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:b ``` The vulnerable write was introduced in commit [cb0a4703bdf01d47508bde1c08aa9a980d70bc20](https://github.com/mermaid-js/mermaid/commit/cb0a4703bdf01d47508bde1c08aa9a980d70bc20) and first shipped in `[email protected]`. 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](https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1) ### Workarounds There are no known workarounds. Please update to a patched version. ### References _Are there any links users can visit to find out more?_ - https://github.com/mermaid-js/mermaid/commit/99af3fc35ef0a9a9c8c6314521344d67523ddccf - https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1

Source published

6 Aug 2026 19:51:59 UTC

Source modified

10 Sep 2026 03:51:12 UTC

Observed here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · 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

Publisher rating · MODERATE