Complex sentence: The pop-up window containing an advertisement is usually generated by JavaScript using cross-site scripting (XSS), sometimes with a secondary payload using Adobe Flash, but can also be generated by other vulnerabilities/security holes in browser security.
Split into simpler sentences: The pop-up window containing an advertisement is usually generated by JavaScript that uses cross-site scripting (XSS), sometimes with a secondary payload that uses Adobe Flash. They can also be generated by other vulnerabilities/security holes in browser security.