A script-based scan engine is embedded in a webpage requested by a client computer from a web server. The script-based scan engine may be embedded in the webpage by injecting the script-based scan engine in a header of the webpage in a computer security device between the client computer and the web server, or by integrating the script-based scan engine in the webpage as a library. The script-based scan engine executes in the client computer when the webpage is received by the client computer. The script-based scan engine scans the webpage for web threats, which may include malicious codes, exploits, and phishing, for example. The webpage is allowed to be rendered by a web browser in the client computer when the webpage is deemed safe by the script-based scan engine.