Mermaid Architecture diagrams are vulnerable to prototype pollution
Rendering an untrusted architecture-beta diagram lets the diagram author write an arbitrary property with the value horizontal or vertical onto Object.prototype. A group id of __proto__ is accepted as a valid parent.
Impact
Any code in the same realm that reads a property of that name from an arbitrary object, or enumerates an object with bare for...in, observes the injected value (which can only be the string horizontal or vertical.
This may mean corrupted option/config defaults, bypassed truthiness checks, causing denial of service or logic corruption in the embedding application.
Because the injected value cannot be an object or function, this is not directly exploitable for remote code execution.
PoC
architecture-beta
group mermaidPrototypePollutionMarker(cloud)[Marker]
service a(server)[A] in __proto__
service b(server)[B] in mermaidPrototypePollutionMarker
a:R -- L:bThe vulnerable write was introduced in commit cb0a4703bdf01d47508bde1c08aa9a980d70bc20 and first shipped in mermaid@11.5.0. The lines are unchanged in every release since.
Patches
This has been patched by https://github.com/mermaid-js/mermaid/commit/99af3fc35ef0a9a9c8c6314521344d67523ddccf, released in Mermaid v11.16.1
Workarounds
There are no known workarounds. Please update to a patched version.
References
Are there any links users can visit to find out more?
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | mermaid | >= 11.5.0, < 11.16.1 | 11.16.1 |
References
- https://github.com/mermaid-js/mermaid/security/advisories/GHSA-3rrr-jr9j-h3q3
- https://github.com/mermaid-js/mermaid/pull/8022
- https://github.com/mermaid-js/mermaid/commit/99af3fc35ef0a9a9c8c6314521344d67523ddccf
- https://github.com/mermaid-js/mermaid
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid@11.16.1
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:H/SI:H/SA:H- Weakness (CWE)
CWE-1321- Platform
- npm
- Package
mermaid- Disclosed
- 06.08.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-3rrr-jr9j-h3q3 - 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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