Apple has removed a top-rated ad blocker from its official Mac App Store after a security researcher discovered it to be quietly collecting and sending detailed user-browsing histories to a domain based in China.

The $4.99 Adware Doctor was until Friday morning listed as the fourth highest-selling app and top-grossing software product in the category of "paid utilities" in the Mac App store.

Its stated purpose is to protect users from malware and having adware served on their browsers. But the app has also been silently exfiltrating browser histories and other sensitive data from systems on which it is installed, says Patrick Wardle, founder and chief research officer of Digita Security and creator of Objective-See, a website for Mac security tools. 

"It also collects system info, a list of the user's currently running processes, and also certain types of files that users have downloaded. It tries to access the user's App Store history — but I believe a bug causes this to fail," he says.

In a blog post Friday, Wardle said he had contacted Apple about the issue one month ago and informed the company about the app's behavior. Even two years ago, in 2016, another security researcher had raised concerns about the same application trying to trick users into granting it administrative privileges on their devices, he said. But until Friday morning, Apple had not removed the app despite promising to investigate, Wardle said.

"There is rather a MASSIVE privacy issue here," Wardle wrote. "The fact that [the] application has been surreptitiously exfiltrating users' browsing history, possibly for years, is, to put it mildly, rather f#@&'d up!"

Apple did not offer any explanation for why it might have waited so long to act. But according to the company, the app has been removed and the issue, which allowed Adware Doctor to access and exfiltrate privacy-sensitive content like browser history and cookies, has been mitigated in Mojave, the next version of the macOS.

A quick check by Dark Reading shows that the app is indeed no longer available for download from the app store for US users, at least.

Wardle, a macOS security veteran and frequent presenter at major security conferences like Black Hat, said he decided to investigate Adware Doctor after another security researcher tweeted an alert about the application stealing private user files last month.

After purchasing a copy of Adware Doctor, Wardle said he used a combination of static and dynamic analysis and quickly found the application to be behaving in a manner completely inconsistent with its stated purpose. Wardle discovered that when a user gives the application permission — by clicking OK — to remove extensions, cookies, and caches from his or her browser, the app ends up surreptitiously stealing the user's browser history.

Apple apps downloaded from the company's official Mac application stores typically are sandboxed, meaning that it is constrained in the kinds of files and user information it can access, Wardle said.

But since Adware Doctor is a malware detection and removal tool, it needs access to user data and files not normally available to other applications. When the application is first launched, it asks the user for permission to access files in his or her home directory and all files and directories under it so the files can be inspected for malware. Once a user has granted that permission, the tool — like any anti-malware product — has free access to files on the devices.

In Adware Doctor's case, however, the app has been using the access to collect and exfiltrate data. "While some (such as a process list), perhaps have a legitimate reason for being collected by an anti-malware or anti-adware product, others such as the user's browsing history seem to be a blatant violation of the user's privacy," Wardle noted.

According to Apple, the issue has been mitigated in the next release of macOS via a sandboxing mechanism that ensures an app won't be able to access privacy-sensitive content after a user grants it permission to the home directory.

The fact that an app like this was allowed on Apple's official app store — supposedly the most secure source for Mac software — should be a wake-up call for the company, says Matt Lock, director of sales engineering at Varonis.

"This isn't the first time an app has collected data for questionable reasons, and it will not be the last," he says. "The irony is that consumers downloaded the app to reduce adware, but got stuck with spyware in the process."

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