Skip to content

Latest commit

 

History

History
39 lines (31 loc) · 3.75 KB

attack-surface.md

File metadata and controls

39 lines (31 loc) · 3.75 KB

Attack Surface Analysis for google/flexbox-layout

Attack Surface: CSS Injection

Description: Injection of malicious CSS code into the application, allowing attackers to manipulate styling and potentially other aspects of the application.

How flexbox-layout Contributes: The library's core mechanism of using JavaScript objects to define CSS styles provides the direct pathway for this attack. If user-supplied data is incorporated into these style objects without proper sanitization or escaping, CSS injection becomes possible. This is the defining characteristic of how flexbox-layout contributes to this vulnerability.

Example:

// Vulnerable Code:
const userProvidedWidth = "100%;} body {background-image: url('https://attacker.com/evil.jpg');} .container {width: "; // Malicious input
const styles = {
  container: {
    width: userProvidedWidth, // Directly using unsanitized input
    display: 'flex',
    // ... other flexbox properties
  }
};
// ... use styles with flexbox-layout

Even a seemingly small injection like manipulating the width property can be leveraged to inject a complete CSS rule that affects other parts of the page. The attacker doesn't need to inject a full <style> tag; they can "break out" of the intended property value.

Impact:

  • Defacement of the application.
  • Data exfiltration (e.g., cookies, session tokens, or content rendered on the page) via CSS techniques (e.g., background-image URLs, attribute selectors).
  • Potential for indirect Cross-Site Scripting (XSS) if the injected CSS can manipulate the DOM structure (e.g., hiding legitimate elements and inserting attacker-controlled ones), creating opportunities for subsequent JavaScript execution.
  • Phishing attacks by altering the application's appearance to mimic legitimate sites.

Risk Severity: High (Potentially Critical if it leads to XSS)

Mitigation Strategies:

  • Strict Input Validation: Never directly embed user input into style objects. Implement rigorous input validation using a whitelist approach whenever possible. Allow only known-safe values and patterns for CSS properties. Reject or strictly sanitize any input containing potentially dangerous characters or CSS keywords.
  • CSS Sanitization: Employ a dedicated CSS sanitization library (e.g., DOMPurify with the FOR_CSS option, or a specialized CSS sanitizer) to remove or escape potentially dangerous CSS properties and values. This is crucial if you must allow any degree of user-controlled styling.
  • Content Security Policy (CSP): Implement a strong CSP with a restrictive style-src directive. Ideally, use style-src 'self' (or a specific, trusted origin) and avoid 'unsafe-inline'. This prevents the execution of inline styles and styles from untrusted sources, significantly limiting the impact of a successful CSS injection.
  • Avoid Direct User Input: Whenever feasible, avoid directly using user input in style objects. Instead, use user input to select from a predefined set of safe styles or options. For example, provide a dropdown list of allowed colors rather than allowing users to enter an arbitrary color value.
  • Context-Aware Output Encoding (Supplementary): While not a primary defense, context-aware output encoding can provide an additional layer of protection in specific situations. For example, if you must insert a user-provided string into a CSS property value, you might use JavaScript's string escaping functions. However, this is not sufficient on its own and should never be relied upon as the sole defense against CSS injection. It's a supplementary measure, not a replacement for sanitization.
  • Escape user input: Escape any user input that is used in style objects.