A malicious software program that encrypts a person's files until a ransom is paid has a crucial error: it leaves the decryption key on the victim's computer.
Symantec analyzed a program called CryptoDefense, which appeared late last month. It's one of an extensive family of malware programs that scramble a person's files until a pricey ransom is paid, a long-running but still profitable scam.
CryptoDefense uses Microsoft's infrastructure and Windows API to generate the encryption and decryption keys, Symantec wrote on its blog.
Files are encrypted by CryptoDefense using a 2048-bit RSA key. The private key needed to decrypt the content is sent back to the attacker's server until the ransom is paid.
But CryptoDefense's developers apparently did not realize that the private key is also stashed 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 wrote.
The decryption key may have been left 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 wrote.
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 be used to host websites on a hidden network that can only be viewed through a web browser configured to use it.
The extortionists demand either US$500 or €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.
Send news tips and comments to [email protected] Follow me on Twitter: @jeremy_kirk