Apple released detection signatures for the iWorm backdoor malware in early October, but researchers now say that the fix doesn't address existing infections, and also leaves open a door to potential new infections, as well.
XProtect might not protect
XProtect is Apple's antivirus system for the OSX operating system. But it only works on new infections, not existing ones, says security researcher Patrick Wardle at Menlo Park, Calif.-based Synack Inc.
"When iWorm came out initially, it didn't have a signature yes, so XProtect didn't protect it," Wardle says.
"And then Apple added the signature but only for the installer application -- so if you were already infected, the signatures wouldn't help you at all."
To help Mac owners and system administrators who are still dealing with the aftermath of the infection, Wardle has released a free, open source tool called Knock Knock that looks for programs that run automatically whenever a computer starts up.
Knock Knock automatically eliminates the most common system applications, and just leaves a list of of potentially suspicious programs.
The iWorm malware, for example, would be listed as "JavaW."
According to Dr. Web, the Russian antivirus company that originally discovered the iWorm malware, there were 18,519 unique IP addresses with infected computers as of September 29. The malware turns infected computers into bots that connect up to a system of Command & Control servers via posts on Reddit -- posts which have since been taken down.
Back door still wide open
An even bigger problem could be that the back door that allowed the iWorm malware to slip through in the first place is still wide open.
Apple's first line of defense, Gatekeeper, checks if newly installed programs are from a trusted source, and warns users if not.
However, the way it knows that a program is newly installed is via flag called a "quarantine attribute" that is voluntarily set by applications that download software, says Wardle.
The major browsers, for example, set this flag correctly. But BitTorrent clients, peer-to-peer messaging systems, and download accelerators might not, he says.
In fact, this is how the iWorm malware originally spread. The malware was attached to highly desirable cracked software such as Photoshop and Microsoft Office and distributed via BitTorrent networks, he says.
To add insult to industry, XProtect also checks that flag to see if software was recently installed before scanning it.
"When you first hear about it, you would think, nobody would possibly architect a security system like this," says Jake Williams, instruction and course author at the SANS Institute and principal consultant at August, Georgia-based Rendition Infosec, LLC.
But it's not a problem unique to Macs, he added.
"We've seen it already play out on Windows, where it's called a zone identifier," he says.
There is nothing that Apple can do to force software developers to set this flag, he confirms. "Download accelerators tend not to set this flag. Torrent clients almost never set that flag."
Look beyond signatures
Even if the signature-based approach was comprehensive, worked for all software downloaders, and applied to existing software as well as to new infections, security experts recommend that enterprises don't put too much reliance on it.
"As soon as Apple releases an update to the XProtect signatures, the malware authors also get those signatures and can immediately change their malware to bypass them," says Williams. "This is a trivially easy process and happens within minutes of the signature being released."
Enterprises need to look to heuristic or behavior-based approaches to combat malware, says Ian Amit, vice president at ZeroFox, a Baltimore-based security firm.
"Security officers who can apply a behavioral fix to the problem will be able to identify already infected devices, and combat similar malware," he says.
User education would help, as well, he says, since users download this malware voluntarily.