javascript.express.security.audit.xss.mustache.escape-function-overwrite.escape-function-overwrite

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The Mustache escape function is being overwritten. This could bypass HTML escaping safety measures built into the rendering engine, exposing your application to cross-site scripting (XSS) vulnerabilities. If you need unescaped HTML, use the triple brace operator in your template: '{{{ ... }}}'.

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Defintion

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rules:
  - id: escape-function-overwrite
    message: "The Mustache escape function is being overwritten. This could bypass
      HTML escaping safety measures built into the rendering engine, exposing
      your application to cross-site scripting (XSS) vulnerabilities. If you
      need unescaped HTML, use the triple brace operator in your template: '{{{
      ... }}}'."
    metadata:
      cwe:
        - "CWE-79: Improper Neutralization of Input During Web Page Generation
          ('Cross-site Scripting')"
      owasp:
        - A07:2017 - Cross-Site Scripting (XSS)
        - A03:2021 - Injection
      references:
        - https://github.com/janl/mustache.js/#variables
      category: security
      technology:
        - express
      cwe2022-top25: true
      cwe2021-top25: true
      subcategory:
        - audit
      likelihood: LOW
      impact: MEDIUM
      confidence: LOW
      license: Commons Clause License Condition v1.0[LGPL-2.1-only]
      vulnerability_class:
        - Cross-Site-Scripting (XSS)
    languages:
      - javascript
      - typescript
    severity: WARNING
    pattern-either:
      - pattern: Mustache.escape = ...
      - patterns:
          - pattern-inside: |
              $MUSTACHE = require("mustache");
              ...
          - pattern: $MUSTACHE.escape = ...

Examples

escape-function-overwrite.js

// cf. https://github.com/janl/mustache.js/#include-templates

// e.g., browser code:
// ruleid: escape-function-overwrite
Mustache.escape = function(val) { return val; }

function renderHello() {
  var template = document.getElementById('template').innerHTML;
  var rendered = Mustache.render(template, { name: 'Luke' });
  document.getElementById('target').innerHTML = rendered;
}

// e.g., Node.js code:
function node() {
  const template = require("mustache");
  // ruleid: escape-function-overwrite
  template.escape = (t) => { return t; }
  let html = template.render(blogItem, { });
}

function ok() {
  // ok: escape-function-overwrite
  const template = require("mustache");
  let html = template.render(blogItem, { });
}