Threat researchers have found a new obfuscation strategy employed by the Hive ransomware gang, which utilises IPv4 addresses and a series of conversions that leads to the download of a Cobalt Strike beacon. Threat actors use code obfuscation to conceal the malicious nature of their code from human reviewers or security software to avoid discovery. 

There are a variety of techniques to create obfuscation, each with its own set of benefits and drawbacks, but a new one identified during an incident response involving Hive ransomware reveals that adversaries are coming up with new, subtler ways to accomplish their objective. 

Analysts at Sentinel Labs describe a new obfuscation technique called "IPfuscation," which is another example of how effective basic but sophisticated tactics can be in real-world malware deployment. The new approach was discovered while examining 64-bit Windows executables, each of which contained a payload that delivered Cobalt Strike. 

The payload is disguised as an array of ASCII IPv4 addresses, giving it the appearance of a harmless list of IP addresses. The list could potentially be misconstrued for hard-coded C2 communication information in malware research. A blob of shellcode arises when the file is handed to a converting function (ip2string.h) that converts the string to binary.

Following this step, the virus executes the shellcode either directly through SYSCALLs or through a callback on the user interface language enumerator (winnls.h), resulting in a normal Cobalt Strike stager. 

The following is an example from the Sentinel Labs report: The first hardcoded IP-formatted string is the ASCII string “252.72.131.228”, which has a binary representation of 0xE48348FC (big-endian), and the next “IP” to be translated is “240.232.200.0”, which has a binary representation of 0xC8E8F0. 

Disassembling these “binary representations” indicates the start of shellcode generated by common penetration testing frameworks. The analysts have uncovered additional IPfuscation variants that instead of IPv4 addresses use IPv6, UUIDs, and MAC addresses, all operating in an almost identical manner as was described above.