How do Content Security Policies (CSPs) work in web security?

Content Security Policies (CSPs) work by restricting the sources of content that a web browser can load.

CSPs are a security measure used in web development to prevent cross-site scripting (XSS), clickjacking and other code injection attacks. They are implemented by web developers and enforced by web browsers. A CSP is essentially a whitelist of sources of content that the browser is allowed to load. This could include scripts, images, stylesheets, and other resources.

When a web page with a CSP is loaded, the browser checks the sources of all the content it is asked to load against the policy. If a piece of content comes from a source not listed in the policy, the browser will block it from loading. This prevents malicious content from being loaded from untrusted sources.

CSPs are implemented using HTTP headers. The server sends the policy to the browser in the 'Content-Security-Policy' HTTP response header. The policy itself is a string of directives, each specifying a type of content and a list of trusted sources for that content. For example, a directive might say 'script-src 'self' https://trusted.com', which means that scripts can only be loaded from the same domain as the web page or from 'https://trusted.com'.

CSPs also support a 'report-only' mode, which doesn't block any content but sends reports about policy violations to a specified URL. This can be useful for testing a new policy or for monitoring a live site for attempted attacks.

CSPs are a powerful tool for web security, but they need to be used correctly to be effective. They should be as specific as possible, to minimise the chance of inadvertently allowing malicious content. They should also be kept up to date as the website changes and new sources of content are added. However, even a well-implemented CSP is not a silver bullet and should be used as part of a broader web security strategy.