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Security Research

Technical Analysis of HijackLoader

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NIKOLAOS PANTAZOPOULOS
September 08, 2023 - 15 Lesezeit: Min

Introduction

HijackLoader is a new malware loader, which has grown in popularity over the past few months. Even though HijackLoader does not contain advanced features, it is capable of using a variety of modules for code injection and execution since it uses a modular architecture, a feature that most loaders do not have. Based on our telemetry, we have observed HijackLoader being used to load different malware families such as Danabot, SystemBC and RedLine Stealer.

In this blog, we examine the inner workings of HighjackLoader, from its initialization to modular design to anti-analysis techniques.

Key Takeaways

- HijackLoader is a new malware loader that ThreatLabz first observed in July 2023.

- The loader is being leveraged to drop numerous malware families, including Danabot, SystemBC, and RedLine Stealer, amplifying its potential threat.

- HijackLoader utilizes syscalls to evade monitoring from security solutions, detects specific processes based on an embedded blocklist, and delays code execution at different stages.

- The malware uses embedded modules that facilitate flexible code injection and execution - a feature uncommon among traditional loaders.

Technical Analysis

The following sections describe each stage and component of HijackLoader. It should be noted that the analysis focuses on one of the many samples observed by ThreatLabz. As a result, certain parts of the analysis might differ from sample to sample (e.g. structure field offsets).

First Stage Loader

Initialization Phase

Upon execution, HijackLoader starts by executing a modified (hooked) function of the Windows C Runtime (CRT), which points to the entry point of the first stage.

During its initialization phase, the loader determines if the final payload has been embedded in the binary or if it needs to download it from an external server. To achieve this, HijackLoader includes an encrypted configuration, which stores information such as:

  • A DWORD hash value to detect the next stage (e.g., the ti module described later in the text) from the modules table.
  • Windows API hashes for dynamic loading.
  • An array of DWORDs, which are used to determine if the loader has to download the final payload. The offsets for these fields might differ from sample to sample.
  • Parameters for several Windows API functions. For example, the constant PAGE_EXECUTE_READWRITE (0x40) for VirtualProtect.
  • A DWORD seed value, which is used for deriving a string based on the compromised host’s username.
  • A DWORD value, which is used for validating the payload, when loaded from disk, by searching it in the payload’s data. 
  • A DWORD value, which is used for detecting all blobs of the encrypted payload.
  • An offset for the payload URL (if any) along with an XOR key to decrypt it.
  • A blocklist of process name hashes (described later in Table 1).

The above configuration block is detected by using hardcoded offsets and then decrypted either with a bitwise XOR or ADD operation. The offsets for the configuration block detection (including the offset of the encryption key) might differ from sample to sample.

Anti-Analysis

The first stage includes a limited set of evasion techniques:

  • Dynamic loading of Windows API functions by leveraging a custom API hashing technique.
  • Performing an HTTP connectivity test to a legitimate website (e.g. mozilla.org). If a connection cannot be made, then HijackLoader does not proceed with the execution and enters an infinite loop until a connection is made.
  • Delaying of code execution at different stages.
  • The first stager checks for the presence of a set of running processes. Depending on which ones are present, it executes different functionality. In Table 1, we summarize the corresponding functionality for each process.
Table 1 - HijackLoader blocklist of processes
PROCESS NAMEPRODUCT NAMEDESCRIPTION
avastsvcAvast AntivirusDelay execution for 40 seconds.
avgsvcAVG Internet SecurityDelay execution for 40 seconds.
a2serviceEmsisoft Anti-MalwareSkip the connectivity test.
wrsaWebroot SecureAnywhereSkip the connectivity test.
msmpengMicrosoft Windows DefenderNo behavioral code change.

Second Stage Loading

HijackLoader locates the payload of the second stage (i.e., the ti module) by following the steps below:

  1. Parses the decrypted configuration block, which was obtained from the initialization phase. Then, HijackLoader locates the encrypted payload URL and decrypts it using a bitwise XOR operation.
  2. Downloads the payload and validates it by checking for the presence of the signature (included in the configuration block) in its data. If the validation passes, it writes it to disk.
  3. Searches for encrypted blobs using the second marker. Each marker represents the start of an encrypted blob along with the size of the blob (which is stored before each occurrence). Moreover, the XOR key is located after the offset of the first encrypted blob.
  4. Once all encrypted blobs have been extracted, they are concatenated together and decrypted with the XOR key.
  5. Finally, the decrypted payload is decompressed using the LZNT1 algorithm.

Pseudocode for this process is shown in Figure 1.

HijackLoader second stage code to download and execute payloadsFigure 1: HijackLoader second stage code to download and execute payloads

The same procedure is followed when the payload is loaded from disk. The only difference is that HijackLoader uses an additional pattern (from the configuration block) for finding the start offset of the embedded payload (Figure 2).

HijackLoader second stage payload execution from a local fileFigure 2: HijackLoader second stage payload execution from a local file

The decrypted payload includes two components:

  • A modules table - This includes the HijackLoader modules along with their settings and the final payload (e.g. SystemBC).
  • Main shellcode and settings, or a list of optional files to use for DLL hijacking.

Next, the first stager needs to load and execute the next stage. This is accomplished by obtaining the file path of the DLL to patch (e.g. mshtml.dll) and a table for the modules of HijackLoader that is included in the decrypted payload. 

Then, HijackLoader loads the specified DLL and locates the next stager (ti module) after searching for its hash (included in the configuration block) in the modules table. Lastly, it copies the module’s data into the code section of the legitimate DLL and executes it.

NOTE: The file paths and names of files that are written to disk by HijackLoader are generated based on the username of the compromised host.

Modules

HijackLoader’s modules assist with the code injection and execution process of the final payload. The table below shows the modules identified by ThreatLabz, along with the corresponding CRC32 values and functionality.

 

Table 2 - HijackLoader modules observed by ThreatLabz
CRC32MODULE NAMEDESCRIPTION
0x78b783caAVDATABlocklist of security products’ process names. The blocklist includes the CRC32 value of each process name.
0x757c9405ESALClears out the shellcode data and executes the final payload.
0x6364a15bESAL6464-bit version of the ESAL module.
0xe7794e15ESLDRAssists with code injection of the main instrumentation shellcode.
0x4fa01ac5ESLDR6464-bit version of ESLDR module.
0x93eb1cb1ESWRClears out the shellcode data and executes the rshell module.
0x699d0c82FIXEDLegitimate executable file (e.g., QQPCMgr), which is used for injecting code into its process.

0xfea2e0eb


 
LauncherLdrDecrypts the stored modules table file from disk. We have only seen the 64-bit version of this module being included.
0xf4f141c2LauncherLdr6464-bit version of LauncherLdr module.
0x74984889rshellRelocates, parses and executes the final payload.
0x7b37e907rshell6464-bit version of rshell module.
0x3ee477f1tiExecuted after the first stage. Performs code injection for other stages and modules.
0x2ab77db8ti6464-bit version of ti module.
0x4eace798tinystubEmpty executable file, which is used for patching during the final payload execution process.
0xa1d724fctinyutilitymodule.dllOverwrites the PE headers of a specified file with null bytes.
0x263596batinyutilitymodule64.dll64-bit version of tinyutilitymodule.dll module.
0x1ae7700aUnknownUnknown module. Not included in any of the observed binaries. Based on the analyzed code, we assess that it includes a file path along with an optional parameter. The current running file is copied into this new location and executed along with the specified parameter.
N/AMain instrumentation shellcodeShellcode injected into the specified target process from the ti module. This module is responsible for executing the final payload.

 

Moreover, each module has a structure that holds information such as:

  • Module name
  • Module offset in the table
  • Module size

Due to the quality of the code and because the rest of the structure fields are not being used, ThreatLabz was not able to identify the purpose for the rest of the structure members. However, ThreatLabz assesses that the following information might be included as well:

  • Module timestamp
  • Internal names (e.g., rLdr64 for the rshell module)

From the modules mentioned above, our analysis focused on the ti and the main instrumentation shellcode modules since these two implement the majority of the core functionality for HijackLoader.

TI module

Anti-Analysis

The anti-analysis techniques are the same as presented in the previous section, but there are two notable differences:

  1. The developers have included the Heaven's gate technique.
  2. The process blocklist remains the same but the code behavior is different as can be seen in Table 3.
Table 3 - Process blocklist in the HijackLoader ti module
PROCESS NAMEDESCRIPTION
avastsvc
  • Adds persistence by creating a shortcut (LNK) file and saving the current executable under a random filename in the Windows folder %AppData%. NOTE: A flag, which is set in the decrypted payload at offset 4 is required too.
  • Generates a random environment variable name with seed 0xE1ABD1C2 and a new random filename. This is used at a later stage for storing the modules table.
  • Writes to disk a legitimate executable file (the FIXED module) and uses it for code injection at a later stage.
avgsvcSame as avastsvc
a2serviceNo behavioral code change.
wrsaNo behavioral code change.
msmpengNo behavioral code change.

 

Execution Phase

The primary role of the ti module is to inject the main instrumentation module, which is responsible for loading the final stage. 

The ti module executes the next stage by using one of the following methods:

  • Re-executes the initial file from a new location and parameter (if the module with the CRC32 value 0x1ae7700a exists).
  • Creates a new process (specified in the configuration), maps the hijacked file into it (e.g. mshtml), and injects the main instrumentation shellcode. In addition (depending on the configuration flags), it executes the FIXED module, which might be used later for code injection.
  • HijackLoader might include an additional set of files, which can be used for DLL hijacking. In this case, it writes them to disk along with the encrypted payload (from stage 1) and executes the hijacked executable.
  • Executes the ESLDR module, which injects the main instrumentation shellcode.

Main Instrumentation Module

The main instrumentation module contains the core functionality for loading and executing the final stage of the infection chain.

Anti-Analysis

The anti-analysis techniques remain the same with the previous aforementioned stages. However, one key difference is the deployment of the AVDATA module. The AVDATA module contains a set of process names and if any of them are detected then the code behavior might change. The process names observed by ThreatLabz and their CRC32 values are presented in the table below.

 

Table 4 - Process name blocklist in the HijackLoader AVDATA module
CRC32PRODUCT NAMEPROCESS NAME
0xb02ef94Avast Antivirusavastsvc.exe
0xc0bfbba0ESET Smart Securityekrn.exe
0x40cb21d3Kaspersky AntiVirusavp.exe
0xc0fe273fSymantec Event Managerccsvchst.exe
0x9e0539f6Norton 360n360.exe
0xe6ef3abAviraavguard.exe
0x8e9e8addAVG Internet Securityavgsvc.exe
0x923d5594AVG Internet Securityavgui.exe
0xce1599c2BitDefender AntiVirusvsserv.exe
0x83ed98a3BitDefender AntiVirusepsecurityservice.exe
0xd50dea99TrendMicro AntiViruscoreserviceshell.exe
0x2fba3706McAfee Antivirusmcshield.exe
0x1235ed11McAfee Antivirusmctray.exe
0x3a39ba4Norton Internet Securitynis.exe
0xe981e279Norton Internet Securityns.exe
0x19e8fad2BitDefender Antivirusbdagent.exe
0x5f1c2fc2Trend Micro Securityuiseagnt.exe
0xc68b2fd8ByteFence Anti-Malwarebytefence.exe
0xefba2118McAfee Security Scan Plusmcuicnt.exe
0xfeb42b97Internet Security Essentialsvkise.exe
0x6274fa64Comodo Internet Securitycis.exe
0x4420ef23Malwarebytes Anti-Malwarembam.exe
0x31c100e7360 Safe Guardzhudongfangyu.exe
0x219b199a360 Total Security360tray.exe
0x64760001N/AUnknown
0x27873423N/AUnknown
0x8bdc7f5bN/AUnknown

 

Each process block in the AVDATA module has the following structure:

struct avdata_process_block
{
  unsigned int CRC32; // CRC32 value of process.
  unsigned char Execution_Type; // Code execution method for the final payload.
  unsigned char LNK_Persistence_Flag; // Adds persistence with an LNK shortcut file.
  unsigned char unknown; // Not used.
  unsigned char unknown_2; // Not used.
  int BITS_Persistence_Flag; // Adds persistence by creating a BITS job.
  int unknown_3; // Not used.
  int unknown_4; // Not used.
  int Injection_Type; // Defines what code injection method to use.
  int overwrite_pe_headers_with_junk_Flag; // Used during module deployment.
};

Persistence

As described in the structure above, persistence on the compromised host is established via the following methods:

  • Creation of a BITS job, which points to the executable file.
  • Creation of a shortcut file (LNK) in the Windows Startup folder. The shortcut’s path is added in a new BITS job and points to the executable file.

Final payload decryption and execution

The embedded payload is decrypted using a bitwise XOR operation with the key being derived from the first 200 bytes. This can be easily represented in Python as follows:

enc_data = data[200:]
key = data[:200]
dec = bytearray()
for idx in range(0, len(enc_data), 4):
    dec_int = struct.unpack("<L", enc_data[idx:idx+4])[0] ^ struct.unpack("<L",
key[idx%200:idx%200 + 4])[0]
    dec.extend(dec_int.to_bytes((dec_int .bit_length() + 7) // 8, byteorder='little'))

HijackLoader’s shellcode then proceeds with the injection or direct execution of the decrypted payload. The technique the shellcode uses depends on a number of different factors such as the payload’s file type and a flag, which is stored in the settings and indicates the injection method to use. In the table below, we describe each case along with a description of the action taken.

 

Table 5 - HijackLoader Code Injection Methods
INJECTION TYPEDESCRIPTION
DLL file type.In the case of a DLL file type, the shellcode parses the PE file and leverages the ESLR module, which erases the shellcode data and executes directly the entry point of the DLL.
Code injection when the injection flag is set to 3 and PE relocation is required (Method 1).

Creates a process of the FIXED module and writes to disk a file, which includes various information such as:

  • Pointer to the address of the rshell module data.
  • Process and thread handles of the created process.
  • Pointer to the address of the final payload.
Then it executes the ESWR module, which injects the rshell module into the created process. As a result, the rshell module reads the written file and therefore the data of the final payload. After relocating and parsing the file, HijackLoader executes the final payload.
Code injection when the injection flag is set to 3 and PE relocation is required (Method 2).
  • Creates a process of the FIXED module, loads the tinystub module, and adds a new PE section to it.
  • The patched stub module is written to the disk. It is important to note that HijackLoader writes the data without including the “MZ” string. Instead, it delays the execution for a few seconds and then writes to the file the MZ signature byte-by-byte.
  • Finally, it proceeds with the execution of the final payload as described in the previous method.
Code injection when the injection flag is set to 3 and no PE relocation is required (Method 1).

The implementation is similar to the previous two cases. However, there are a few notable differences:

  • HijackLoader adds a new section in the final payload file. This new section has the rshell module.
  • In the case of a .NET file, HijackLoader creates an msbuild process instead of using the FIXED module.
  • The code searches for certain values in the rshell module and replaces them with the same values that are used in the file, which is written to disk (i.e., egg-hunting).
  • In order to evade detection by security products, it writes random data to the injected process.
Code injection when the injection flag is set to 3 and no PE relocation is required (Method 2).Same code injection technique as with the previous case, but the data injection takes place with timing delays.
Code injection when the injection flag is set to 3 with a .NET PE file.
  • Creates an msbuild process, and injects the rshell module and the payload. 
  • Searches for certain values in the rshell module and replaces them to point to the address of the decrypted payload along with the injection type and the payload size. NOTE: No file is written on disk.
  • Writes random data in the injected process.
Code injection when the injection flag is set to 4.
  • Creates an msbuild process and injects the rshell module and the payload. 
  • Searches for certain values in the rshell module and replaces them to point to the address of the decrypted payload along with the injection type and the payload size. NOTE: No file is written on disk.
  • Writes random data in the injected process.

 

Conclusion

In summary, HijackLoader is a modular loader with evasion techniques, which provides a variety of loading options for malicious payloads. Moreover, it does not have any advanced features and the quality of the code is poor. However, considering the increasing popularity of HijackLoader, we expect code improvements and further usage from more threat actors, especially to fill the void left by Emotet and Qakbot.

In addition to staying on top of these threats, Zscaler's ThreatLabz team continuously monitors for new threats and shares its findings with the wider community.

Cloud Sandbox

Zscaler’s multilayered cloud security platform detects indicators related to HijackLoader at various levels.

A screenshot of Zscaler coverage

Indicators of Compromise (IOCs)

Host Indicators

SHA256 HashDESCRIPTION
7bd39678ac3452bf55359b44c5192b79412ce61a82cd72eef88f91aba5792ee6HijackLoader
6b1621bded06b082f83c731319c9deb2fdf751a4cec1d1b2b00ab9e75f4c29caHijackLoader
e67790b394f5238908fcc326a9db940b200d9b50cbb45f0bfa94038db50beeaeHijackLoader
693cace37b4b6fed2ca67906c7a4b1c11273110561a207a222aa4e62fb4a184aHijackLoader
04c0a4f3b5f787a0c9fa8f6d8ef19e01097185dd1f2ba40ae4bbbeca9c3a1c72HijackLoader

 

Network Indicators

IOCDESCRIPTION

hxxps://www.4sync[.]com/web/directDownload/KFtZys

VO/4jBKM7R0.baa89a7b43a7b73227f22ae561718f7f

Payload URL, which HijackLoader uses to load Danabot.

hxxps://geupdate-service[.]bond/img/3344379399.png

Payload URL, which HijackLoader uses to load RedLine stealer.
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