CSP: Introducing New Security Risks

G5 Cyber Security

2 weeks ago

TL;DR

Yes, a Content Security Policy (CSP) can introduce new security risks if not carefully configured. While designed to *prevent* attacks like XSS, overly permissive or incorrectly defined CSPs can inadvertently allow malicious code execution or weaken existing protections.

Understanding the Risk

A CSP is a powerful tool that tells the browser which sources of content are trusted for your website. If you’re too lenient with what’s allowed, you might open up vulnerabilities. The core issue isn’t that CSP *inherently* creates risks, but that a poorly written one can be worse than no CSP at all.

How CSP Can Introduce New Risks

Overly Broad Directives: Using directives like script-src '*' or style-src '*' effectively disables script and style protection. This is equivalent to having no CSP.
```
# BAD - Allows all scripts from any source
script-src *;
```
Unintended Allowances: Allowing a trusted domain can sometimes indirectly allow malicious code if that domain is compromised or hosts user-supplied content. For example, allowing a CDN might be risky if the CDN itself gets hacked.
```
# Potentially Risky - Allows scripts from a CDN
script-src https://cdn.example.com;
```
Nonce Misuse: Using nonces (random strings) to allow inline scripts is good, but if the nonce isn’t truly random or is predictable, it can be bypassed.
```
# Good - Inline script allowed with a strong nonce
script-src 'nonce-{random_value}';
```
Report-Only Mode Pitfalls: While Report-Only mode is useful for testing, relying on it long-term without addressing the reported violations can create a false sense of security.
```
# Report-Only - Doesn't block, just reports. Don't rely on this forever!
Content-Security-Policy-Report-Only: default-src 'self'; report-uri /csp-report;
```
Ignoring `unsafe-inline` and `unsafe-eval`: These keywords should be avoided whenever possible. They significantly weaken CSP protection.
```
# BAD - Avoid these!  They disable important protections
script-src 'self' 'unsafe-inline' 'unsafe-eval';
```
Incorrect `style-src` Configuration: Allowing `’unsafe-inline’` for styles can open up XSS vulnerabilities. Consider using hashes or nonces instead.
```
# BAD - Avoid unsafe-inline for styles
style-src 'self' 'unsafe-inline';
```
Missing `frame-ancestors` Directive: Without a proper frame-ancestors directive, your site could be vulnerable to clickjacking attacks.
```
# Good - Prevent framing from untrusted sources
frame-ancestors 'self' https://trusted.example.com;
```
Third-Party Script Risks: Allowing third-party scripts (e.g., analytics, ads) can introduce vulnerabilities if those scripts are compromised or contain malicious code.
```
# Risky - Be careful with third-party scripts
script-src 'self' https://third-party.example.com;
```

Steps to Mitigate Risks

Start Strict: Begin with a very restrictive CSP and gradually relax it as needed, testing thoroughly at each step.
Use Nonces or Hashes: Prefer nonces or hashes for inline scripts and styles instead of `’unsafe-inline’`.
Regularly Review Your Policy: Periodically review your CSP to ensure it still meets your security requirements.
Monitor Reports: Pay attention to CSP violation reports (if you’re using Report-Only mode or have a reporting endpoint configured).
Test Thoroughly: Test your CSP with various browsers and scenarios to identify any unexpected behavior.
Understand Your Dependencies: Know where your scripts are coming from and the potential risks associated with each source.
Use a CSP Generator: Consider using a CSP generator tool as a starting point, but always review and customize the generated policy.

Resources

MDN Web Docs – Content Security Policy
CSP Evaluator (tool for testing CSPs)