December 01, 2010, 10:53 AM — Ransomware is making a comeback, plaguing users with extortion demands of up to $120 to return documents or drives to their control, security experts said today.
There appear to be two different campaigns underway, said Chet Wisniewski, a senior security adviser at antivirus vendor Sophos.
"It looks like we're looking at different samples," said Wisniewski, referring to analyses done by Sophos and other security firms, including Kaspersky Lab and CA.
Last week, Sophos came across malware that used malicious PDF documents to exploit one or more since-patched vulnerabilities in Adobe Reader. If successful, the malware sniffed out a wide range of file formats -- including numerous media formats, Microsoft Office and OpenOffice.org document formats and image formats -- then partially encrypted those files to make then unreadable.
To unlock the encryption, the scammers demanded $120, and for good measure threatened users.
"Remember: Don't try to tell someone about this message if you want to get your files back!" an on-screen warning stated. "Just do all we told."
Wisniewski said that as of today, there was no way to decrypt the modified files, although Sophos' researchers were working on a solution. "It's not likely you're going to be able to brute force it," he said, talking about the 1024-bit encryption key that locks the files. "We'll look at how it's generating the keys, and if that's predictable, we may be able to offer some kind of tool that creates the key."
Other security companies have reported seeing the same ransomware, the term used to describe a scheme where hackers plant malware that encrypts files and then displays a message demanding money to unlock the data.
On Monday, Russian antivirus vendor Kaspersky Lab said that it, like Sophos, had analyzed the new ransomware , which it claimed was a stronger version of a long-running bad apple dubbed "GpCode."
Unlike the previous variants, it doesn't delete files after encryption," wrote Kaspersky researcher Vitaly Kamluk on the company's blog. "Instead, it overwrites data in the files, which makes it impossible to use data-recovery software such as PhotoRec, which we suggested during the last attack."
As Kamluk noted, GpCode has a long history, first surfacing six years ago and then reappearing in 2008.
Many of the GpCode variants have been "mostly hot air," Kamluk said.
In 2007, a GpCode Trojan said it had locked encrypted files with a 4096-bit key, a claim Kaspersky later revealed as bogus . The company has had success coming up with tools to help infected users reclaim their files.