Security Experts:

Flashpack Exploit Kit Uses Ad Networks to Deliver Cryptowall, Dofoil Malware

Researchers at Trend Micro have spotted a campaign in which attackers abuse advertising networks and the Flashpack exploit kit in an effort to distribute various pieces of malware, including the information-stealing malware Zeus, the Dofoil Trojan, and the Cryptowall ransomware.

The security firm says Flashpack uses free ads to distribute the threats. Researchers have been monitoring multiple URLs utilized by the exploit kit as landing pages and determined that they have been accessed mostly by users in North America.

One of the pieces of malware distributed by cybercriminals via the malicious advertising (malvertising) campaign is TROJ_DOFOIL, for which the number of infections peaked in October. According to Trend Micro, 41% of compromised devices are located in the Asia Pacific region, 27% in North America, and 17% in the Europe, Middle East and Africa (EMEA) region.

Flashpack Exploit Used in Malvertising CampaignsWhen users access a website that serves malicious ads, they are taken via multiple redirects to a Flashpack exploit kit landing page, which is set up to serve a variant of the Dofoil Trojan (TROJ_DOFOIL.WYTU).

The Cryptowall ransomware, which encrypts files found on infected computers and keeps them that way until a ransom is paid, is distributed via SWF files containing exploit code for a Flash Player vulnerability (CVE-2014-0515) which Adobe patched in April following reports that it had been exploited in watering hole attacks.

Trend Micro explains that the distribution of threats such as Defoil and Cryptowall via malvertising poses a serious risk if we consider the combination of this attack technique with free applications that display ads.

"Ad-enabled free applications pose a serious threat to users and enterprises as attackers leverage this to distribute threats like Ransomware and DOFOIL. As such, this may lead to system infection and possible information and data theft. End-users are recommended to be cautious with the applications that they install. Similarly, in enterprise setting, employees should  be educated on what kind of application can be installed on their desktops.  If possible, create IT policies (like Acceptable Usage Policies) that could be drafted by their internal governing bodies, such as their InfoSec department," Trend Micro researchers said in a blog post.

"Aside from this, the combination of ensuring that third party applications like Flash and Java that loaded is by Web browsers is something to note for end-user to enterprise users alike. If possible, use security software with web filtering functionalities that has the capability to block malware-related and ad-related sites," they added.

A similar campaign affecting several high-profile websites, including Yahoo and Match.com, was analyzed last month by researchers at Proofpoint.

A report published recently by researchers from Ruhr-University Bochum, University College London and the University of California, Santa Barbara (UCSB) shows that 1% of online ads are malicious.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.