The Cerber ransomware has been around for quite a while, and many variants are out in the wild, but one in particular, which has been observed over the past few months, contains a function that allows it to avoid encrypting files from any security products on your device. This includes firewalls, antivirus software or antispyware products.
Article Link: http://news.softpedia.com/news/new-cerber-variant-spares-files-of-security-programs-from-encryption-512968.shtml