According to trusted third-party reporting, HIDDEN COBRA actors have likely been using FALLCHILL malware since 2016 to target the aerospace, telecommunications, and finance industries. The malware is a fully functional RAT with multiple commands that the actors can issue from a command and control (C2) server to a victim’s system via dual proxies. FALLCHILL typically infects a system as a file dropped by other HIDDEN COBRA malware or as a file downloaded unknowingly by users when visiting sites compromised by HIDDEN COBRA actors. HIDDEN COBRA actors use an external tool or dropper to install the FALLCHILL malware-as-a-service to establish persistence. Because of this, additional HIDDEN COBRA malware may be present on systems compromised with FALLCHILL.

During analysis of the infrastructure used by FALLCHILL malware, the U.S. Government identified 83 network nodes. Additionally, using publicly available registration information, the U.S. Government identified the countries in which the infected IP addresses are registered.

Technical Details

FALLCHILL is the primary component of a C2 infrastructure that uses multiple proxies to obfuscate network traffic between HIDDEN COBRA actors and a victim’s system. According to trusted third-party reporting, communication flows from the victim’s system to HIDDEN COBRA actors using a series of proxies as shown in figure 1.

Figure 1. HIDDEN COBRA Communication Flow

FALLCHILL uses fake Transport Layer Security (TLS) communications, encoding the data with RC4 encryption with the following key: [0d 06 09 2a 86 48 86 f7 0d 01 01 01 05 00 03 82]. FALLCHILL collects basic system information and beacons the following to the C2:

• operating system (OS) version information,
• processor information,
• system name,
• local IP address information,
• unique generated ID, and
• media access control (MAC) address.

FALLCHILL contains the following built-in functions for remote operations that provide various capabilities on a victim’s system:

• retrieve information about all installed disks, including the disk type and the amount of free space on the disk;
• create, start, and terminate a new process and its primary thread;
• search, read, write, move, and execute files;
• get and modify file or directory timestamps;
• change the current directory for a process or file; and
• delete malware and artifacts associated with the malware from the infected system.

Detection and Response

This alert’s IOC files provide HIDDEN COBRA indicators related to FALLCHILL. DHS and FBI recommend that network administrators review the information provided, identify whether any of the provided IP addresses fall within their organizations’ allocated IP address space, and—if found—take necessary measures to remove the malware.

When reviewing network perimeter logs for the IP addresses, organizations may find instances of these IP addresses attempting to connect to their systems. Upon reviewing the traffic from these IP addresses, system owners may find some traffic relates to malicious activity and some traffic relates to legitimate activity.

Network Signatures and Host-Based Rules

This section contains network signatures and host-based rules that can be used to detect malicious activity associated with HIDDEN COBRA actors. Although created using a comprehensive vetting process, the possibility of false positives always remains. These signatures and rules should be used to supplement analysis and should not be used as a sole source of attributing this activity to HIDDEN COBRA actors.

Network Signatures

alert tcp any any -> any any (msg:"Malicious SSL 01 Detected";content:"|17 03 01 00 08|";  pcre:"/\x17\x03\x01\x00\x08.{4}\x04\x88\x4d\x76/"; rev:1; sid:2;)

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alert tcp any any -> any any (msg:"Malicious SSL 02 Detected";content:"|17 03 01 00 08|";  pcre:"/\x17\x03\x01\x00\x08.{4}\x06\x88\x4d\x76/"; rev:1; sid:3;)

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alert tcp any any -> any any (msg:"Malicious SSL 03 Detected";content:"|17 03 01 00 08|";  pcre:"/\x17\x03\x01\x00\x08.{4}\xb2\x63\x70\x7b/"; rev:1; sid:4;)

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alert tcp any any -> any any (msg:"Malicious SSL 04 Detected";content:"|17 03 01 00 08|";  pcre:"/\x17\x03\x01\x00\x08.{4}\xb0\x63\x70\x7b/"; rev:1; sid:5;)

YARA Rules

The following rules were provided to NCCIC by a trusted third party for the purpose of assisting in the identification of malware associated with this alert.

THIS DHS/NCCIC MATERIAL IS FURNISHED ON AN “AS-IS” BASIS.  These rules have been tested and determined to function effectively in a lab environment, but we have no way of knowing if they may function differently in a production network.  Anyone using these rules are encouraged to test them using a data set representitive of their environment.

rule rc4_stack_key_fallchill[出自:jiwo.org]
{
meta:
description = "rc4_stack_key"
strings:
$stack_key = { 0d 06 09 2a ?? ?? ?? ?? 86 48 86 f7 ?? ?? ?? ?? 0d 01 01 01 ?? ?? ?? ?? 05 00 03 82 41 8b c9 41 8b d1 49 8b 40 08 48 ff c2 88 4c 02 ff ff c1 81 f9 00 01 00 00 7c eb }
condition:
(uint16(0) == 0x5A4D and uint16(uint32(0x3c)) == 0x4550) and $stack_key
}

rule success_fail_codes_fallchill

{
meta:
description = "success_fail_codes"
strings:
$s0 = { 68 7a 34 12 00 }
$s1 = { ba 7a 34 12 00 }
$f0 = { 68 5c 34 12 00 }
$f1 = { ba 5c 34 12 00 }
condition:
(uint16(0) == 0x5A4D and uint16(uint32(0x3c)) == 0x4550) and (($s0 and $f0) or ($s1 and $f1))
}

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