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Medium, CVSS 5.3amended

Mermaid: Improper sanitization of configuration leads to CSS injection

CVE-2026-41159GHSA-87f9-hvmw-gh4p

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (mermaid 11.15.0, 10.9.6), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm mermaid

    Affected
    >= 11.0.0-alpha.1, <= 11.14.0<= 10.9.5
    Fixed in
    11.15.0, 10.9.6

Sources

Severity and exploitation
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-2026-41159: 0.49% estimated exploitation activity in the next 30 days; percentile 41.94 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Impact 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: 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. Patches - v11.15.0 (see 64769738d5b59211e1decb471ffbaca8afec51aa) - v10.9.6 (see a9d9f0d8eb790349121508688cd338253fd80d76) 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

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-87f9-hvmw-gh4p

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-87f9-hvmw-gh4p

Agent brief

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# Patch Next advisory brief: CVE-2026-41159

Advisory: Mermaid: Improper sanitization of configuration leads to CSS injection
IDs: CVE-2026-41159, GHSA-87f9-hvmw-gh4p
Record: https://patchnext.com/advisories/cve-2026-41159 (revision 3, source checked 26 Oct 2026, 12:53 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 11.15.0, 10.9.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mermaid: affected >= 11.0.0-alpha.1, <= 11.14.0; <= 10.9.5; fixed in 11.15.0, 10.9.6

## 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://osv.dev/vulnerability/GHSA-87f9-hvmw-gh4p (OSV record GHSA-87f9-hvmw-gh4p lists affected ranges: npm mermaid >=11.0.0-alpha.1 =0 <10.9.6 (fixed))
- Verified: https://github.com/advisories/GHSA-87f9-hvmw-gh4p (GitHub-reviewed advisory GHSA-87f9-hvmw-gh4p lists affected ranges: npm mermaid >=11.0.0-alpha.1 =0 <=10.9.5)

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