While precise statistics are hard to come by, researchers at Symantec say they are seeing hundreds of thousands of spam email messages a day distributing the threat, with hundreds of infections per day. Ransomware scams are still in vogue, but where CryptoLocker makes its mark is its use of asymmetric encryption -- and don't be surprised if security vendors are not the only ones taking notice. Other attackers will move in this direction as well.
"It's not a revolution, but a natural evolution," says Lance James, head of intelligence at Vigilant by Deloitte. "Putting it bluntly, I think we expected this sooner and should be surprised it took so long. Yes, others will move in this direction, or they will sell CryptoLocker base code to enable the development of related ransomware, thus spawning in the underground a new widespread standard, if you will, for ransomware."
Unlike other ransomware, CryptoLocker's authors have properly implemented an asymmetric system (2048 bit RSA) and 256 bit AES-CBC using the native Microsoft Windows crypto system, which is the basis for legitimate tools such as BitLocker, he explains.
"Most encryption uses a symmetric [one key] key system or simply locks access to the files but does not fully encrypt the data," James says. "A reverse engineer can simply build tools that recover the key or leverage knowledge of how the software works to unlock the files. Encryption mechanisms found in other ransomware are of a homebrew variety -- they include errors and vulnerabilities that reversers and infosec professionals can identify, thereby enabling the creation of workarounds to neutralize the intent of the ransomeware."
Once on the system, the malware can encrypt files located within shared network drives, USB drives, external hard drives, network file shares, and even some cloud storage drives. If one computer on a network becomes infected, then mapped network drives could become infected as well. CryptoLocker then connects to the attackers' command-and-control server to put the asymmetric private encryption key "out of the victim's reach," according to a warning from US-CERT.
"I wouldn't say it is necessarily any more sophisticated, but perhaps just better executed," notes Chet Wisniewski, senior security adviser at Sophos. "They aren't pretending to be the cops. They are simply encrypting your files, demanding money, and mostly honoring their end of the bargain -- simple, straight to the point of extortion."
Ransomware that was popular early in the year didn't even perform encryption -- it just locked the screen with a "scary law enforcement message and demanded money," he adds.
Ransomware can be a very profitable type of operation. In a paper (PDF) released last year, Symantec estimated that one particular group was extorting nearly $400,000 a month from victims.
Ransomware attacks have been on the uptick for the past several quarters. According to McAfee's third quarter threat report (PDF), more than 312,000 new, unique samples were detected during that three-month period -- less than the previous quarter, but still the second-highest figure the firm has seen.
"Ransomware is not new, but evidently its creators are making money from it, and that is the key to its persistence," observes Roger Thompson, chief emerging threat researcher at ICSA Labs. "In fact, it seems to have replaced fake antivirus as a common form of monetization. I can't remember the last time I saw a fake AV. You'd think that the interaction required to pass money would get more people caught, but I suspect it is a function of small amounts combined with multiple jurisdictions. In other words, it seems too much trouble for the police to be bothered."
The good news, Wisniewski notes, is that businesses and home users can take a number of precautions.
"Keep your antivirus up to date and be sure not to allow EXE files to come in as email attachments," he says. "Block EXE files inside of archives, like ZIP and RAR, at the mail gateway. CryptoLocker is primarily being installed through existing Zeus/ZBot infections, and Zeus comes in through email and drive-by installs on booby-trapped websites. Do your backups. Don't pay the crooks or depend on their honesty to decrypt your files. Ensure the important information in your organization is backed up regularly."
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