On 27 August 2021, cybersecurity researchers discovered a malware loader dubbed Colibri being sold on an underground Russian forum. The actors claim that the loader is stealthy and can be used to target Windows systems, to drop other malware onto the infected system.
The features of the loader, as listed in the advertisement, include the following:
Threat actor’s post on the cybercrime forum
Unpacking the loader
Colibri loader comes packed in a trojanized executable file. By using x64dbg (debugger) and putting breakpoints on the function VirtualAlloc we were able to extract the actual payload of the Colibri loader.
Packed sample | 74c4f24e9c025d55c4dd8aca8b91fce3 |
Colibri unpacked sample | 58FEE16BBEA42A378F4D87D0E8A6F9C8 |
By testing the extracted payload with PEStudio it is evident that the payload has only two sections, .text (Code Section) and .reloc (Relocation Section). The results of scanning the payload also show the existence of a self-modifying section in the code. This implies that the payload is capable of dynamically resolving other parts of the code that are not accessible through static analysis of the payload.
By running the payload in an IDA debugger we will be able to resolve the self-modifying code section of the payload.
To avoid detection by AVs statically, the payload’s author hashes all WinAPI functions, ignoring the Import Address Table (IAT), which aids in recognising the malware’s activity statically. The payload resolves the function names dynamically using XOR and Shift operations. After resolving the function name, the address of the function is stored in eax register and a call function is created.
Before running on the system, the payload creates a mutex by calling the function CreateMutexW and then tests if there is an instance of the payload already running on the infected system. If there is an existing running process of payload on the system, the payload calls the ExitProcess function and exits the execution. If there is no instance of the payload running on the system, the payload continues the execution and calls the Sleep function to sleep for 3 seconds as a simple way to evade protection.
To make the static analysis more difficult and to evade detection, the author of this malware has encrypted all the strings. After resolving the function names dynamically and using the debugger, the strings extracted from the payload will have the following artifacts:
The payload first initializes the use of WinInet functions by calling InternetOpenW, followed by which it opens the HTTP section with the function InternetConnectW. The payload creates an HTTP request by calling the function HttpOpenRequestW under the following parameters:
/GET
/gate.php?type=check&uid=59045F4FF04F133112200
HTTP/1.1
After this, the payload sends the GET request to the server by calling the function HttpSendRequestW. Then, the payload calls the function InternetQueryDataAvailable
to determine the amount of requested data. Based on the results of the previous function calls, the payload reads the data by calling the function InternetReadFile
. Our assumption is that the payload requests the C2 server and downloads another payload on the system.
The payload calls the function CryptStringToBinary
, to decrypt data after downloading the data from the C2 server, which indicates that the data could be encrypted.
MD5 | 74c4f24e9c025d55c4dd8aca8b91fce3 |
58FEE16BBEA42A378F4D87D0E8A6F9C8 | |
IP | 80.92.205.102 |
URL | 80.92.205.102/gate.php?type=check&uid=59045F4FF04F133112200 |
Colibri loader is a type of malware that is used to load more types of malware into the infected system. This loader has multiple techniques that help avoid detection. This includes, omitting the IAT (Import Address Table) along with the encrypted strings to make the analysis more difficult. Like any other loader malware, the Colibri can be used to install information-stealing malware which may result in substantial loss of sensitive information. Thus, users should be wary of any unknown files on their systems.
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