QBOT Malware Analysis

• Elastic Security Labs is releasing a QBOT malware analysis report from a recent campaign
• This report covers the execution chain from initial infection to communication with its command and control containing details about in depth features such as its injection mechanism and dynamic persistence mechanism.
• From this research we produced a YARA rule, configuration-extractor, and indicators of compromises (IOCs)

This section describes the QBOT execution flow in the following three stages:

• First Stage: Initialization
• Second Stage: Installation
• Third Stage: Communication

The sample is executed with the regsvr32.exe binary, which in turn will call QBOT’s DllRegisterServer export:

After execution, QBOT checks if it’s running under the Windows Defender sandbox by checking the existence of a specific subdirectory titled: C:\\INTERNAL\\__empty, if this folder exists, the malware terminates itself:

The malware will then enumerate running processes to detect any antivirus (AV) products on the machine. The image below contains a list of AV vendors QBOT reacts to:

AV detection will not prevent QBOT from running. However, it will change its behavior in later stages. In order to generate a seed for its pseudorandom number generator (PRNG), QBOT generates a fingerprint of the computer by using the following expression:

fingerprint = CRC32(computerName + CVolumeSerialNumber + AccountName)

If the “C:” volume doesn’t exist the expression below is used instead:

fingerprint = CRC32(computerName + AccountName)

Finally, QBOT will choose a set of targets to inject into depending on the AVs previously detected and the machine architecture:

QBOT will try to inject itself iteratively, using its second stage as an entry point, into one of its targets– choosing the next target process if the injection fails. Below is an example of QBOT injecting into explorer.exe.

QBOT begins its second stage by saving the content of its binary in memory and then corrupting the file on disk:

The malware then loads its configuration from one of its resource sections:

QBOT also has the capability to load its configuration from a .cfg file if available in the process root directory:

After loading its configuration, QBOT proceeds to install itself on the machine– initially by writing its internal configuration to the registry:

Shortly after, QBOT creates a persistence subdirectory with a randomly-generated name under the %APPDATA%\Microsoft directory. This folder is used to drop the in-memory QBOT binary for persistence across reboot:

At this point, the folder will be empty because the malware will only drop the binary if a shutdown/reboot event is detected. This “contingency” binary will be deleted after reboot.

QBOT will attempt the same install process for all users and try to either execute the malware within the user session if it exists, or create a value under the CurrentVersion\Run registry key for the targeted user to launch the malware at the next login. Our analysis didn’t manage to reproduce this behavior on an updated Windows 10 machine. The only artifact observed is the randomly generated persistence folder created under the user %APPDATA%\Microsoft directory:

QBOT finishes its second stage by restoring the content of its corrupted binary and registering a task via Schtask to launch a QBOT service under the NT AUTHORITY\SYSTEM account.

The first stage has a special execution path where it registers a service handler if the process is running under the SYSTEM account. The QBOT service then executes stages 2 and 3 as normal, corrupting the binary yet again and executing commands on behalf of other QBOT processes via messages received through a randomly generated named pipe:

QBOT begins its third stage by registering a window and console event handler to monitor suspend/resume and shutdown/reboot events. Monitoring these events enables the malware to install persistence dynamically by dropping a copy of the QBOT binary in the persistence folder and creating a value under the CurrentVersion\Run registry key:

At reboot, QBOT will take care of deleting any persistence artifacts.

The malware will proceed to creating a watchdog thread to monitor running processes against a hardcoded list of binaries every second. If any process matches, a registry value is set that will then change QBOT behavior to use randomly generated IP addresses instead of the real one, thus never reaching its command and control:

QBOT will then load its domains from one of its .rsrc files and from the registry as every domain update received from its C2 will be part of its configuration written to the registry. See Extracted Network Infrastructure in Appendix A.

Finally, the malware starts communicating with C2 via HTTP and TLS. The underlying protocol uses a JSON object encapsulated within an enciphered message which is then base64-encoded:

Below an example of a HTTP POST request sent by QBOT to its C2:

Through this communication channel, QBOT receives commands from C2 — see Appendix B (Command Handlers). Aside from management commands (update, configuration knobs), our sample only handles binary execution-related commands, but we know that the malware is modular and can be built with additional features like a VNC server, a reverse shell server, proxy support (to be part of the domains list), and numerous other capabilities are feasible.

The MTRNG engine is then used by various functions to generate different types of data, for example for generating registry key values and persistence folders. As QBOT needs to reproduce values, it will almost always use the computer fingerprint and a “salt” specific to the value it wants to generate:

All QBOT strings are XOR-encrypted and concatenated in a single blob we call a “string bank”. To get a specific string the malware needs a string identifier (identifier being an offset in the string bank), a decryption key, and the targeted string bank.

As this sample has two string banks, it has four GetString' functions currying the string bank and the decryption key parameters: One C string function and one wide string function for each string bank. Wide string functions use the same string banks, but convert the data to utf-16.

See Appendix C (String Deciphering Implementation).

The malware resolves the library name through its GetString function and then resolves the hash table with a classic library’s exports via manual parsing, comparing each export to the expected hash. In this sample, the hashing comparison algorithm use this formula:

CRC32(exportName) XOR 0x218fe95b == hash

See Appendix D (Resource Deciphering Implementation).

AVG and Avast share the same antivirus engine. Thus if QBOT detects one of those antivirus running, it will also check at the installation stage if one of their DLLs is loaded within the malware memory space. If so, QBOT will skip the installation phase.

If QBOT is running under SYSTEM account, it will add its persistence folder to the Windows Defender exclusion path in the registry. It will also do this for the legacy Microsoft Security Essential (MSE) exclusion path if detected.

Each second, QBOT parses running processes looking for one matching the hardcoded exception list. If any is found, a “fuse” value is set in the registry and the watchdog stops. If this fuse value is set, QBOT will not stop execution– but at the third stage, the malware will use randomly generated IP and won't be able to contact C2.

QBOT uses the aforementioned method to inject libraries received from C2. The difference is that as well as mapping itself, the malware will also map the received binary and use a library loader as entry point.

Part of the QBOT installation process is installing itself within others users’ accounts. To do so, the malware enumerates each user with an account on the machine (local and domain), then dumps its configuration under the user’s Software\Microsoft registry key, creates a persistence folder under the users’ %APPDATA%\Microsoft folder, and finally tries to either launch QBOT under the user session if the session exist, or else creates a run key to launch the malware when the user will log in.

QBOT registers a window handler to monitor suspend/resume events. When they occur, the malware will install/uninstall persistence.

QBOT registers a console event to handle shutdown/reboot events as well.

QBOT has a mechanism to verify the signature of every message received from its command and control. The verification mechanism is based on a public key embedded in the sample. This public key could be used to identify the campaign the sample belongs to, but this mechanism may not always be present.

The public key comes from a hardcoded XOR-encrypted data blob.

Part of QBOT communication with its command and control is sending information about the computer. Information are gathered through a set Windows API calls, shell commands and Windows Management Instrumentation (WMI) commands:

One especially interesting procedure listed installed antivirus via WMI:

QBOT can receive updates from its command and control. The new binary will be written to disk, executed through a command line, and the main process will terminate.

QBOT has a system to keep track of processes injected with binaries received from its command and control in order to manage them as the malware receives subsequent commands. It also has a way to serialize and save those binaries on disk in case it has to stop execution and recover execution when restarted.

To do this bookkeeping, QBOT maintains two global structures — a list of all binaries received from its command and control, and a list of running injected processes:

The QBOT malware family is highly active and still part of the threat landscape in 2022 due to its features and its powerful modular system. While initially characterized as an information stealer in 2007, this family has been leveraged as a delivery mechanism for additional malware and post-compromise activity.

Elastic Security provides out-of-the-box prevention capabilities against this threat. Existing Elastic Security users can access these capabilities within the product. If you’re new to Elastic Security, take a look at our Quick Start guides (bite-sized training videos to get you started quickly) or our free fundamentals training courses. You can always get started with a free 14-day trial of Elastic Cloud.