ruby.rails.security.audit.xss.templates.alias-for-html-safe.alias-for-html-safe

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The syntax <%== ... %> is an alias for html_safe. This means the content inside these tags will be rendered as raw HTML. This may expose your application to cross-site scripting. If you need raw HTML, prefer using the more explicit html_safe and be sure to correctly sanitize variables using a library such as DOMPurify.

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rules:
  - id: alias-for-html-safe
    message: The syntax `<%== ... %>` is an alias for `html_safe`. This means the
      content inside these tags will be rendered as raw HTML. This may expose
      your application to cross-site scripting. If you need raw HTML, prefer
      using the more explicit `html_safe` and be sure to correctly sanitize
      variables using a library such as DOMPurify.
    metadata:
      cwe:
        - "CWE-79: Improper Neutralization of Input During Web Page Generation
          ('Cross-site Scripting')"
      references:
        - https://medium.com/sumone-technical-blog/a-pretty-way-to-unescape-html-in-a-ruby-on-rails-application-efc22b850027
        - https://stackoverflow.com/questions/4251284/raw-vs-html-safe-vs-h-to-unescape-html#:~:text===
      category: security
      technology:
        - rails
      owasp:
        - A07:2017 - Cross-Site Scripting (XSS)
        - A03:2021 - Injection
      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:
      - generic
    paths:
      include:
        - "*.erb"
    severity: WARNING
    patterns:
      - pattern: <%== ... %>
      - pattern-not: <%== $...A.to_json %>

Examples

alias-for-html-safe.erb

@custom_page_title = “Page <strong>Title</strong>”
<div>
  <!-- ruleid: alias-for-html-safe -->
  <h1><%== @custom_page_title %></h1>
  <!-- ok: alias-for-html-safe -->
  <h1><%= @custom_page_title %></h1>
  <!-- ok: alias-for-html-safe -->
  <h1><%== @custom_page_title.to_json %></h1>
</div>