The Annoying MacOS Threat That Won't Go Away

Mac users generally tend to be better protected against malware and other online threats than Windows users. That doesn't mean they are immune, however.

Shlayer, a malware tool for distributing unwanted advertisements on MacOS systems, is a case in point. Since first surfacing in February 2018, the malware has emerged as the most widely distributed threat on the MacOS platform. Among those most impacted by the malware are MacOS users in the US, Germany, France, and the UK.

Kaspersky, which has been tracking Shlayer for some time, this week described it as currently infecting at least one in 10 Mac users. Though the malware has little to separate it from other malicious software from a purely technical standpoint, it continues to remain as active as when it first surfaced.

According to Kaspersky, in 2019 Shlayer-related attacks accounted for nearly 30% of all attacks on MacOS devices protected by the company's products. Worse, almost all of the other remaining top 10 MacOS threats were adware products distributed by Shlayer. Among them were AdWare.OSX.Bnodlero, AdWare.OSX.Geonei, AdWare.OSX.Pirrit, and AdWare.OSX.Cimpli, the security vendor noted.

One reason for Shlayer's continuing prevalence is the manner in which it is being distributed. Currently, over 1,000 "partner" websites distribute Shlayer on behalf of the malware's authors. Unsuspecting users who arrive on these sites — many of which hawk pirated content — are typically redirected to fake Flash Player update pages from where the malware gets downloaded on MacOS systems. The partner sites get paid for each download.

"The affiliate network is an intermediate link between the creators of the Trojan and those who are willing to distribute it for a fee," says Vladimir Kuskov, head of advanced threat research and software classification at Kaspersky. "The role of partner sites is to attract users to their resource and instill the need to download and run a malicious file." 

Shlayer is being distributed in a variety of other ways, including malicious links to fake Adobe Flash update sites embedded in article references on Wikipedia and video descriptions on YouTube. Kasperksy researchers have so far found links to at least 700 malicious domains for downloading Shlayer hidden in a variety of legitimate sites.

Users looking for pirated content are more likely to get infected, Kuskov says. At the same time, even those clicking on links below a YouTube video or while searching for something on Wikipedia are at risk, he notes.

Annoying but Less Harmful
Shlayer is distributed under the guise of a Flash Player installer and, at first sight, looks pretty legitimate. Like other installers, the malware installs software, except that in this case it installs adware instead of legitimate software.

One alleviating fact is that Shlayer does not load on its own. Users have to actively click and download the installer for it to load on a system. But those distributing the malware have employed a variety of social engineering tricks to redirect users to fake Flash Player update sites to get users to download the malware, Kuskov notes.

Shlayer itself is also not persistent on an infected system. A user who discovers the malware can simply delete the installation file to get rid of it, he says.

The real problem is the adware it installs. "It's important to understand that Shlayer itself performs only the initial stage of the attack — it penetrates the system, loads the main payload, and runs it," Kuskov says. The installed adware is not easy for the average user to remove. It can be especially challenging because of the multiple adware family Shlayer can install on a single system.

Also, some adware like AdWare.OSX.Cimpli can intercept a user's HTTP and HTTPS traffic and inject code into the Web pages requested by the user. "In theory, that means that Cimpli can steal any data entered by the user on the Web page," Koskov said.

Even so, Shlayer is relatively innocuous compared to other more destructive malware. It is also an example of how attackers are constantly looking for ways to earn money by attacking MacOS systems.

The threat landscape for Apple devices is changing, and the amount of malicious and unwanted software is growing, Kaspersky said. Since at least 2012, the volume of malicious and potentially unwanted files targeted at MacOS has been doubling each year. 

"But instead of full-fledged malware, MacOS users increasingly receive annoying, but less harmful, adware," Kuskov says. "It seems that this way of monetizing an infection allows attackers to make more profit and save on expenses."

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