Mermaid allows CSS injection applying to sibling elements of the diagram

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.3 · 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 · score source

EPSS

• CVE-2026-50159: 0.60% estimated exploitation activity in the next 30 days; percentile 48.89 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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…

Published by the source

6 Aug 2026, 19:55 UTC

Last modified by the source

6 Aug 2026, 19:55 UTC

Source last checked

26 Oct 2026, 13:00 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-6x64-9x62-f2gx

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# Patch Next advisory brief: CVE-2026-50159

Advisory: Mermaid allows CSS injection applying to sibling elements of the diagram
IDs: CVE-2026-50159, GHSA-6x64-9x62-f2gx
Record: https://patchnext.com/advisories/cve-2026-50159 (revision 1, source checked 26 Oct 2026, 13:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mermaid 11.16.1, 10.9.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mermaid: affected >= 11.0.0-alpha.1, < 11.16.1; < 10.9.8; fixed in 10.9.8, 11.16.1

## Suggested inspection (read-only)
- `npm ls mermaid --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mermaid.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-6x64-9x62-f2gx (GitHub-reviewed advisory GHSA-6x64-9x62-f2gx lists affected ranges: npm mermaid >=11.0.0-alpha.1 =0 <10.9.8)

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