Dell Secureworks estimated that 250,000 systems have been infected globally in the first 100 days of the CryptoLocker threat and more than a million dollars has been given to buy the cyber criminals off. CryptoLocker is ransomware which encrypts user's important files and then demands money in return for decrypting them.

Dell Secureworks said that the criminals have managed to amass 1,216 bitcoins since September. Had the hackers immediately exchange bitcoins into dollars, the cash pile would have amounted to $380,000. If however, they held onto the money and exchanged them as of last week, that figure rises to $980,000, based on the current weighted price of $804/BTC. Even if Cryptolocker is removed, there is no way the encrypted files can be decrypted. Decryption keys are stored on one of many Cryptolocker servers. The files can only be restored by paying the ransom.

The authors of Cryptolocker have created a robust program that is difficult to circumvent," SecureWorks said in a blog post. "Instead of using a custom, cryptographic implementation like many other malware families, Cryptolocker uses strong third-party certified cryptography offered by Microsoft's CryptoAPI."

The malware has targeted English-speakers, specifically those located in the United States. "Malware authors from Russia and Eastern Europe, where the CryptoLocker authors are thought to originate, commonly target victims in North America and Western Europe," the researchers said.

Early versions of the malware were distributed through spam targeting businesses and threatened them with a customer complaint to get them to open the file.