Since October, millions of internet users have been exposed to malicious code served from the pixels in tainted banner ads meant to install Trojans and spyware, according to security firm ESET.
The attack campaign, called Stegano, has been spreading from malicious ads in a “number of reputable news websites,” ESET said in a Tuesday blog post. It’s been preying on Internet Explorer users by scanning for vulnerabilities in Adobe Flash and then exploiting them.
The attack is designed to infect victims with malware that can steal email password credentials through its keylogging and screenshot grabbing features, among others.
The attack is also hard to detect. To infect their victims, the hackers were essentially poisoning the pixels used in the tainted banner ads, ESET said in a separate post.
ESET The malicious banner ad.
The hackers concealed their malicious coding in the parameters controlling the pixels’ transparency on the banner ad. This allowed their attack to go unnoticed by the legitimate advertising networks.
Victims will typically see a banner ad for a product called “Browser Defense” or “Broxu.” But in reality, the ad is also designed to run some Javascript that will secretly open a new browser window to a malicious website designed to exploit vulnerabilities in Flash that will help carry out the rest of the attack.
Hackers have used similar so-called malvertising tactics to secretly serve malicious coding over legitimate online advertising networks. It’s an attack method that has proven to be a successful at quickly spreading malware to potentially millions.
The makers behind the Stegano attack were also careful to create safeguards to prevent detection, ESET said. For instance, the banner ads will alternate between serving a malicious version or a clean version, depending on the settings run on the victim’s computer. It will also check for any security products or virtualization software on the machine before proceeding with the attack.
ESET declined to name the news websites that were found unknowingly displaying the malicious ads, but cautioned that the attack was widespread, and could have been hosted through other popular sites as well.
The security firm is advising that users upgrade their computers and software to the latest security patches, to avoid becoming victims.