Q. I keep getting these “Congratulations!” messages about winning prizes from Amazon.com, even when I’m not using the site. Why is this?

A. Those persistent pop-up messages are not from Amazon, but are part of a wave of malicious display advertisements — also known as “malvertising” — invading desktop, Android and iPhone browsers. The intrusive ads can also appear when browsing links that are posted on Facebook.

Malvertising has been a problem for years, but lately has become more pervasive on mobile devices. Symantec, a security company, noted this year that the “congratulations” malware has been making a strong push onto Android devices with a Trojan horse called Android.Fakeyouwon.

In January, the cybersecurity company Confiant also outlined how the recent malware was spread across the web by fraudulent advertising agencies, which placed tainted ads on legitimate sites. If you select the pop-up’s O.K. button or try to dismiss the box, the browser is typically redirected to a page that promises prizes for participation in a quiz.