Angler Exploit Kit to redirect visitors to malicious websites hosting the Asprox malware.
AppNexus servers process 16 billion ad buys per day, making it the biggest reach on the open web after Google. Back in May, AppNexus was serving malicious ads targeting Microsoft's Silverlight platform. The world's largest Internet Video Subscription service Netflix runs on Silverlight, and because of its popularity, hackers have been loading exploit kits with Silverlight.
As part of this campaign, users of several high-profile websites including Java.com, Deviantart.com, TMZ.com, Photobucket.com, IBTimes.com, eBay.ie, Kapaza.be and TVgids.nl, last week were redirected to websites serving malicious advertisements that infected visitors by installing botnet malware on their computer, said security company Fox-IT.
"These websites have not been compromised themselves, but are the victim of malvertising. This means an advertisement provider, providing its services to a small part of a website, serves malicious advertisement aimed at infecting visitors with malware," researchers at Fox-IT said in a blog post.
Angler exploit kits are available on the underground black forums and are used in various malicious campaigns to own websites and redirect users off to websites hosting banking malware and other types of malicious code in order to victimize them.
"Please note, a visitor does not need to click on the malicious advertisements in order to get infected. This all happens silently in the background as the ad is loaded by the user's browser," researchers warned.
According to the Researchers, Angler first checks whether the victim's browser supports an outdated versions of Java, Adobe Flash Player or Microsoft Silverlight, and then silently install a variant of the Asprox botnet malware.
Asprox is generally a spam botnet that was involved in multiple high-profile attacks on various websites in order to spread malware. The malware recently has been modified for click-fraud and cyber criminals are using it to spread malware through email attachments with exploit kits. It also has other malicious functionality including scanning websites for vulnerabilities and stealing log-in credentials stored on computers.
"Asprox has gone through many changes and modifications which includes spam modules, website scanning modules and even credential stealing modules," Fox-IT said. "This history and current events show Asprox is still actively being developed and used."
Once visited on a site hosting the malicious ad, users are redirected in the background to ads[.]femmotion[.]com, which then redirects to the exploit kit on a number of other domains, the gloriousdead[.]com and taggingapp[.]com.
"All the exploit kit hosts were observed using port 37702. Running exploit kits on high ports at best prevents certain network tools from logging the HTTP connections, as these are typically configured to monitor only HTTP ports," Fox-IT said. "It does mean this exploit kit is blocked on a lot of corporate networks as they do not allow for browsing outside the normal HTTP ports, port 80 (or proxy ports) and 443 for SSL."
In order to show targeted advertisements to users, advertisers engage in an automatic, real-time bidding process, which makes malicious advertisements more difficult to track. "In the case of this malvertising campaign the malicious advertisers were the highest bidders," Fox-IT says.
Hackers used a method called "retargeting", which is actually used by Digital Advertising agencies to rotate the ads shown to the same visitor when they access a specific page multiple times.
"The way it works is that a user with an interesting set of tracking cookies and other metadata for a certain adprovider is retargetted from the original advertisement content on the website to the modified or personalized data," Fox-IT researchers said. "We have seen examples where the website that helped with the ad redirect to infect a user had no idea it was helping the delivery of certain content for a certain ad provider."