Summary
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.
PoC
architecture-beta
group mermaidPrototypePollutionMarker(cloud)[Marker]
service a(server)[A] in __proto__
service b(server)[B] in mermaidPrototypePollutionMarker
a:R -- L:b
The vulnerable write was introduced in commit cb0a4703bdf01d47508bde1c08aa9a980d70bc20 and first shipped in [email protected]. The lines are unchanged in every release since.
Workarounds
There are no known workarounds. Please update to a patched version.
References
Are there any links users can visit to find out more?
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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This has been patched by https://github.com/mermaid-js/mermaid/commit/99af3fc35ef0a9a9c8c6314521344d67523ddccf, released in Mermaid v11.16.1
Frequently Asked Questions
- What is CVE-2026-71437? CVE-2026-71437 is a medium-severity security vulnerability in mermaid (npm), affecting versions >= 11.5.0, < 11.16.1. It is fixed in 11.16.1.
- Which versions of mermaid are affected by CVE-2026-71437? mermaid (npm) versions >= 11.5.0, < 11.16.1 is affected.
- Is there a fix for CVE-2026-71437? Yes. CVE-2026-71437 is fixed in 11.16.1. Upgrade to this version or later.
- Is CVE-2026-71437 exploitable, and should I be worried? Whether CVE-2026-71437 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-71437 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-71437? Upgrade
mermaidto 11.16.1 or later.