π What is Content Security Policy (CSP)?
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. These attacks are used for everything from data theft to site defacement or distribution of malware.
π History and Background
The need for CSP arose from the increasing prevalence of XSS attacks, which exploit vulnerabilities in web applications to inject malicious scripts into trusted websites. Traditional methods of preventing XSS, such as input validation and output encoding, were often insufficient. CSP was introduced as a more robust, declarative approach to controlling the resources that a web page is allowed to load.
π Key Principles of CSP
- π‘οΈ Explicit Whitelisting: Instead of trusting all sources by default, CSP operates on the principle of explicitly whitelisting trusted sources of content.
- π« Restricting Inline Scripts: CSP discourages the use of inline JavaScript and CSS, as these are common vectors for XSS attacks.
- π Policy Enforcement: CSP allows web developers to define and enforce policies that dictate which resources the browser should load for a given web page.
βοΈ How CSP Works
CSP is implemented by adding an HTTP response header or a <meta> tag in the HTML. The header or tag contains a policy that defines the sources from which the browser is allowed to load resources.
Here's a basic example of a CSP HTTP header:
Content-Security-Policy: default-src 'self'
This policy tells the browser to only load resources from the website's own origin (i.e., the same domain, protocol, and port).
π οΈ Directives in CSP
CSP directives specify the types of resources and their allowed sources. Some common directives include:
- π
default-src: Sets the default source for all resource types. -
To allow scripts only from your own domain and a trusted CDN, you can use the following policy:
It is important to test the CSP implementation thoroughly before deploying it to a production environment. Tools like the browser's developer console and online CSP validators can help identify and resolve any issues.
Content Security Policy (CSP) is a powerful tool for preventing XSS attacks and improving website security. By explicitly whitelisting trusted sources of content, CSP can help protect against a wide range of attacks. While implementing CSP can be complex, the benefits of increased security and reduced risk of attacks make it a worthwhile investment.