Malware infects users through trusted Chrome extensions

Shady online marketers have a new trick up their sleeve: buying trusted Chrome browser extensions with a large installed-base and exploiting their auto-update status to push out adware.

To spread pesky 'potentially unwanted programs' like adware, online ad affiliate marketers are exploiting Google’s “autoupdating” policy for Chrome extensions, which is actually designed to improve user security by automatically installing fixes to Chrome extensions without asking the user's permission.

Over the weekend Google removed two Chrome extensions from its web store for breaching its developer policies after public outcry in response to a developer who appears to have unwittingly sold his extension to an adware vendor.

Amit Agarwal, the maker of one of the removed extensions, “Add to Feedly”, on Friday explained in a blog post why he regretted selling his product.

Agarwal built the Feedly RSS extension to patch-up a missing feature in Feedly, the RSS feed service that stepped up to replace Google Reader after Google killed it off last year. The extension allowed Chrome users to add a website to Feedly with one-click.

At last count it had more than 30,000 users, according to Agarwal, who says he sold it after being offered a four-figure sum.

“I had no clue about the buyer and was also curious to know why would anyone pay this kind of money for such a simple Chrome extension,” wrote Agarwal.

A month later, and still without a clue who the buyer was, he found out why: the new owners updated the extension in Google’s Chrome store, which packaged up unusual advertising into the extension.

“These aren’t regular banner ads that you see on web pages, these are invisible ads that work the background and replace links on every website that you visit into affiliate links. In simple English, if the extension is activated in Chrome, it will inject adware into all web pages,” wrote Agarwal.

The extension’s behaviour appears to violate a number of Google’s Chrome web store developer policies, but chief among them are that “ads do not interfere with any native ads or functionality of the website”.

The practice of buying trusted Chrome extensions with a large installed-base and using them for nefarious purposes apparently isn’t that uncommon.

Following Agarwal’s post, a developer (gemusan) behind the Chrome extension Honey, which offers a one-click coupon-code service, claimed in a Reddit AMA they had been approached multiple times by malware companies interested in buying the extension. Unlike Agarwal, they refused.

But gemusan added that data collection companies have offered “6 figures a month” for Honey to share the data on its 700,000 users.

The other app Google removed was “Tweet This Page” following a Ars Technica piece that cited similar behavioural changes from an extension that was once widely trusted.

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