A sequence of interconnected bugs could allow hackers to hijack devices running on macOS using little more than an infected Office document and a .zip file, an expert has warned.
The vulnerability was identified by ex-NSA researcher Patrick Wardle, now working for security firm Jamf, who found that even fully-patched macOS Catalina systems were at risk.
The exploit uses a rigged Office document, saved in an archaic format (.slk), to trick the target machine into allowing Office to activate macros without consent and without notifying the user.
The attack then takes advantage of two further vulnerabilities in order to seize control of the machine. By including a dollar sign at the start of the filename, a hacker can break free of the restrictive Office sandbox, while compressing the file within a .zip folder bypasses macOS controls that prevent downloaded items from accessing user files.
Apple’s macOS has long enjoyed a stellar reputation from a security and data privacy perspective, but Apple devices are by no means unhackable. This misconception, Wardle suggests, could lead both users and security personnel to underestimate the potential threat level.
“In the world of Windows, macro-based Office attacks are well understood (and frankly are rather old news). However, on macOS, though such attacks are growing in popularity and are quite en vogue, they have received far less attention from the research and security community,” he wrote in a recent blog post.
“Triggered by simply opening a malicious (macro-laced) Office document, no alerts, prompts, nor other user interactions were required in order to persistently infect even a fully-patched macOS Catalina system.”
The researcher did concede that the attack requires the target individual to log in and out of their device twice, with a further step in the process fulfilled with each login. However, this does not necessarily make the attack any less feasible for criminals, who are content to play the long game.
According to Wardle, Apple did not respond to his disclosure. Microsoft, for its part, has conducted an investigation into the issue and verified the researcher’s findings.
“[The company has] determined that any application, even when sandboxed, is vulnerable to misuse of these APIs. We are in regular discussion with Apple to identify solutions to these issues and support as needed,” said a Microsoft spokesperson.
The vulnerabilities have now been patched with the latest versions of Office for Mac. Users are therefore advised to update their Office software and operating system as soon as possible, to shield against attack.