Adblocking extensions with more than 300,000 active users have been surreptitiously uploading user browsing data and tampering with users’ social media accounts thanks to malware its new owner introduced a few weeks ago, according to technical analyses and posts on Github.

Hugo Xu, developer of the Nano Adblocker and Nano Defender extensions, said 17 days ago that he no longer had the time to maintain the project and had sold the rights to the versions available in Google’s Chrome Web Store. Xu told me that Nano Adblocker and Nano Defender, which often are installed together, have about 300,000 installations total.

Four days ago, Raymond Hill, maker of the uBlock Origin extension upon which Nano Adblocker is based, revealed that the new developers had rolled out updates that added malicious code.

The first thing Hill noticed the new extension doing was checking if the user had opened the developer console. If it was opened, the extension sent a file titled “report” to a server at https://def.dev-nano.com/. “In simple words, the extension remotely checks whether you are using the extension dev tools—which is what you would do if you wanted to find out what the extension is doing,” he wrote.

The most obvious change end users noticed was that infected browsers were automatically issuing likes for large numbers of Instagram posts, with no input from users. Cyril Gorlla, an artificial intelligence and machine learning researcher at the University of California in San Diego, told me that his browser liked more than 200 images from an Instagram account that didn’t follow anyone. The screenshot to the right shows some of the photos involved.

Nano Adblocker and Nano Defender aren’t the only extensions that have been reported to tamper with Instagram accounts. User Agent Switcher, an extension that had more than 100,000 active users until Google removed it earlier this month is reported to have done the same thing.

Many Nano extension users in this forum reported that their infected browsers were also accessing user accounts that weren’t already open in their browsers. This has led to speculation that the updated extensions are accessing authentication cookies and using them to gain access to the user accounts. Hill said he reviewed some of the added code and found that it was uploading data.

“Since the added code was able to collect request headers in real-time (through websocket connection I guess), this means sensitive information such as session cookies could be leaked,” he wrote in a message. “I am not a malware expert so I can’t come up with *all* that is possible when having real-time access to request headers, but I do get that it’s really bad.”

Other users reported that sites other than Instagram were also being accessed and tampered with, in some cases, even when the user hadn’t accessed the site, but these claims couldn’t immediately be verified.

Alexei, an Electronic Frontier Foundation senior staff technologist who works on the Privacy Badger extension, has been following the discussions and provided me with the following synopsis:

The gist is that the Nano extensions were updated to surreptitiously upload your browsing data in a remotely configurable way. Remotely configurable means that there was no need to update the extensions to modify the list of websites whose data would be stolen. In fact, the list of websites is unknown at this time as it was remotely configured. There are many reports of users’ Instagram accounts being affected, however.

Evidence collected to date shows that the extensions are covertly uploading user data and gaining unauthorized access to at least one website, in violation of Google terms of service and quite possibly applicable laws. Google has already removed the extensions from the Chrome Web Store and issued a warning that they aren’t safe. Anyone who had either of these extensions installed should remove them from their machines immediately.

Nano Adblocker and Nano Defender are available in the extension stores hosted by both Firefox and Microsoft Edge. Xu and others say that neither of the extensions available in these other locations are affected. The caveat is that Edge can install extensions from the Chrome Web Store. Any Edge users who used this source are infected and should remove the extensions.

The possibility that the extensions may have uploaded session cookies means that anyone who was infected should at a minimum fully log out of all sites. In most cases this should invalidate the session cookies and prevent anyone from using them to gain unauthorized access. Truly paranoid users will want to change passwords just to be on the safe side.

The incident is the latest example of someone acquiring an established browser extension or Android app and using it to infect the large user base that already has it installed. It’s hard to provide actionable advice for preventing this kind of abuse. The Nano extensions weren’t some fly-by-night operation. Users had every reason to believe they were safe until, of course, that was no longer the case. The best advice is to routinely review the extensions that are installed. Any that are no longer of use should be removed.