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Sophos details delivery method of financial malware Gootkit

Sophos has published new research, “Gootloader Expands Its Payload Delivery Options,” that details how the delivery method for the six-year-old Gootkit financial malware has been developed into a complex and stealthy delivery system for a wide range of malware, including ransomware. Sophos researchers have named the platform, “Gootloader.” Gootloader is actively delivering malicious payloads through tightly targeted operations in the U.S., Germany and South Korea. Previous campaigns also targeted internet users in France.

The Gootloader infection chain begins with sophisticated social engineering techniques that involve hacked websites, malicious downloads, and manipulated search engine optimization (SEO). When someone types a question into a search engine such as Google, the hacked websites appear among the top results. To ensure targets from the right geographies are captured, the adversaries rewrite website code “on the go” so that website visitors who fall outside the desired countries are shown benign web content, while those from the right location are shown a page featuring a fake discussion forum on the topic they’ve queried. The fake websites are visually identical regardless of whether they are in English, German or Korean.

The fake discussion forum includes a post from a “site administrator,” with a link to a download. The download is a malicious Javascript file that initiates the next stage of compromise.

From this point on, the attack proceeds covertly, using a wide range of complicated evasion techniques, multiple layers of obfuscation, and fileless malware that is injected into memory or the registry where conventional security scans cannot reach it. Gootloader is currently delivering Kronos financial malware in Germany, and the post-exploitation tool, Cobalt Strike, in the US and South Korea. It has also delivered REvil ransomware and the Gootkit trojan itself.

“The developers behind Gootkit appear to have shifted resources and energy from delivering just their own financial malware to creating a stealthy, complex delivery platform for all kinds of payloads, including REvil ransomware,” said Gabor Szappanos, threat research director at Sophos. “This shows that criminals tend to reuse their proven solutions instead of developing new delivery mechanisms. Further, instead of actively attacking endpoint tools as some malware distributors do, the creators of Gootloader have opted for convoluted evasive techniques that conceal the end result.

“Gootloader’s creators use a number of social engineering tricks that can fool even technically skilled IT users. Fortunately, there are a few warning signs internet users can look out for. These include Google search results that point to websites for businesses that have no logical connection to the advice they appear to offer; advice that precisely matches the search terms used in the initial question; and a ‘message board’-style page that looks identical to the examples shown in the Sophos research, featuring text and a download link that also precisely matches the search terms used in the initial Google search.”

The best overall protection against Gootloader attacks is a comprehensive security solution that can scan for suspicious activity in memory and protect against fileless malware. Windows users can also turn off the “Hide Extensions for Known File Types” view setting in the Windows file explorer as this will allow them to see that the .zip download delivered by the attackers contains a file with a .js extension. Script blockers such as NoScript for Firefox can help web surfers remain safe by preventing the replacement for the hacked web page from appearing in the first place.

The first stage javascript files is detected as: AMSI/GootLdr-A. The PowerShell loader is detected as: AMSI/Reflect-H. Sophos researchers have posted Indicators of compromise to SophosLabs Github.

Further information on Gootloader and other cyberthreats is available on SophosLabs Uncut where Sophos researchers regularly publish their latest research and breakthrough findings, such as Egregor Ransomware: Maze’s Heir Apparant and Conti Ransomware: Evasive By Nature. Threat researchers can follow SophosLabs Uncut in real time on Twitter at @SophosLabs.