5.3

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-41149

CVE-2026-41149: Mermaid: Improper sanitization of `classDef` in state diagrams leads to HTML injection

Impact

Under the default configuration, Mermaid state diagram's classDef allow DOM injection that escapes the SVG, although <script> tags are removed, preventing XSS.

Proof-of-concept

stateDiagram-v2
  classDef xss fill:red</style></svg><style>*{x:x;y:y;overflow:visible!important;contain:none!important;transform:none!important;filter:none!important;clip-path:none!important}</style><div style="x:x;y:y;color:red;font:5em/1 monospace;display:grid;place-items:center;z-index:2147483647;width:100vw;height:100vh;position:fixed;top:0;left:0;background:black">HACKED</div><svg><style>a:b
  [*] --> A:::xss

Patches

Workarounds

If you can not update to a patched version, setting "securityLevel": "sandbox" will prevent this, by rendering the mermaid diagram in a sandboxed <iframe>.

Credits

Thanks to @zsxsoft from @KeenSecurityLab for reporting this vulnerability.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-41149

CVE-2026-41149:

5.3

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
mermaid	npm	>= 11.0.0-alpha.1, <= 11.14.0	11.15.0
mermaid	npm	<= 10.9.5	10.9.6

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the improper construction of CSS styles from user-provided classDef definitions in Mermaid diagrams. The core of the issue is in the createCssStyles function within packages/mermaid/src/mermaidAPI.ts. Previously, this function used simple string concatenation to build a stylesheet from diagram definitions. This allowed an attacker to craft a classDef with malicious content that could break out of the SVG's <style> block and inject arbitrary HTML into the DOM. The provided patches (37ff937f1da2e19f882fd1db01235db4d01f4056 and 4e2d512bf5bf6f9de1a8f0a48da78dc4d09ac4f3) address this by refactoring createCssStyles to use the CSSStyleSheet API. This ensures that only valid CSS rules are created, effectively sanitizing the input and preventing the injection. The function createUserStyles is also relevant as it is a public API function that calls createCssStyles, and would therefore be part of the stack trace during exploitation.

Vulnerable functions

createCssStyles

packages/mermaid/src/mermaidAPI.ts

This function was vulnerable because it constructed CSS style strings by concatenating user-provided input from `classDef` definitions without proper sanitization. This allowed for the injection of arbitrary HTML and CSS, leading to a DOM injection vulnerability. The patch mitigates this by using the `CSSStyleSheet` API to programmatically and safely construct stylesheets, which prevents the injection of malicious content.

createUserStyles

packages/mermaid/src/mermaidAPI.ts

This function is an exported part of the mermaid API that invokes the vulnerable `createCssStyles` function. During an exploit, this function would likely appear in the call stack, making it a key runtime indicator of the vulnerable code path being triggered. It takes configuration and class definitions as input and passes them down to the underlying vulnerable function.

Vulnerability Intelligence
Miggo AI

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WAF Rule

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Reasoning

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