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[knsv]

XSS vulnerability

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Vulnerability description

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In version 8.6.0, Mermaid added a support of directives to add more control over styles(themes) applied to the diagrams.
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Syntax for declaring the directive is %%{init: {<JSON_OBJECT>}}%%
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Directives can be used to overwrite default theme properties like fontFamily or fontSize to the graph.
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Behind the scenes, library takes JSON_OBJECT from directive and merges it with config object. Later that config is used to generate new CSS rules:

  let userStyles = '';
  // user provided theme CSS
  if (cnf.themeCSS !== undefined) {
    userStyles += `\n${cnf.themeCSS}`;
  }
  // user provided theme CSS
  if (cnf.fontFamily !== undefined) {
    userStyles += `\n:root { --mermaid-font-family: ${cnf.fontFamily}}`;
  }
  // user provided theme CSS
  if (cnf.altFontFamily !== undefined) {
    userStyles += `\n:root { --mermaid-alt-font-family: ${cnf.altFontFamily}}`;
  }

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Problem is that there is no sanitization of user-supplied values, which are added to style tag via innerHTML method afterwards:

  const stylis = new Stylis();
  const rules = stylis(`#${id}`, getStyles(graphType, userStyles, cnf.themeVariables));
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  const style1 = document.createElement('style');
  style1.innerHTML = rules;
  svg.insertBefore(style1, firstChild);

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This leads to Cross-Site Scripting attack via following directive:

%%{init: { 'fontFamily': '\"></style><img src=x onerror=alert(document.cookie)>'} }%%

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Steps to reproduce

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Create mermaid diagram with following payload:

%%{init: { 'fontFamily': '\"></style><img src=x onerror=alert(document.cookie)>'} }%%
sequenceDiagram
Alice->>Bob: Hi Bob
Bob->>Alice: Hi Alice

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Or visit following PoC:
https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoiJSV7aW5pdDogeyAnZm9udEZhbWlseSc6ICdcXFwiPjwvc3R5bGU-PGltZyBzcmM9eCBvbmVycm9yPWFsZXJ0KGRvY3VtZW50LmNvb2tpZSk-J30gfSUlXG5zZXF1ZW5jZURpYWdyYW1cbkFsaWNlLT4-Qm9iOiBIaSBCb2JcbkJvYi0-PkFsaWNlOiBIaSBBbGljZSIsIm1lcm1haWQiOnsidGhlbWUiOiJkZWZhdWx0In0sInVwZGF0ZUVkaXRvciI6ZmFsc2V9
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​
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Prototype pollution vulnerability

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Vulnerability description

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In version 8.6.0, Mermaid added a support of directives to add more control over styles(themes) applied to the diagrams.
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Syntax for declaring the directive is %%{init: {<JSON_OBJECT>}}%%
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The issue is that directive JSON_OBJECT is lacking proper sanitization before merging with default config which means we can specify __proto__ attribute to overwrite Object prototype.
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For example, if we use following payload, it will add attribute polluted to every new object in the application:

%%{init: { '__proto__': {'polluted': 'asdf'}} }%%
sequenceDiagram
Alice->>Bob: Hi Bob
Bob->>Alice: Hi Alice

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This can break the app leading to client-side DOS or can be used to achieve XSS. In some rare cases it can even lead to Remote Code Execution.
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Steps to reproduce

Open https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoiJSV7aW5pdDogeyAnX19wcm90b19fJzogeydwb2xsdXRlZCc6ICdhc2RmJ319IH0lJVxuc2VxdWVuY2VEaWFncmFtXG5BbGljZS0-PkJvYjogSGkgQm9iXG5Cb2ItPj5BbGljZTogSGkgQWxpY2VcbiIsIm1lcm1haWQiOnt9LCJ1cGRhdGVFZGl0b3IiOmZhbHNlfQ
Then open developer console in the browser and create a new object:

a={}
a.polluted 

You will see that newly created object already has polluted value.
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Resources

You can learn more about this type of vulnerability here:
https://portswigger.net/daily-swig/prototype-pollution-the-dangerous-and-underrated-vulnerability-impacting-javascript-applications

[knsv]

Af ic for this issue is released in version 8.9.2