In the web context, CSP stands for Content Security Policy. It is a security feature that helps protect web applications from various types of attacks, such as cross-site scripting (XSS) and data injection. CSP is implemented using an HTTP header that instructs the web browser about what sources of content are allowed to be loaded and executed on a web page.
The primary purpose of CSP is to mitigate the risks associated with code injection attacks. By defining a policy for what content can be executed on a page, CSP helps prevent malicious code from being executed, even if it somehow makes its way into the application (e.g., through user input or other vulnerabilities).
Here’s how CSP works:
Enabling CSP: To use CSP, a web server sends the appropriate HTTP header in its response to a web browser. The header is called Content-Security-Policy and contains the policy rules that the browser should enforce for that page.
Defining the Policy: The CSP policy is a set of directives that define which sources are allowed for different types of content, such as scripts, styles, images, fonts, etc. The sources can be specified using various methods like exact URLs, wildcards, or even specific domains.
Policy Directives: Some common CSP directives include:
default-src: Specifies the default sources for all content types.
script-src: Defines the sources from which JavaScript can be loaded and executed.
style-src: Specifies the sources allowed for loading stylesheets.
img-src: Defines the sources allowed for loading images.
font-src: Specifies the sources for loading web fonts.
connect-src: Defines the allowed sources for making network requests (e.g., AJAX, WebSockets).
Reporting: CSP can also include a report-uri directive or the report-to directive (a new standard replacing report-uri) to report violations back to the server. This helps website owners monitor and understand any attempted violations.
Fallbacks: Since CSP is not universally supported by all browsers, it’s essential to include a fallback mechanism for browsers that don’t understand CSP. This is achieved by using the Content-Security-Policy-Report-Only header or by providing a non-CSP version of the site.
By implementing CSP, web developers can significantly reduce the risk of XSS attacks and other code injection vulnerabilities. However, it’s important to carefully define the policy to avoid unintended blocking of legitimate content and to ensure that the application functions as intended while still maintaining a robust security posture.