CVE-2026-50159 mermaid-js CVE debrief

Who should care

Developers and administrators using Mermaid for diagram generation, especially in user-facing applications or with untrusted input, should review and apply updates. Vulnerability management and security teams should monitor for suspicious activity and implement compensating controls as needed.

Technical summary

Mermaid, a JavaScript tool for generating diagrams, is vulnerable to CSS injection via sibling combinator selectors. An attacker who can supply diagram text can inject arbitrary CSS into the rendered page, potentially altering appearance or behavior of unrelated page elements. Fixed in versions 10.9.8 and 11.16.1. Affected systems may include user-facing applications or those with untrusted input.

Defensive priority

Medium-priority defensive review recommended due to potential CSS injection vulnerability.

Recommended defensive actions

• Review and apply Mermaid updates to versions 10.9.8 or 11.16.1
• Inventory systems using Mermaid for diagram generation
• Monitor for suspicious diagram submissions
• Implement Content Security Policy (CSP) to mitigate CSS injection
• Restrict user-supplied diagram text input
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

Evidence from official CVE and NVD sources indicates a CSS injection vulnerability in Mermaid prior to versions 10.9.8 and 11.16.1. Limited detail available on attack scenarios and affected systems. Defenders should verify Mermaid versions, review diagram text inputs, and monitor for suspicious activity. The vulnerability allows an attacker to inject arbitrary CSS into the rendered page, potentially altering appearance or behavior of unrelated page elements.

Official resources