CamScanner is a scan-to-PDF tool for mobile devices that has had in excess of 100M downloads and comes in a free (ad-supported) and in a paid version.
Researchers from Kaspersky have identified an issue with a recent version of CamScanner. The malware resides in the advertising plug-in, which is why the paid version continues to be safe. The actual CamScanner software continues to be safe, but it is obviously impossible to avoid the advertising module when the free version is used.
CamScanner has been available via the Google Play Store for some considerable time and it is only in the past few months that reports have surfaced of 'odd' behaviour. It appears that the developers added a new advertising management module that contained the module "Trojan-Dropper.AndroidOS.Necro.n" which has previously been observed in apps pre-installed on some Chinese smartphones.
According to Kaspersky, the module is a trojan dropper, meaning that "the module extracts and runs another malicious module from an encrypted file included in the app's resources. This 'dropped' malware, in turn, is a Trojan Downloader that downloads more malicious modules depending on what its creators are up to at the moment."
This means that it is capable of connecting to malicious servers to access whatever 'bad' modules the controllers want to execute on your phone.
More details of the malicious code and other important information may be found on Kaspersky's posting on SecureList.
Google has removed the free version of CamScanner from the Play Store; the paid version remains.