Mermaid: Improper sanitization of configuration leads to CSS injection
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:
%%{init: {"fontFamily": "x;a{b} :not(&){background:green !important} c{d}"}}%%
flowchart LR
A --> BThe 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
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | mermaid | >= 11.0.0-alpha.1, < 11.15.0, < 10.9.6 | 11.15.0, 10.9.6 |
References
- https://github.com/mermaid-js/mermaid/security/advisories/GHSA-87f9-hvmw-gh4p
- https://nvd.nist.gov/vuln/detail/CVE-2026-41159
- https://github.com/mermaid-js/mermaid/commit/64769738d5b59211e1decb471ffbaca8afec51aa
- https://github.com/mermaid-js/mermaid/commit/a9d9f0d8eb790349121508688cd338253fd80d76
- https://github.com/mermaid-js/mermaid
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.15.0
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid@11.15.0
- https://github.com/mermaid-js/mermaid/releases/tag/v10.9.6
- CVSS vector
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- Weakness (CWE)
CWE-94- Platform
- npm
- Package
mermaid- Disclosed
- 11.05.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-87f9-hvmw-gh4p - CC BY 4.0
Vulnerability history: mermaid
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 16
- First → latest
- 09.2020 08.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 3
- Medium 12
- Low 1
Disclosures per year
- 2020: 1
- 2021: 1
- 2022: 2
- 2023: 0
- 2024: 1
- 2025: 2
- 2026: 9
-
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