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Ransomware can be brutal to treat.

After a user falls victim to the software program that encrypts a person’s files until the victim pays a ransom, it could take quite a bit of time to unlock the computer. That is, of course, unless the malware leaves the decryption key on the victim’s computer.

CryptoDefense is one of a family of malware programs that scramble a person’s files until a pricey ransom is paid, a long-running but still profitable scam, said researches at Symantec that analyzed the program.

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CryptoDefense uses Microsoft’s infrastructure and Windows API to generate the encryption and decryption keys, Symantec researchers wrote in a blog post.

Cyber Security

Files end up encrypted by CryptoDefense using a 2048-bit RSA key. The private key needed to decrypt the content goes back to the attacker’s server until the victim pays the ransom.

But CryptoDefense’s developers apparently did not realize the private key is also on the user’s computer in a file folder with application data.

“Due to the attacker’s poor implementation of the cryptographic functionality they have quite literally left their hostages with a key to escape,” Symantec said.

The decryption key may be under the door mat, but it’s doubtful an average user infected with CryptoDefense would have the technical skills to figure it out.

CryptoDefense has been seen sent out in spam messages, masquerading as a PDF document. If a user installs it, the malware tries to communicate with four domains and uploads a profile of the infected machine, Symantec said.

It then encrypts files, inserting an additional file in folders with encrypted ones with instructions for how to free the files. The attackers have created a “hidden” website to receive payments using the TOR (The Onion Router) network, an anonymity tool.

TOR offers users a greater degree of privacy when browsing the Internet by routing encrypted traffic between a user and a website through a network of worldwide servers. TOR can also host websites on a hidden network only viewed through a web browser configured to use it.

The extortionists sometimes demand $500 within four days. If the victim doesn’t pay in that time frame, the ransom doubles.

Since the ransom is payable in bitcoin, Symantec looked at the virtual currency’s public ledger, called the blockchain, to see how many bitcoins have flowed into their coffers.

The company estimated the cybercriminals received more than $34,000 worth of bitcoin in just a month, showing the effectiveness of their scam.

Symantec said it has blocked 11,000 CryptoDefense infections in more than 100 countries, with the majority of those infection attempts in the U.S., followed by the U.K., Canada, Australia, Japan, India, Italy and the Netherlands.

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