mermaid-js Mermaid up to 10.9.7/11.16.0 Configuration Merge Helper prototype pollution

Summaryinfo

A vulnerability classified as critical has been found in mermaid-js Mermaid up to 10.9.7/11.16.0. Affected by this vulnerability is the function mermaid.initialize/mermaidAPI.setConfig/mermaidAPI.updateSiteConfig of the component Configuration Merge Helper. The manipulation leads to prototype pollution. This vulnerability is listed as CVE-2026-71438. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability, which was classified as critical, has been found in mermaid-js Mermaid up to 10.9.7/11.16.0. This issue affects the function mermaid.initialize/mermaidAPI.setConfig/mermaidAPI.updateSiteConfig of the component Configuration Merge Helper. The manipulation with an unknown input leads to a prototype pollution vulnerability. Using CWE to declare the problem leads to CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-71438 since 08/06/2026. The exploitation is known to be easy. The attack may be initiated remotely. Technical details of the vulnerability are known, but there is no available exploit. The attack technique deployed by this issue is T1059 according to MITRE ATT&CK.

Upgrading to version 10.9.8 or 11.16.1 eliminates this vulnerability.

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Productinfo

Vendor

• mermaid-js

Name

• Mermaid

Version

• 10.9.0
• 10.9.1
• 10.9.2
• 10.9.3
• 10.9.4
• 10.9.5
• 10.9.6
• 10.9.7
• 11.0
• 11.1
• 11.2
• 11.3
• 11.4
• 11.5
• 11.6
• 11.7
• 11.8
• 11.9
• 11.10
• 11.11
• 11.12
• 11.13
• 11.14
• 11.15
• 11.16.0

Website

• Product: https://github.com/mermaid-js/mermaid/

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