Malicious Chrome and Edge add-ons had a novel way to hide on 3 million devices

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In December, Ars reported that as many as 3 million people had been infected by Chrome and Edge browser extensions that stole personal data and redirected users to ad or phishing sites. Now, the researchers who discovered the scam have revealed the lengths the extension developers took to hide their nefarious deeds.

As previously reported, the 28 extensions available in official Google and Microsoft repositories advertised themselves as a way to download pictures, videos, or other content from sites including Facebook, Instagram, Vimeo, and Spotify. Behind the scenes, they also collected user’s birth dates, email addresses, and device information and redirected clicks and search results to malicious sites. Google and Microsoft eventually removed the extensions.

Researchers from Prague-based Avast said on Wednesday that the extension developers employed a novel way to hide malicious traffic sent between infected devices and the command and control servers they connected to. Specifically, the extensions funneled commands into the cache-control headers of traffic that was camouflaged to appear as data related to Google analytics, which websites use to measure visitor interactions.