GHSA-87f9-hvmw-gh4p
Fix: mermaid-js/mermaid@6476973GHSA-87f9-hvmw-gh4p is a security vulnerability in mermaid. O3 Security confirms whether GHSA-87f9-hvmw-gh4p is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Mermaid: Improper sanitization of configuration leads to CSS injection
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.
mermaidnpmDescription
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 --> B
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | mermaid | ≥ 11.0.0-alpha.1&&< 11.15.0 | 11.15.0 |
| 📦npm | mermaid | all versions | 10.9.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mermaid. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update mermaid to 11.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-87f9-hvmw-gh4p is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-87f9-hvmw-gh4p is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-87f9-hvmw-gh4p. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-87f9-hvmw-gh4p in your dependencies?
O3 detects GHSA-87f9-hvmw-gh4p across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.