More than two dozen malicious programs posing as third-party extensions for top social media sites have been downloaded some 3 million times, surreptitiously redirecting users to phishing sites, displaying advertisements, and stealing data, antivirus firm Avast reports.
"This could be used to gather credentials and other sensitive corporate data from the websites visited by the victim," he says. "We are preparing a technical blog post with more technical information and IoCs, but for now, we can share the ... malicious domains."
The malicious extensions are the latest attempt by cybercriminals to hide code in add-ons for popular browsers. In February, independent researcher Jamila Kaya and Duo Security announced they had discovered more than 500 Chrome extensions that infected millions of users' browsers to steal data. In June, Awake Security reported more than 70 extensions in the Google Chrome Web store were downloaded more than 32 million times and which collected browsing data and credentials for internal websites.
In its latest research, Avast found the third-party extensions would collect information about users whenever they clicked on a link, offering attackers the option to send users to an attacker-controlled URL before forwarding them to their destinations. The extensions also collect the users' birthdates, e-mail addresses, and information about the local system, including name of the device, its operating system, and IP addresses.
The cyberattackers behind the extensions appear to be focused on generating affiliate fees and advertising money, Avast said.
"Our hypothesis is that either the extensions were deliberately created with the malware built in or the author waited for the extensions to become popular and then pushed an update containing the malware," says Rubín.
The attacks were first noticed last month, but if the extensions were originally published without the malicious functionality, they could have easily escaped notice, Avast said. The extensions advertised themselves as add-ons for various social media services, with names such as Video Downloader for Facebook, Vimeo Video Downloader, and Instagram Story Downloader. The extensions' backdoors are well-hidden and the extensions only start to exhibit malicious behavior days after installation, which made it hard for any security software to discover, Rubín says.
One interesting feature of the malicious extensions is they detect whether the user is searching for information about one of its domains. If so, it won't execute any malicious code. In addition, the extensions attempt to determine whether the user might be an analyst or a developer, he says.
"If the malware is running in a browser with many extensions for web development installed, it deactivates itself in order to lower the chances of being detected," Rubín says, adding that it checks whether the user is accessing locally hosted websites or has development tools installed.
The malicious extensions mainly target users in Ukraine, but Avast has also detected a large number of downloads in Brazil, Argentina, and Spain.
Both Google and Microsoft have been notified and are investigating the issues, the company said in a statement.