Identification of Content Security Policy CSP bypass

Content Security Policy (CSP) is a security standard introduced to prevent various types of attacks, including Cross-Site Scripting (XSS) and data injection attacks. It allows web developers to control resources the user agent is allowed to load for a given page. CSP bypass occurs when an attacker finds a way to execute unauthorized scripts or styles, despite a site's CSP rules. This can be due to misconfigurations or oversights in defining the policy.

Common Indicators of CSP Bypass:

  1. Unsafe Directives: Using unsafe directives like 'unsafe-inline' or 'unsafe-eval' can create loopholes in CSP, allowing inline scripts or dynamic script execution.
  2. Overly Permissive Sources: If the CSP includes overly broad source whitelists (e.g., allowing any subdomain or using wildcards), it might provide attackers with opportunities to inject malicious content.
  3. Trusting Unreliable Sources: Including third-party domains in the CSP without proper vetting can introduce risks if those domains are compromised.
  4. Improper Nonce or Hash Use: If nonces or hashes are used incorrectly or predictably, attackers might be able to bypass the CSP by matching these values.

How to Identify CSP Bypass:

  1. Manual Testing: Manually review the site's CSP header to identify overly permissive or unsafe policies. Test for known bypass techniques, such as exploiting 'unsafe-inline' or finding ways to inject code through whitelisted sources.
  2. CSP Evaluator Tools: Utilize CSP evaluator tools that analyze a site's CSP and highlight potential weaknesses or recommendations for tightening the policy.
  3. Code Review: During code reviews, pay special attention to how the CSP is defined and implemented, checking for common pitfalls and ensuring that the policy aligns with the application's security needs.
  4. Automated Scanning: Use automated tools that can test for CSP implementation flaws by attempting to inject scripts or load resources in violation of the defined policy.

Mitigation Strategies:

  1. Strict Policy Definition: Define CSP policies strictly to ensure that only necessary resources are allowed, minimizing the use of unsafe directives and broad source lists.
  2. Use Nonces or Hashes: Apply nonces (unique tokens) or hashes to allow specific inline scripts or styles, ensuring they are generated securely and unpredictably.
  3. Regularly Review and Update CSP: Regularly review the CSP to adjust for changes in the application and the evolving threat landscape, removing unnecessary sources and tightening policies.
  4. Content Whitelisting: Carefully whitelist trusted content sources and avoid using wildcards or overly broad URLs. Regularly audit the integrity and security of the whitelisted sources.
  5. Monitor and Report Violations: Implement CSP reporting mechanisms to receive alerts when violations occur, helping to detect and respond to bypass attempts.

Addressing CSP bypass is crucial for maintaining the integrity and security of web applications, as it strengthens the defense against various injection attacks and ensures that the CSP provides the intended level of protection.