We discovered spam mails abusing EGG (.egg) files to deliver the GandCrab v4.3 ransomware (detected by Trend Micro as Ransom_GANDCRAB.TIAOBHO). EGG is a compressed archive file format (like .zip) that is widely used in South Korea. Additionally, the operators behind the spam mails appear to be specifically going after South Korean users, as evidenced by the use of Hangul in the spam mails’ subject, body, and filename attachment.
Figure 1. Spam mail sample from the campaign. The subject reads “[Fair Trade Commission] Notice of Investigation of Violation of E-Commerce Transaction” in English.
The spam mail is under the guise of an investigation on an “e-commerce transaction” violation. In English, an excerpt from the email body roughly translates to “'Unfair e-commerce notification' has been filed against your head office, I will let you know that I am going to do it.” Meanwhile, the attached EGG file is named “Notification of e-commerce violation.” It’s important to note that files in EGG format can only be decompressed by ALZip.
We first observed the spam mails on August 7, 2018. In our analysis of the samples, the attached EGG (TROJ_GANDCRAB.TICABAK) contains three files: two shortcut .lnk files (LNK_GANDCRAB.E) that are disguised to appear as documents, and an .exe file that will disappear once the user decompresses the EGG file. Within the .lnk files, “VenusLocker_korean.exe” is inscribed, which could mean that the VenusLocker group was behind the distribution of spam mails.
Figure 2. Content of the EGG file
If the user was tricked into decompressing the EGG file and subsequently opens either of the two .doc-disguised .lnk files, the hidden file, which is actually the GandCrab malware, executes. Upon connecting to its C&C server, the GandCrab v4.3 ransomware will encrypt files in the infected machine.
Figure 3. Infection chain
Figure 4. GandCrab v4.3 ransom note
Defending against GandCrab ransomware
Data from Trend Micro shows that GandCrab was the second-highest detected ransomware family globally from March to July 2018. Users and enterprises can adopt these best practicesto eliminate the risk of infection by this increasingly pervasive ransomware.
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