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Malware Increasingly Being Signed With Stolen Certificates

Cybercriminals target legitimate software vendors looking for the keys needed to sign software, helping them defeat security measures

Earlier this year, a Trojan infected a machine at software maker Blackbaud. But rather than steal money or source code or customer details, the attack targeted another valuable but often overlooked piece of information: the company's private keys for signing its software.

Blackbaud, which makes software to help nonprofits run their businesses, discovered the theft when a security firm approached the company and revealed it had encountered malware signed with the company's private keys. Ironically, Blackbaud had already been working to bolster its corporate security, says Jana Eggers, senior vice president of products and marketing for the Charleston, S.C.-based software company.

"Companies are very aware of their customer information, but there are other pieces of information that need to be protected," Eggers says. "A virus can be tweaked to search for something different, and a security certificate is not what you think of protecting in the same way as customer information."

Blackbaud is not alone. Cybercriminals are increasingly targeting developers' systems to steal the private keys used to sign software. Programs signed with a digital certificate are considered safer by the operating system and security software, and the authors of malicious software have caught on.

Perhaps the most high-profile piece of malware to use digital certificates is Stuxnet. The Trojan infected industrial control systems in 2009 and 2010 and is widely considered to have been a nation-state attack on Iran's nuclear processing facility. The first stage of the Stuxnet attack used code signed by certificates belonging to two Taiwanese companies to appear more innocuous to security software.

In 2010, a version of the well-known Zeus banking Trojan used a digital signature belonging to software security firm Kaspersky Labs to lessen the chance of the program being identified as malware. And a third attack in the same year used an Adobe flaw and was signed with a certificate from a credit union.

"They are going right after the keys to the kingdom," says Jeff Hudson, CEO at Venafi, a maker of enterprise key and certificate management software. "They are not trying to siphon off pennies; they are going right for the heart."

Security firm Symantec has documented a piece of malicious software known as Infostealer.Nimkey that is designed to steal private keys and keystrokes.

Thousands of certificates have been stolen and are being used by malware, says Yuval Ben-Itzhak, chief technology officer for security firm AVG. In a quarterly security report released on Tuesday, AVG documented two attacks that used stolen certificates: one belonging to Blackbaud and the other to German gaming company GameForge Productions. GameForge did not respond to a request for comment.

In the first half of 2011, three times as many certificates were used to sign malware than the first half of 2010, according to AVG's report. Companies need to better protect their certificates, and security software should become more skeptical of signed code, Ben-Itzhak says.

"The vendors are not rushing to revoke stolen certificates," he says. "Blindly trusting certificates today is not a good practice."

For large companies, certificate management systems such as Venafi's can make sure that companies do not suffer from certificates that accidentally expire or that are stored in insecure places. Many times, developers just keep the private keys needed to sign software on their work systems, Venafi's Hudson says.

"We advocate people to, number one, know what you've got on your network," he says. "Number two, make sure that you separate out private keys from pubic keys and be able to replace them immediately, if you are compromised."

Treating digital certificates as valuable as the attackers is a sentiment with which Blackbaud's Eggers can now certainly agree. "More people need to be aware of the threat," she says.

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