Highly malleable, highly sophisticated and over 10,000 bytes of machine code. This is what Unit 42 researchers were met with during code analysis of this “bear” of a file. The code behaviour and features strongly correlate with that of the WaterBear malware family, which has been active since as early as 2009.
Analysis by Trend Micro and TeamT5 unveiled WaterBear as a multifaceted, stage-two implant, capable of file transfer, shell access, screen capture and much more. The malware is associated with the cyber espionage group BlackTech, which many in the broader threat research community have assessed to have ties to the Chinese government, and is believed to be responsible for recent attacks against several East Asian government organizations. Due to the similarities with WaterBear, and the polymorphic nature of the code, Unit 42 named this novel Chinese shellcode “BendyBear.” It stands in a class of its own in terms of being one of the most sophisticated, well-engineered and difficult-to-detect samples of shellcode employed by an Advanced Persistent Threat (APT).
The BendyBear sample was determined to be x64 shellcode for a stage-zero implant whose sole function is to download a more robust implant from a command and control (C2) server. Shellcode, despite its name, is used to describe the small piece of code loaded onto the target immediately following exploitation, regardless of whether or not it actually spawns a command shell. At 10,000+ bytes, BendyBear is noticeably larger than most, and uses its size to implement advanced features and anti-analysis techniques, such as modified RC4 encryption, signature block verification, and polymorphic code.
The sample analysed in this blog was identified by its connections to a malicious C2 domain published by Taiwan’s Ministry of Justice Investigation Bureau in August 2020. It was discovered absent additional information regarding the exploit vector, potential victims or intended use.
Palo Alto Networks customers can be protected from the attacks outlined in this blog with the Next-Generation Firewall alongside DNS Security, URL Filtering and WildFire security subscriptions, and Cortex XDR.
A New Class of Shellcode
At a macro level, BendyBear is unique in that it:
- Transmits payloads in modified RC4-encrypted chunks. This hardens the encryption of the network communication, as a single RC4 key will not decrypt the entire payload.
- Attempts to remain hidden from cybersecurity analysis by explicitly checking its environment for signs of debugging.
- Leverages existing Windows registry key that is enabled by default in Windows 10 to store configuration data.
- Clears the host’s DNS cache every time it attempts to connect to its C2 server, thereby requiring that the host resolve the current IP address for the malicious C2 domain each time.
- Generates unique session keys for each connection to the C2 server.
- Obscures its connection protocol by connecting to the C2 server over a common port (443), thereby blending in with normal SSL network traffic.
- Employs polymorphic code, changing its runtime footprint during code execution to thwart memory analysis and evade signaturing.
- Encrypts or decrypts function blocks (code blocks) during runtime, as needed, to evade detection.
- Uses position independent code (PIC) to throw off static analysis tools.
For an in-depth technical breakdown of each of these capabilities, visit https://unit42.paloaltonetworks.com/bendybear-shellcode-blacktech/.
