How to use external JavaScript libraries in extensions considering the strict CSP policy?

Modern browser extensions often require the use of third-party libraries for working with the interface, network requests, or data analysis. However, introducing external code comes with Content Security Policy (CSP) limitations, which in the context of extensions prohibit the execution of dynamically generated code and the loading of scripts from remote sources. This is done to protect users from malicious code injection.

The main rule for the secure use of third-party libraries is that all code files must be locally packaged inside the extension directory. Loading scripts over the network from third-party servers via script tags directly from the internet is blocked in Manifest V3. Therefore, developers must download the required versions of libraries and include them in the project at the build stage.

When configuring the extension manifest, special attention must be paid to security policy parameters. The manifest specifies allowed sources and script execution rules. If complex template engines or libraries requiring dynamic code evaluation are used, the CSP rules must be configured accordingly, although in modern practices it is recommended to completely avoid functions like eval.

To automate builds and dependency management, developers often use modern module bundlers such as Webpack or Vite. They allow combining local dependencies and minifying the code before packaging the extension. This guarantees that only verified files meeting all marketplace security requirements make it into the release version.

When connecting third-party libraries to various extension components, such as popups or content scripts, execution context isolation should be taken into account. Using isolated worlds for content scripts prevents global variable conflicts between the extension library and the website's own scripts.