Yet determining whether a file, script, or executable binary is malicious is not an easy task, and often defensive technologies have missed the more subtle or sophisticated attacks that insert exploit code as part of a common file format, such as Office or portable document format (PDF) files. While some companies banned scripts as a way to fend off macro viruses in the late '90s and early 2000s, now security firms are finding ways to sanitize common file formats, removing or modifying executable code within the files to stymie attackers.
Last week, for example, security firm Symantec announced it had added a feature, known as Disarm, to its messaging-security gateway to remove executable code from Office and PDF files. Such sanitizing would have rendered harmless 98 percent of the zero-day exploits used in targeted attacks in the last year, says Darren Shou, director of Symantec Research Labs.
"With these previous technologies, you had to make a determination about the goodness and badness of something, and then you had to make the determination of whether to allow it to go through or not," he says. "What turns this idea on its head is that we are not going to make any determination, and that is the fundamental difference."
Targeted attacks, often referred to as advanced persistent threat-type attacks, are designed to evade traditional signature-based detection and often operate under the threshold of behavioral-based pattern recognition systems. Commonly, attackers will insert exploit code in common files attached to e-mail messages or link to the files, which are then downloaded from the Web.
Yet the process of sanitizing files can remove the vector of attacks, if not the attack itself. The technique promises to defeat a class of attacks that traditional passive defenses have had a hard time beating.
Symantec is not the only company using sanitization to clean files of executable code. For the past four years, Israeli security firm Votiro has used a similar concept to render ineffective any exploit code in common file formats. Rather than try to remove exploitable code, the company makes what it calls "microchanges" to particular portions of the file to block any potential malicious code, says Itay Glick, founder and CEO of Votiro.
"We are not looking for bad stuff because we don't know what the bad stuff will look like," he says. "What we do is look at the file itself and make microchanges to create interference and prevent the exploit from running correctly."
In addition to Office and PDF files, the company currently sanitizes image files. In the future, it plans to also render audio and video files safe as well, says Glick.
[Overly accommodating platforms and protocols let attackers use inputs like code, essentially allowing attackers to program an unintentional virtual machine. See Taming Bad Inputs Means Taking Aim At 'Weird Machines'.]
Microsoft has also had a history of finding ways to make its software more resistant to exploits, adding technologies such as address space layout randomization (ASLR) to make exploits less reliable.
Sanitizing files should be used as another layer in a company's defense-in-depth strategy, says Amanda Grady, senior product manager for Symantec. Companies that have multiple employees using a single account or that are prone to targeted attacks may want to go beyond just blocking executable files to blocking any scripts or code embedded within files, she says.
"If they want to have a clean room type of environment, where they don't want to let any executable content in over e-mail, they could do that with this technology," Grady says.
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