New Variant of macOS Threat XCSSET Spotted in the Wild

Attackers are wielding a new variant of one of the biggest threats to the macOS platform, malware called XCSSET, Microsoft is warning. The fresh version has so far been seen in a handful of attacks targeting Apple developers, but its reach could grow much longer in the coming weeks.

XCSSET can read and dump data from Safari browsers; inject JavaScript backdoors into websites; steal information from the victim’s Skype, Telegram, WeChat, Notes, and other apps; take screenshots; encrypt files; and exfiltrate data to attacker-controlled systems. The new variant — which features enhanced obfuscation methods, updated persistence mechanisms, and new infection strategies — is the first known update to the malware since 2022, Microsoft Threat Intelligence revealed in a post on X this week.

“These enhanced features add to this malware family’s previously known capabilities, like targeting digital wallets, collecting data from the Notes app, and exfiltrating system information and files,” according to the post.

Researchers at Trend Micro first discovered XCSSET in 2020 when investigating a security incident related to Xcode developer projects; the malware in the past has targeted software developers by exploiting vulnerabilities and then infecting their projects, using this as a means to spread. If one of the infected projects is downloaded and built by another developer, XCSSET also infects their projects, which could in turn be downloaded by others. This gives the malware wormable capability, and the potential for a broader supply chain attack.

Significant Enhancements to macOS Malware

The variant appears to be a significant update to the modular malware, with various new features that make it easier for attackers to spread XCSSET and also obscure their malicious activities.

Enhanced obfuscation methods present in XCSSET use “a significantly more randomized approach for generating payloads to infect Xcode projects,” randomizing both its encoding technique and a number of encoding iterations, according to Microsoft.

And while older XCSSET variants only used xxd (hexdump) for encoding, the latest one also incorporates Base64 and obfuscates module names. This makes it more challenging to determine the intent of the malware’s modules, Microsoft said.

Its operators also have outfitted the variant with two distinct new persistence mechanisms: the “zshrc” method and the “dock” method. In the former method, the malware creates a file named ~/.zshrc_aliases that contains the payload, according to Microsoft. “It then appends a command in the ~/.zshrc file to ensure that the created file is launched every time a new shell session is initiated, guaranteeing the malware’s persistence across shell sessions,” according to the post.

The dock method involves downloading a signed dockutil tool from a command-and-control (C2) server to manage the dock items, and then creating a fake Launchpad application, replacing the legitimate Launchpad’s path entry in the dock with this fake one.

“This ensures that every time the Launchpad is started from the dock, both the legitimate Launchpad and the malicious payload are executed,” according to Microsoft.

The variant also employs new infection methods that determine where the payload is placed in Xcode projects. The method is chosen from one of the following options: TARGET, RULE, or FORCED_STRATEGY, while an additional method involves placing the payload inside the TARGET_DEVICE_FAMILY key under build settings and running it at a later phase.

Advice for macOS Cyber Defenders

Though traditionally not a target for threat actors, the macOS platform has become increasingly more at risk to malware and other security threats in recent years, mainly due to Apple’s growing market share in a shrinking PC market.

To avoid downloading Xcode projects infected with XCSSET, Microsoft recommends that developers and users “always inspect and verify any Xcode projects downloaded or cloned from repositories” that potentially will spread the malware.

“They should also only install apps from trusted sources, such as a software platform’s official app store,” according to Microsoft.

Users of Microsoft Defender for Endpoint on Mac should be protected against XCSSET, including its new variant, the company added, because it can detect all currently known versions of the malware.