attack-surface.md

Attack Surface Analysis for thoughtbot/bourbon

Attack Surface: CSS Injection via Unsanitized Input

• Description: Malicious CSS code is injected into the application's stylesheets, potentially altering the appearance, behavior, or even exfiltrating data.
  • Bourbon's Contribution: Bourbon mixins and functions, when misused with unsanitized user input provided as Sass variables, become the direct mechanism for this injection. The vulnerability isn't in Bourbon itself, but in how it's used to process untrusted input.
  • Example:

    // Vulnerable Sass (highly unlikely scenario)
    $user_color: $_POST['color']; // Directly using unsanitized POST data!
    .element {
      background-color: $user_color; // Bourbon mixin or function using the variable
      @include some-bourbon-mixin($user_color); //Another example
    }

    If $_POST['color'] contains red; } body { display: none; } /*, the malicious CSS would be injected.
  • Impact:
    • Defacement of the website.
    • Injection of malicious styles that could phish users.
    • Limited data exfiltration (using CSS techniques).
    • Potentially, in very specific and rare cases with older browsers or unusual configurations, a pathway to XSS might be created through highly crafted CSS, although this is not a typical CSS injection outcome.
  • Risk Severity: High (conditional on the presence of the architectural flaw allowing unsanitized input into Sass)
  • Mitigation Strategies:
    • Strict Input Validation and Sanitization: Never directly incorporate user input into Sass variables. All user-supplied data must be rigorously validated and sanitized on the server-side before being used in any context, including Sass compilation. Employ a whitelist approach (allowing only known-good values) rather than a blacklist.
    • Architectural Enforcement: The application's architecture should prevent any direct path from user input to Sass variables. This is a fundamental security design principle. Sass variables should be derived from trusted, server-side sources.
    • Content Security Policy (CSP): A well-configured CSP can mitigate the impact of CSS injection, even if it occurs, by restricting style sources and potentially disallowing inline styles.

Attack Surface: Supply Chain Attack

• Description: A compromised version of the Bourbon library itself is installed, introducing malicious code that executes during the build process.
  • Bourbon's Contribution: Bourbon is the direct target of the attack. The attacker compromises the Bourbon package (e.g., on npm) to inject their code.
  • Example: An attacker gains control of the Bourbon package on npm and publishes a malicious version that, during installation or build, steals environment variables or injects malicious JavaScript into the compiled CSS (though the latter is less likely, the build process itself is compromised).
  • Impact:
    • Compromise of the build environment.
    • Potential for injection of malicious code into the final application.
    • Theft of sensitive data (API keys, secrets) accessible during the build.
  • Risk Severity: High
  • Mitigation Strategies:
    • Package Manager Integrity Checks: Use npm with package-lock.json or yarn with yarn.lock. These files record the exact versions and cryptographic hashes of installed dependencies, ensuring that the same, verified code is installed each time.
    • Dependency Auditing: Regularly audit dependencies for known vulnerabilities. Use tools like npm audit or dedicated Software Composition Analysis (SCA) tools.
    • Version Pinning: Pin Bourbon (and other dependencies) to specific versions in your package.json to prevent unexpected updates that might introduce malicious code or vulnerabilities.
    • Software Composition Analysis (SCA): Employ SCA tools to continuously monitor and identify vulnerabilities in your dependencies, including Bourbon.
    • Vendoring (Optional, High Control): For the highest level of control, consider vendoring Bourbon (copying its source code directly into your project). This eliminates reliance on external package repositories but requires manual updates.