How malware developers could bypass Mac’s Gatekeeper without really trying

In September, Ars reported a drop-dead simple exploit that completely bypassed an OS X security feature known as Gatekeeper. Apple shipped a fix, but now the security researcher who discovered the original vulnerability said he found an equally obvious work-around.

Patrick Wardle said the security fix consisted of blacklisting a small number of known files he privately reported to Apple that could be repackaged to install malicious software on Macs, even when Gatekeeper is set to its most restrictive setting. Wardle was able to revive his attack with little effort by finding a new Apple trusted file that hadn't been blocked by the Apple update. In other words, it was precisely the same attack as before, except it used a new, previously unblocked Apple-trusted file. Notably, that file was offered by security company Kaspersky Lab. Late on Thursday, Apple released an update blocking that file, too.

"It literally took me five minutes to fully bypass it," Wardle, who is director of research of security firm Synack, told Ars, referring to the updated Gatekeeper. "So yes, it means that the immediate issue is mitigated and cannot be abused anymore. However the core issue is not fixed so if anybody finds another app that can be abused we are back to square one (full gatekeeper bypass)."

Wardle's attack in September was almost identical to the one he'll be discussing this weekend at the Shmoocon security conference in Washington, DC. It uses a binary file already trusted by Apple to pass through Gatekeeper. Once it's on the other side, the Apple-trusted file executes one or more malicious files that are included in the same folder. The bundled files can install a variety of nefarious programs not trusted by Apple, including password loggers, apps that capture audio and video, and botnet software.

The exploit works with Apple-trusted executable apps that are bundled with, and are programmed to execute, one or more additional apps. The hack works by renaming the Apple-trusted file but otherwise making no other changes to it. Wardle then packages it inside an Apple disk image that contains any executables he wants. Gatekeeper inspects only the first executable file and allows the remaining bundled apps to be executed with no questions asked.

An Apple representative said the new files Wardle privately reported have been blocked using XProtect, an antimalware feature that's a complement to Gatekeeper. The representative said Apple is aware of Wardle's research and continues to work on ways to make Gatekeeper more effective.

Wardle criticized Apple's approach blacklisting only a small number of apps that can be used to exploit the vulnerability rather than correcting the underlying cause of the failure. Researchers frequently compare such approaches to the game of whackamole, since new threats pop up almost as quickly as old ones are retired. He said he has developed a proof-of-concept tool that would trigger a Gatekeeper inspection any time a new process was started. Any process that was started by a file downloaded from the Internet would require the file to be digitally signed by an Apple-trusted developer.