Google has removed "Add to Feedly" and "Tweet This Page" from the Chrome web store after users complained that both extensions silently served them ads while using the browser.
Add to Feedly allowed users to add any RSS feed on the web page they were browsing to the Feedly reader. The second extension allowed users to share pages to Twitter.
Both were originally built by independent developers and sold off to third parties after amassing thousands of users.
The new owners then quietly introduced updates which meant the extensions began serving ads without asking the user's permission - resulting in lower user ratings and complaints.
Buying users
Using a Chrome extension to inject more ads into web pages is against Google's policies, but appears to be difficult to police.
Extensions can update in the background and, in some cases, introduce new features without asking the user's permission. It can also be difficult to connect the dots between an influx of ads and a malicious extension.
The developer behind Add to Feedly, Amit Agarwal, sold his extension off after being offered "four figures".
But he found the extension had been subsequently updated to alter normal links to affiliate links. "In simple English, if the extension is activated in Chrome, it will inject adware into all web pages," he said.
Agarwal said it had been a "mistake" to sell Add to Feedly, and apologised to the extension's existing users.
Meanwhile, Tweet this Page began hijacking Google web searches after being sold, according to an Ars Technica report.
Buying up users
Several other developers have come forward claiming to have been courted by data-focused buyers.
The developer of discount service Honey, George Ruan, said malware vendors, adware developers and data collection companies had all been interested in his company's user base. The Chrome extension has almost 270,000 users.
"[One] data collection company did throw a dollar figure our way. It's over six figures a month," he said. He added that doing anything "shady" would kill user trust.
Google has yet to issue any advice on avoiding malicious extensions, though it's possible to end the problem simply by removing the add-on.
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