How to Use Content Security Policy (CSP) to Mitigate Risks

November 26, 20234 min read


Content Security Policy (CSP) is an added layer of security that helps to detect and mitigate certain types of attacks, including Cross Site Scripting (XSS) and data injection attacks. Implementing CSP involves adding the Content-Security-Policy HTTP header to a web page and defining the domains that the browser should consider as valid sources for executable scripts. This guide aims to detail the steps necessary to effectively implement CSP to strengthen your website’s defense against malicious attacks.

Understanding CSP

  • CSP Overview: CSP is a browser feature that can help protect your web application from content injection vulnerabilities by declaring which dynamic resources are allowed to load.
  • Directives: CSP uses directives to specify the types of content that can be safely executed or rendered within a web page.
  • Sources: Sources are the origins from where specific content types can be loaded. They can be as granular as a full URL or as broad as a keyword like ‘self’.

Implementing Content Security Policy

Step 1: Define Your Policy

  • Assess Your Needs: Determine which scripts or content types you use and where they come from.
  • Identify Sources: List the domain sources you wish to allow for each content type.
  • Draft a Policy: Begin drafting your CSP, being as specific as possible regarding the sources.

Step 2: Set Up the CSP Header

  • Add the Header: Implement the Content-Security-Policy HTTP header either through server configurations or meta tags.
  • Use Multiple Policies: If needed, you can define several policies that will be enforced together.

Step 3: Test Your Policy

  • Use Report-Only Mode: Before enforcing CSP, test it in “report-only” mode, which will not block any content but will report violations.
  • Monitor Reports: Collect and analyze violation reports to adjust and finetune your policy.

Step 4: Refine Your Policy

  • Iterative Refinement: Adjust your policy based on the reports until you’re confident that it reflects your site’s needs and doesn’t break any functionality.
  • Update Regularly: Be prepared to update your CSP as you add or change resources on your web page.

Step 5: Enforce Your Policy

  • Switch to Enforce Mode: Once tested, switch your policy from report-only mode to block offending resources to provide the intended protection.
  • Monitor for Issues: Regularly check for any new violations or any negative impact on user experience.

Best Practices for Managing CSP

Simplify Your Sources

  • Use Fewer Sources: Minimize the number of different sources to reduce the attack surface.
  • Use Nonces or Hashes: Where possible, use nonces or hashes instead of allowing whole domains to be more specific about what is allowed.

Update and Review

  • Regularly Review Policies: Review and update your policies to reflect changes in your web application and to respond to emerging threats.
  • Communicate with Third-Parties: If your site relies on third-party scripts, communicate with those parties to understand any changes that might affect your CSP.

Automate Monitoring

  • Tools for Automation: Utilize tools such as web application firewalls (WAFs) and CSP monitoring services to automate detection and alerting of CSP violations.

Provide Fallback Content

  • Plan for Blocks: Create fallback content or functionality that will be displayed when the primary resource is blocked by the CSP.