In a series of blogs a few years back, we covered how malware could abuse and circumvent online services that use CAPTCHA tests as part of their security (1 2). In this blog, we take a look at a recent malware variant from the wild caught on camera that shows CAPTCHA tests used by some online services may still be weak.
The image below (Picture 1) shows this malware’s ecosystem, which we’ll describe step by step. Step 1: The starting point of an infection is a banking Trojan variant known as Cridex. This variant is propagated via malicious email messages that hold shortened links leading to exploit kits (see this example), in our case the Blackhole exploit kit. Step 2: If the exploit is successful, the Cridex variant is downloaded to the machine. Step 3: Cridex runs on the machine. Step 4: Cridex is a data-stealing Trojan that is similar to Zeus in the way it operates: It logs content from Web sessions and alters them to harvest information from the infected user. The Cridex configuration file downloaded by this variant (safe to view and download and shortened here) shows which websites the variant monitors and steals data from, along with Web form injection points (data alteration injected into Web forms to harvest additional data like ATM PIN numbers). We have observed that Facebook, Twitter, and many banking services are targets. A partial list of targeted websites can be found here. Step 5: Any stolen data from the system is uploaded to a command and control server.
Picture 1: The Cridex ecosystem:
Step 6: One of the components downloaded by Cridex with the configuration file is a propagation module or spamming module that allows the botmaster to send spam/malicious emails to infect other systems and increase the bot size. The spamming module holds backdoor components that allow browsing activities in the name of the user. The module opens Web sessions to online mail services and registers new email accounts that are later used by the bot to send spam/malicious emails. As we know, online mail services hold security checks like CAPTCHA challenges to verify that a human is indeed behind any account registration. Step 7: According to our findings, CAPTCHA challenges in some cases can be broken with the help of a CAPTCHA-breaking server, which allows the bot to register a mail account or address after only a few attempts. This video documents the registration of an online mail account by the bot on an infected machine:
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