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Hackers infect quarter of a million PCs with ransomware

Criminals estimated to have made a million dollars in bitcoins.

Security researchers said that over 250,000 computers have been infected by Cryptolocker malware and have managed to extort almost $1 million from victims.

The ransomware encrypts user's important files and then demands money in return for decrypting them. Researchers from Dell Secureworks estimated that "200,000 to 250,000 systems were infected globally in the first 100 days of the CryptoLocker threat."

The researchers 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.

Unlike other malware, 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.

"By using a sound implementation and following best practices, 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."

According to the researchers, 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.

The security researchers said that the early versions of the malware were distributed through spam emails targeting business professionals rather than home internet users. The malware used the lure of a customer complaint against the recipient to start the infection and encryption process.

According to Secureworks, the brains behind the malware have previous experience  in malware development and distribution, especially of ransomware.

"Based on the duration and scale of attacks, they also appear to have the established and substantial "real world" infrastructure necessary to "cash out" ransoms and launder the proceeds," the researchers said.

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