CVE-2026-26226: beautiful-mermaid XSS Vulnerability

CVE-2026-26226 Overview

CVE-2026-26226 is an SVG attribute injection vulnerability affecting the beautiful-mermaid npm package versions prior to 0.1.3. This flaw allows attackers to achieve cross-site scripting (XSS) when rendering attacker-controlled Mermaid diagrams. User-controlled values from Mermaid style and classDef directives are interpolated into SVG attribute values without proper escaping, enabling crafted input to break out of an attribute context and inject arbitrary SVG elements and attributes into the rendered output.

Critical Impact

When the generated SVG is embedded in a web page, this vulnerability enables script execution in the context of the embedding origin, potentially allowing session hijacking, credential theft, and malicious actions on behalf of authenticated users.

Affected Products

• beautiful-mermaid versions prior to 0.1.3

Discovery Timeline

• February 13, 2026 - CVE-2026-26226 published to NVD
• February 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-26226

Vulnerability Analysis

This vulnerability represents a classic input validation failure in the context of SVG rendering. The beautiful-mermaid library processes Mermaid diagram syntax and generates SVG output for display in web applications. When handling user-supplied values within style and classDef directives, the library fails to properly sanitize or escape these inputs before interpolating them into SVG attribute values.

The lack of proper escaping creates an injection point where an attacker can craft malicious input that terminates the current attribute context and introduces arbitrary SVG content. Since SVG supports inline JavaScript through various event handlers and the <script> element, this attribute injection vulnerability escalates directly to cross-site scripting.

Root Cause

The root cause is improper input validation (CWE-79) in the SVG generation process. The library directly interpolates user-controlled values from Mermaid style and classDef directives into SVG attributes without implementing proper output encoding or sanitization. Specifically, special characters such as quotes, angle brackets, and event handler attributes are not escaped, allowing attackers to break out of the intended attribute context.

Attack Vector

This vulnerability is exploitable over the network, requiring user interaction (the victim must view a page containing the malicious Mermaid diagram). An attacker would craft a malicious Mermaid diagram containing specially-crafted style or classDef directives with injection payloads. When a victim views a web page that renders this diagram using a vulnerable version of beautiful-mermaid, the injected SVG content executes arbitrary JavaScript in the victim's browser context.

The attack flow involves embedding malicious attribute values that include quote characters to escape the current attribute, followed by event handlers such as onload or onclick containing JavaScript payloads. When the SVG is rendered in a victim's browser, these event handlers execute, compromising the user's session.

Detection Methods for CVE-2026-26226

Indicators of Compromise

• Presence of beautiful-mermaid package versions below 0.1.3 in package.json or package-lock.json files
• SVG output containing unexpected event handler attributes such as onload, onclick, onerror, or onmouseover
• User-generated Mermaid diagrams containing unusual characters in style or classDef directives, particularly quote characters and angle brackets
• JavaScript errors or unexpected script execution when rendering Mermaid diagrams

Detection Strategies

• Implement dependency scanning tools to identify vulnerable versions of beautiful-mermaid in your codebase
• Deploy web application firewalls (WAF) with rules to detect XSS patterns in requests containing Mermaid diagram syntax
• Monitor for Content Security Policy (CSP) violations that may indicate attempted XSS exploitation
• Review application logs for requests containing suspicious patterns in Mermaid diagram inputs

Monitoring Recommendations

• Enable browser-side CSP reporting to detect inline script execution attempts
• Implement server-side logging for all user-submitted Mermaid diagram content
• Monitor npm audit and security advisory feeds for beautiful-mermaid package updates
• Set up alerts for dependency version mismatches between development and production environments

How to Mitigate CVE-2026-26226

Immediate Actions Required

• Upgrade beautiful-mermaid to version 0.1.3 or later immediately
• Audit any user-generated Mermaid diagrams stored in your application for potential exploitation attempts
• Review rendered SVG output for unexpected attributes or elements
• Implement Content Security Policy headers to mitigate the impact of any successful XSS attacks

Patch Information

The vulnerability has been addressed in beautiful-mermaid version 0.1.3. The fix implements proper escaping of user-controlled values before interpolation into SVG attributes. Organizations should update to this version or later to remediate the vulnerability.

For detailed patch information, see the GitHub Pull Request and the GitHub Release v0.1.3.

Additional technical details are available in the VulnCheck Advisory on SVG Injection.

Workarounds

• Sanitize all user input before processing with beautiful-mermaid by removing or encoding special characters in style and classDef directive values
• Implement a strict Content Security Policy that prevents inline script execution (script-src 'self')
• Render SVG content in sandboxed iframes with the sandbox attribute to isolate potential XSS execution
• Disable user-controlled Mermaid diagram rendering until the package can be updated

# Update beautiful-mermaid to the patched version
npm update beautiful-mermaid@0.1.3

# Verify the installed version
npm list beautiful-mermaid