python.django.security.audit.xss.var-in-script-tag.var-in-script-tag

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Detected a template variable used in a script tag. Although template variables are HTML escaped, HTML escaping does not always prevent cross-site scripting (XSS) attacks when used directly in JavaScript. If you need this data on the rendered page, consider placing it in the HTML portion (outside of a script tag). Alternatively, use a JavaScript-specific encoder, such as the one available in OWASP ESAPI. For Django, you may also consider using the 'json_script' template tag and retrieving the data in your script by using the element ID (e.g., document.getElementById).

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Defintion

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rules:
  - id: var-in-script-tag
    languages:
      - generic
    severity: ERROR
    message: Detected a template variable used in a script tag. Although template
      variables are HTML escaped, HTML escaping does not always prevent
      cross-site scripting (XSS) attacks when used directly in JavaScript. If
      you need this data on the rendered page, consider placing it in the HTML
      portion (outside of a script tag). Alternatively, use a
      JavaScript-specific encoder, such as the one available in OWASP ESAPI. For
      Django, you may also consider using the 'json_script' template tag and
      retrieving the data in your script by using the element ID (e.g.,
      `document.getElementById`).
    patterns:
      - pattern-inside: <script ...> ... </script>
      - pattern: "{{ ... }}"
      - pattern-not-inside: nonce = '...'
      - pattern-not-inside: nonce = "..."
    paths:
      include:
        - "*.html"
    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://adamj.eu/tech/2020/02/18/safely-including-data-for-javascript-in-a-django-template/?utm_campaign=Django%2BNewsletter&utm_medium=rss&utm_source=Django_Newsletter_12A
        - https://www.veracode.com/blog/secure-development/nodejs-template-engines-why-default-encoders-are-not-enough
        - https://github.com/ESAPI/owasp-esapi-js
      category: security
      technology:
        - django
      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]

Examples

var-in-script-tag.html

<!DOCTYPE html>
<html>
    <head></head>
    <body>
        <script>
            // ruleid: var-in-script-tag
            const mydata = {{ mydata_json|safe }};
            // ruleid: var-in-script-tag
            const moredata = {{ mydata_json }};
        </script>
        <div>
            <!-- ok: var-in-script-tag -->
            <p>{{ this_is_fine }}</p>
        </div>

        <!-- ok: var-in-script-tag -->
        <script nonce="{{ request.csp_nonce }}">
            console.log('inline script running');
        </script>
    </body>
</html>