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Malvertising, or malicious advertising, is the use of online, malicious advertisements to spread malware and compromise systems. Generally this occurs through the injection of unwanted or malicious code into ads. Malicious actors then pay legitimate online advertising networks to display the infected ads on various websites, exposing every user visiting these sites to the potential risk of infection. Generally, the legitimate advertising networks and websites are not aware they are serving malicious content.

How does malvertising work?

Malicious actors hide a small piece of code deep within a legitimate looking advertisement, which will direct the user’s machine to a malicious or compromised server. When the user’s machine successfully makes a connection to the server, an exploit kit hosted on that server executes. An exploit kit is a type of malware that evaluates a system, determines what vulnerabilities exist on the system, and exploits a vulnerability. From there, the malicious actor is able to install malware by utilizing the security bypass created by the exploit kit. The additional software could allow the attacker to perform a number of actions including, allowing full access to the computer, exfiltrating financial or sensitive information, locking the system and holding it ransom via ransomware, or adding the system to a botnet so it can be used to perform additional attacks. This entire process occurs behind the scenes, out of sight of the user and without any interaction from the user.

The Most Popular Exploit Kit

One of the most popular exploit kits currently in use is the Angler Exploit Kit. Angler employs a number of evasion techniques in order to avoid being detected. For example, the URL of the landing page the user’s computer connects to, where the exploit kit is hosted, is often generated dynamically. This makes it difficult to detect because the URL is constantly changing. Angler also has the functionality to determine if it is being run inside of a virtual machine, thus making it difficult for cybersecurity analysts to perform analysis on it. Finally, multiple layers of obfuscation exist in Angler, built on top of each other with various encoding schemes (base64, RC4, etc.) to hide the code that executes when the vulnerable user visits the server.

Angler uses a variety of vulnerabilities in Adobe Flash, Microsoft Silverlight, and Oracle Java. These are all extremely common extensions running on many popular web browsers. When the user’s computer visits the server hosting the exploit kit, the system is scanned to determine which versions of the above software are running on the user’s browser. From there, Angler picks the best vulnerability for exploiting the victim.

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