A campaign has been detected that uses alerts about out of date security certificates to fool unsuspecting web users into downloading malware. The warnings have been placed on several legitimate websites that have been compromised by cybercriminals.

When visitors arrive on the compromised websites they are presented with an error message that tells them the digital security certificate has expired and they need to download an updated one. Downloading and running the file results in malware being installed on the user’s device – The Mokes backdoor (aka Smoke Loader) and the Buerak malware downloader.

This tactic of malware distribution is nothing new. Cybercriminals have been using this method for years to fool users into downloading malware under the guide of a browser or Flash update, but this is the first time that expired website security certificate error messages have been used for malware distribution.

The NET::ERR_CERT_OUT_OF_DATE error message is delivered via an iframe that is overlaid over the website using a jquery.js script. The warning matches the size of the original page, so it is all the visitor sees when they land on the website. If they want to be able to view the content, they are told they should update their security certificate to allow the connection to the website to be made. The content of the message is loaded from a third-party web resource, but the URL displayed is of the legitimate website the user has navigated to.

It is not clear how the threat actors compromised the websites. Oftentimes websites are compromised using brute force tactics to guess weak passwords, or exploits are used for vulnerabilities that have not been patched. It is also unclear how people are being sent to the websites. Typically, traffic is sent to the compromised websites through phishing scams or malicious web adverts (malvertising), but visitors could simply navigate to the website through a Google search.

Since the warnings are appearing on legitimate websites, users may think the messages are genuine. One of the compromised websites is the official website of a zoo, another identified by Kaspersky Lab was for a legitimate auto parts dealer. The campaign has been active for at least two months.

Protecting against this method of malware distribution requires a combination of security solutions. Up-to-date anti-virus software is a must to ensure that any files downloaded to business computers are scanned for malware. A web filtering solution such as WebTitan will also provide protection by preventing users from visiting compromised websites that are being used to distribute malware and also blocking downloads of dangerous file types.

Contact TitanHQ today to find out more about web filtering and how you can protect your business from web-based attacks.