Either way, clicking on the link or attachment leads to a site where targets download the malicious file, but only after completing the CAPTCHA (which is short for completely automated public Turing test to tell computers and humans apart). The purpose: to thwart automated analysis defenders use to detect and block attacks and get attack campaigns shut down. Typically the analysis is performed by what are essentially bots that download malware samples and run and analyze them in virtual machines. Requiring the successful completion of a CAPTCHA means analysis will only happen when a live human being downloads the sample. Without the automation, the chances of the malicious file flying under the radar are much better. Microsoft has dubbed Chimborazo’s ongoing attack campaign Dudear.
CAPTCHAs, those puzzles with muffled sounds or blurred or squiggly letters that websites use to filter out bots (often unsuccessfully), have been annoying end users for more than a decade. Now, the challenge-and-response tests are likely to vex targets in malware attacks. From a report: Microsoft recently spotted an attack group distributing a malicious Excel document on a site requiring users to complete a CAPTCHA, most likely in an attempt to thwart automated detection by good guys. The Excel file contains macros that, when enabled, install GraceWire, a trojan that steals sensitive information such as passwords. The attacks are the work of a group Microsoft calls Chimborazo, which company researchers have been tracking since at least January. Previously, Microsoft observed Chimborazo distributing the Excel file in attachments included in phishing messages and later spreading through embedded Web links. In recent weeks, the group has begun sending phishing emails that change things up again. In some cases, the phishes include links that lead to redirector sites (usually legitimate sites that have been compromised). In other cases, the emails have an HTML attachment that contains a malicious iframe tag.