New and "improved" version of Cryptowall found in exploit kits
Cryptowall 4.0 is the most recent iteration of this malicious software that encrypts users' files and holds them for ransom, and it makes it harder for victims to recover their data without paying up. File names are encrypted as well as the contents, so that even with a forensic data recovery, it's difficult or impossible to know which files are which.
Now it has found its way into at least one of the many exploit kits that automate the process by which attackers exploit vulnerabilities: Nuclear. This is one of the most widely used attack tools, along with Angler.
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