Chinese Hackers Distributing Nim language Malware in SMS Bomber Tool

This newly discovered malware campaign is attributed to a Chinese hacking group called Tropic Trooper.

Cybersecurity researchers at Check Point have shared details of a new malware campaign suspected to be launched by a Chinese hacking group Tropic Trooper.

The malware operators are using a unique loader Nimbda, written in Nim language, and a new variant of Yahoyah trojan.

Researchers state that the hackers possess extensive cryptographic knowledge as they have extended the AES specification in a customized implementation.

Info-Stealing Trojan Embedded in SMS Bomber Tool

According to Check Point’s analysis, the info stealing trojan is hidden inside a Chinese language greyware tool called SMS Bomber. This tool is used for targeting cellphones with Denial of Service attacks (DoS attacks).

SMS Bomber tool allows users to enter any phone number to flood their phones with a message, rendering the devices unusable. Novice hackers typically use such tools to compromise websites.

Attack Scenario

When the infected version of SMS Bomber (equipped with standard functionalities and the tool’s binary) is downloaded to the device, the attack sequence is immediately initiated. The downloaded tool also contains additional coding injected into a notepad.exe process.

In a blog post, researchers explained that This executable is the Nimbda loader, which uses the SMS Bomber as an icon and an executable while the loader injects shellcode in the notepad process in the background. The process then reaches a GitHub repository, fetches an obfuscated executable, decodes it, and executes it through process hollowing in Dllhost.exe, the new Yahoyah variant. 

This variant collects host-related data and transmits it to the attacker-operated C2 server. The final payload that Yahoyah executable drops is encoded in a JPG file through steganography. Researchers identified it as TClient. It is a backdoor used in previous campaigns by Tropic Trooper.

The interface of SMS Bomber (left) – Infection chain of the malware (right)

What Information is Collected

Yahoyah can collect device names, local wireless networks’ SSIDs located within the target device’s vicinity, MAC address, antivirus products installed on the device, OS version, and presence of Tencent and WeChat files.

The encryption used for Yahoyah is a custom AES implementation to perform the inverted sequence of round operations twice. Therefore, Check Point named it AEES. Though it doesn’t make encryption any stronger, it makes analyzing it complicated for researchers.

Potential Targets

Tropic Trooper has mainly focused on espionage in their previously identified phishing campaigns targeting Russian entities. However, in this campaign, the hackers have trojanized the SMS Bomber tool; hence they have narrowed down their targets.

Researchers believe their target could be based on the intelligence information the group collected during past espionages. Tropic Trooper also uses KeyBoy, Earth Centaur, and Pirate Panda monikers.

The group has a history of targeting targets in Hong Kong, Taiwan, and the Philippines. Moreover, their prominent targets are linked to the government, transportation, healthcare, and technology sectors.

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