The company disclosed the effort after several cyber security firms reported finding a malicious program dubbed XcodeGhost that was embedded in hundreds of legitimate apps.

It is the first reported case of large numbers of malicious software programs making their way past Apple’s stringent app review process. Prior to this attack, a total of just five malicious apps had ever been found in the App Store, according to cyber security firm Palo Alto Networks (PANW)

The hackers embedded the malicious code in these apps by convincing developers of legitimate software to use a tainted, counterfeit version of Apple’s software for creating iOS and Mac apps, which is known as Xcode, Apple said.

“We’ve removed the apps from the App Store that we know have been created with this counterfeit software,” Apple spokeswoman Christine Monaghan said in an email. “We are working with the developers to make sure they’re using the proper version of Xcode to rebuild their apps.”

She did not say what steps iPhone and iPad users could take to determine whether their devices were infected.

Palo Alto Networks Director of Threat Intelligence Ryan Olson said the malware had limited functionality and his firm had uncovered no examples of data theft or other harm as a result of the attack.

Still, he said it was “a pretty big deal” because it showed that the App Store could be compromised if hackers infected machines of software developers writing legitimate apps. Other attackers may copy that approach, which is hard to defend against, he said.

“Developers are now a huge target,” he said.

Researchers said infected apps included Tencent’s popular mobile chat app WeChat, car-hailing app Didi Kuaidi and a music app from Internet portal NetEase.

In a statement released later, Tencent said that the malware had only infected its relatively old version of WeChat – v6.2.5, and that newer versions hadn’t been affected.

“A preliminary investigation into the flaw has revealed that there has been no theft and leakage of users’ information or money, but the WeChat team will continue to closely monitor the situation,” Tencent said.

The tainted version of Xcode was downloaded from a server in China that developers may have used because it allowed for faster downloads than using Apple’s U.S. servers, Olson said.

Chinese security firm Qihoo360 Technology said on its blog that it had uncovered 344 apps tainted with XcodeGhost.

Apple (AAPL) declined to say how many apps it had uncovered.

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