Several users have complained that ads served through Microsoft's Skype app are serving malicious downloads, which if opened, can trigger ransomware.

News of the issue came from a Reddit thread on Wednesday, in which the original poster said that Skype's home screen -- the first screen that shows up on consumer versions of the software -- was pushing a fake, malicious ad, purporting to be a critical update for the Flash web plug-in.

According to the thread, the ad triggered a download of an HTML application, designed to look like a legitimate app. The app, when opened, would download a malicious payload, which locks the user's computer and encrypts its files for ransom.
Skype users hit by ransomware through in-app malicious ads | ZDNet