A file-encrypting ransomware program called CryptoWall infected over 600,000 computer systems in the past six months and held 5 billion files hostage, garnering more than $1 million for its creators , researchers found. The Counter Threat Unit (CTU) at Dell SecureWorks performed an extensive analysis of CryptoWall that involved gathering data from its command-and-control (C&C) servers, tracking its variants and distribution methods and counting payments made by victims so far.