `\u0026 + * { /* my CSS here */}` and see if it\u0027s applied outside of your mermaid `\u003csvg\u003e`.\n\n```mermaid-example\n---\nconfig:\n themeCSS: |-\n \u0026 + * { background:red !important; width:100vw !important; height:100vh !important; position:fixed !important; inset:0 !important; }\n---\ninfo\n```\n\n### References\n\n- GHSA-87f9-hvmw-gh4p/CVE-2026-41159 (related vulnerability)\n- https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed\n- https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1\n- https://github.com/mermaid-js/mermaid/commit/7e83f1533318b307764d961906a73377266f4c5e\n- https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8",
"id": "GHSA-6x64-9x62-f2gx",
"modified": "2026-08-06T19:55:31Z",
"published": "2026-08-06T19:55:31Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/security/advisories/GHSA-6x64-9x62-f2gx"
},
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/pull/8022"
},
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/commit/12d472c9ed43f94814b110da8d7a9ae6dd5266ed"
},
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/commit/7e83f1533318b307764d961906a73377266f4c5e"
},
{
"type": "PACKAGE",
"url": "https://github.com/mermaid-js/mermaid"
},
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/releases/tag/mermaid@11.16.1"
},
{
"type": "WEB",
"url": "https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "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",
"type": "CVSS_V4"
}
],
"summary": "Mermaid allows CSS injection applying to sibling elements of the diagram"
}
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Vulnerability timeline
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