CVE-2026-71438: Mermaid prototype pollution in configuration APIs

Executive brief

Mermaid, a popular JavaScript-based diagramming and charting tool, contains a vulnerability in how it handles configuration settings. If an application using Mermaid allows untrusted users to provide configuration data directly, an attacker could potentially manipulate the tool's internal behavior. While the risk is considered low because these settings are typically only managed by developers, it could lead to unauthorized changes in how diagrams are rendered or processed.

Technical details

A prototype pollution vulnerability exists in Mermaid's configuration setters, including mermaid.initialize, mermaidAPI.setConfig, and mermaidAPI.updateSiteConfig. The root cause is the use of a vulnerable deep-merge helper, assignWithDepth, which fails to properly sanitize property keys when merging caller-supplied objects into the internal configuration. An attacker could exploit this if an integrating application passes untrusted, user-controlled data directly into these configuration entry points. Successful exploitation could allow an attacker to modify the global Object prototype, potentially leading to property injection or other side effects. The issue is patched in versions 11.16.1 and 10.9.8.

Affected products

• mermaid-js mermaid >= 11.0.0-alpha.1, < 11.16.1; < 10.9.8

Timeline

• 2026-08-04: disclosed
• 2026-08-06: advisory: GitHub Advisory published

References

• https://api.github.com/users/Str1ckl4nd
• https://github.com/Str1ckl4nd
• https://api.github.com/users/Str1ckl4nd/gists%7B/gist_id%7D
• https://api.github.com/users/Str1ckl4nd/repos
• https://avatars.githubusercontent.com/u/73695323?v=4
• https://api.github.com/users/Str1ckl4nd/events%7B/privacy%7D