Detected by Palo Alto Networks, a Trojan-like malware CryptoJack replaces a wallet address the user copied to the clipboard with an attacker’s address which sends funds into the attacker’s wallet.
First this malspam campaign was noticed on the morning of February 25, 2018, by Unit 42 and Proofpoint researchers. The campaign seemed to specifically target Japanese and American users.
Besides cryptocurrencies, ComboJack targets digital currencies such as WebMoney and Yandex Money. ComboJack understands which form of currency a wallet address is using by the length of the text and the starting letter or number.
This type of hack was first utilized by CryptoShuffler malware in 2017.
How it operates
In order to install the malware and hijack the victim’s machine, the hacker(s) sent out malspam campaigns urging users to open a PDF in order to identify whether or not they know the owner of a supposed lost passport. The PDF contained an embedded RTF file which contained an embedded remote object. This remote object in its turn contained encoded PowerShell commands which downloaded and finally executed a malware file.
The initially downloaded file is a self-extracting executable (SFX). Once ComboJack is extracted it begins by copying itself to a specified location on a user’s system drive. It then uses the built-in Windows tool, attrib.exe (used for setting file attributes), to set both hidden and system attributes to itself. This hides the file from the user and allows it to execute with system-level privileges. After making an entry in the system registry, ComboJack enters into an infinite loop: Every half second it checks the contents of the clipboard and tries to determine if the victim has copied any wallet information for various digital currencies.
When found, ComboJack will replace the address with a hardcoded attacker’s address hoping the victim will accidentally send money to the wrong address.
The exploit being used by ComboJack has previously been patched by Microsoft, so users can protect themselves by updating the operating system. But as cryptocurrencies continue to rise, it is likely we will see more and more malware targeting owners of crypto funds.
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