N/A

CVSS Score

-

CVSS Score

Basic Information

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CVE-2026-41159

CVE-2026-41159: Mermaid: Improper sanitization of configuration leads to CSS injection

Impact

Mermaid's default configuration allows injecting CSS that applies outside of the Mermaid diagram via the fontFamily, themeCSS, and altFontFamily configuration options.

Live demo: mermaid.live

Example code:

%%{init: {"fontFamily": "x;a{b} :not(&){background:green !important} c{d}"}}%%
flowchart LR
    A --> B

The injected CSS exploits stylis's & (scope reference) handling. :not(&) escapes the #mermaid-xxx automatic scoping, applying styles to all page elements. Global at-rules (@font-face, @keyframes, @counter-style) are also injectable as stylis hoists them to top level.

This allows page defacement and DOM attribute exfiltration via CSS :has() selectors.

Patches

Workarounds

If you can't upgrade mermaid, you can set the secure config value in the mermaid config to avoid allowing diagrams to modify fontFamily, themeCSS, altFontFamily, and themeVariables.

Setting "securityLevel": "sandbox" will also prevent this.

Credits

Reported by @zsxsoft on behalf of @KeenSecurityLab

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-41159

CVE-2026-41159:

N/A

CVSS Score

-

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 is a CSS injection in the mermaid library, caused by improper sanitization of CSS in configuration options. The analysis of the provided patches points to the createUserStyles function in packages/mermaid/src/mermaidAPI.ts as the culprit. This function was responsible for compiling user-provided CSS. The vulnerability was that it didn't prevent CSS rules from "escaping" the container of the mermaid diagram. The patch introduces a new function, compileCSS, which uses a middleware to properly scope the CSS, and createUserStyles was updated to use it. Therefore, createUserStyles is the vulnerable function that would appear in a runtime profile during exploitation.

Vulnerable functions

createUserStyles

packages/mermaid/src/mermaidAPI.ts

The `createUserStyles` function is responsible for processing user-provided CSS from the mermaid configuration. Before the patch, it used `stylis` to compile the CSS without properly sanitizing it. This allowed an attacker to use special CSS selectors like `:not(&)` to make the styles apply outside of the intended mermaid diagram, leading to a CSS injection vulnerability. The patch fixes this by replacing the direct `stylis` call with a new `compileCSS` function that includes a middleware to enforce proper namespacing.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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