How It Works: Steganography Hides Malware in Image Files

Lordian Mosuela


Copyright © 2016 Virus Bulletin



One of the more insidious new obfuscation techniques identified by our research team in the past year, which we believe will grow in popularity, uses a new 'digital steganography', or
concealment technique, to evade detection by conventional security tools. Digital steganography is a method of concealing a file, message, image or video within another file, message, image or video. In this case the technique is used by a piece of malware popularly known as Stegoloader (detected by CYREN as W32/Gatak), which is a trojan or downloader for stealing data and delivering ransomware.

Overview: 'New and Improved' Hiding Technique

Gatak/Stegoloader is a good illustration of the ongoing arms race between malware writers and your Internet security. A precursor to the new digital steganography technique was seen in the Duqu [1] malware (discovered in 2011), which was found to transmit encrypted data appended to an image (.JPG) file. The Zeus/Zbot [2] malware also uses similar tactics, appending its encrypted configuration file to an image file for exfiltration. However, this technique has proven easy to block using content filtering rules, because the configuration file is simply appended to the image file.

The Gatak/Stegoloader malware, which emerged in 2015 [3], improves on this steganography technique – it completely hides its malicious code within an image (.PNG) file. So far, we have seen this threat bundled in software licence cracking tools that are used (illegally) to generate software licence keys (typically to extend software trials or unlock software features without payment), but there is a high probability that new distribution mechanisms will appear.

How It Works

Figure 1 shows a Gatak sample (SHA256: 0a58b98205c8542ae0516b4fe3ff8a4a6d6e9c199ec2d4e0de0aa8f9e1290328) in which two executable files are included inside a compressed archive. 

stenography-1.jpgFigure 1: Gatak sample showing two executable files inside a compressed archive.

Upon execution of the installer file, the software licence cracking tool (1237.exe) runs and the window shown in Figure 2 is displayed – this is used to generate software keys for a specific software program. Of course, the Gatak malware (9604.exe) is also executed, but without the user's knowledge. 

stenography-2.jpgFigure 2: The cracking tool.

The malware installation is complex, with many steps, but there are two main parts:

  1. Initial decryption and installation of the malware.
  2. Download of the image and use of the hidden image data to establish encrypted communications with the C&C server and to download/upload data and/or further malware.

Step 1: Initial installation of the malware

The Gatak malware first assembles its code into memory by decrypting nine segments of the encrypted code embedded in the malware itself, and then transfers its execution to the decrypted code in memory. Figure 3 shows the size and virtual address of nine parts of encrypted code that are included in the three sections of the malware: .data, .adata and sync. 


Figure 3: Size and virtual address of nine parts of encrypted code that are included in three sections of the malware.

The key for decryption is two bytes in size and is hard coded in the malware body. Decryption is accomplished by alternating the two bytes in the decryption key and subtracting them from the encrypted code:

  1. Subtract a byte of encrypted code using the byte key.
  2. Shift the byte key and use another byte key.
  3. Store the decrypted byte in the allocated buffer.
  4. Increment a pointer to the next byte of encrypted code.
  5. Increment a pointer to the next byte of allocated buffer.
  6. Loop to #1.

The decrypted code then rebuilds itself by first obtaining the location of the process environment block (PEB) using the FS:[30] register. It then acquires the address of InLoadOrderModuleList (see Figure 4) in order to find the virtual address of ntdll.dll. This procedure is used to construct the required import addresses of the ZwAllocateVirtualMemory, ZwFreeVirtualMemory and LdrLoadDll APIs, which are used to load more DLLs and retrieve the required APIs from them through their API hashes. 

stenography-4.jpgFigure 4: Code snippet of getting InLoadOrderModuleList.

Figure 5 shows the list of APIs gathered with their corresponding hashes. 

stenography-5.jpgFigure 5: List of APIs gathered with their corresponding hashes.

The malware then gets the ftCreationTime.dwLowDateTime of the %USERPROFILE% folder of the infected system using the FindFirstFileA API and stores it as a variable for later use.

Next, it gets the ftCreationTime of the Windows folder using the GetWindowsDirectoryA and FindFirstFileA APIs, and tries to compare it with the hard-coded array of the Windows timestamp. If a match is found, it will not infect the system. It also tries to determine whether the system is based on Linux running a Windows compatibility layer (Wine) by checking for the existence of the following registry key:


If this registry key exists, it will not infect the system, it will terminate and delete itself using the following command (where %% is the path and filename of the malware):


It tries to get the processID of the explorer.exe process in the system by using a combination of the FindWindowA and GetWindowThreadProcessId APIs with the following parameters:

  • lpClassName = "Shell_TrayWnd"
  • lpWindowName = NULL

If this method fails, it will create a rundll32.exe process using the following command:

rundll32.exe shell32.dll,Control_RunDLL

It gets the processID of the newly created process using the CreateProcessA and GetProcessId APIs, then transfers its execution to the injected code in either the system's explorer.exe or the newly created rundll32.exe process using the CreateRemoteThread API and proceeds to terminate and delete itself from the system.

The malware checks whether it is already installed in the system using the following conditions:

  • Finding the AtomName = "1234554321" using the GlobalFindAtomA API
  • Opening the registry key: HKEY_CURRENT_USER\Software\Microsoft\[unique_string_through_hash_of_computer_name]

As shown in Figure 6, the malware sends a step-by-step status update to its C&C server. 

stenography-6.jpgFigure 6: The malware sends a step-by-step status update to its C&C server.

It then activates InternetOpenA with the User-Agent:

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0)

and InternetOpenUrlA with the following URL string structure:



  • [HEX1] is the hex string of the computer name
  • [HEX2] is the ftCreationTime.dwLowDateTime of the %USERPROFILE% folder
  • [COUNTER] is the status counter of the procedure
  • [STATUS] is the string that represents the status of the procedure used.

Step 2: Downloading the image with the encrypted message

The malware then attempts to download an image file from one of the following URLs:

  • http://www.imagesup. net/?di=1514---3927715
  • http://hostthenpost. org/uploads/a7f5c7e67-----8ff9dbfac6a3f28d3be.png


Figure 7: Gatak/Stegoloader's downloaded image file including hidden data.

After downloading the image file and before proceeding to the steganography algorithm to retrieve the hidden encypted data, the malware runs through the following integrity checklist:

  1. It verifies that the image is a PNG file by checking that the header starts with '0x89PNG' bytes.
  2. It checks whether it properly initializes Windows GDI+ via the GdiplusStartup API.
  3. It checks whether it can create a bitmap object for the downloaded image to retrieve the pixel data using the CreateStreamOnHGlobal and GdipCreateBitmapFromStream APIs.

The malware then retrieves the hidden encrypted data from the image by getting the pixel data of the image. The pixel data serves as variables in a stream generator algorithm to form the hidden encrypted data. 'Pixel data' is the colour of the pixel at position (x,y) in a bitmap object. The malware uses a combination of the GdipGetImageHeight, GdipGetImageWidth and GdipBitmapGetPixel APIs to obtain the pixel data of the image.


Figure 8: Image's hidden encrypted stream.

It decrypts the encrypted stream in the image with an RC4 algorithm and a hard-coded eight-byte key at offset 0x19 of the injected code in either explorer.exe or rundll32.exe (see below for description).


Figure 9: Start of injected code.

Next, it checks the CRC32 of the decrypted stream to verify the correctness of the resolved stream. This resolved stream is a shellcode. The shellcode contains:

  • A command code located at offset 0x4 which instructs the injected remote code on how to process the hidden data.
  • The length (located at offset zero DWORD size) and CRC32 value (located at offset [length_of_shellcode + 4] DWORD size) of the shellcode that serves as a variable for its CRC32 checker function.
  • The hard-coded byte key (located at offset 0xb) that will be used to encrypt/decrypt the communicated message in the malware's C&C server.
  • The hard-coded byte key ID of the image shellcode located at offset 0x21.


Figure 10: Image's hidden decrypted stream.

The command codes and their descriptions are as follows:

  • 0x10: Execute payload code in memory
  • 0x20: Create then execute payload code in [binary file]
  • 0x21: Create then execute then delete payload code in [binary file]

where [binary file] has the form %temp%\~XX[random_number].tmp.

The image shellcode is a backdoor that communicates with the following command and control servers:

  • http://deid.sharpfans. org/calibre/view?present=09---67
  • http://bpp.bppharma. com/calibre/view?present=09---67
  • http://reader.lifeacademyinc. com:80/encourage/help?pointed=85----4
  • http://5.135.233. 16:80/file/photos?handle=68--77
  • http://cod.chezsimone971. com:80/encourage/help?pointed=85----4

The message sent by the image shellcode is encrypted with an RC4 algorithm prepended with a 16-byte key ID. The key ID serves as the RC4 key for the encrypted message. The structure of the decrypted message sent to the C&C server is shown in Figure 11


Figure 11: Structure of the decrypted message sent to the C&C server.

  • @ Offset 0 = 16-byte key ID of the shellcode.
  • @ Offset 0x10 = 4-byte CRC32 value starting at offset 0x14 until the end of the message.
  • @ Offset 0x14 = 16-byte command ID of the shellcode, randomly generated using the SystemFunction036 API.
  • @ Offset 0x24 = 16-byte function ID of shellcode, randomly generated using the SystemFunction036 API at the start of the shellcode execution.
  • @ Offset 0x34 = byte header for the message.
  • @ Offset 0x35 = 8-byte session ID of the message, which initializes to zero in the first contact with the command and control server.
  • @ Offset 0x3D = BOT command.
  • @ Offset 0x3E = success/error flag, set to zero for success of operation.
  • @ Offset 0x3F = information flag, set to one if information length is greater than 1,024 bytes.
  • @ Offset 0x40 = length of information.
  • @ Offset 0x44 = start of the information.

Information greater than 1,024 bytes will be compressed with the LZMA algorithm.

The information sent to the C&C server includes sensitive information stolen from the infected system in the following format: "{"[Information Description]": "[Base64 Encoded Information]"}".

Figure 12 shows the structure of the decrypted message received from the C&C server. 


Figure 12: Structure of the decrypted message received from the C&C server.

  • @ Offset 0 = 4-byte CRC32 value starting at offset 4 until the end of the message.
  • @ Offset 4 = 16-byte command ID of the C&C server.
  • @ Offset 0x14 = byte header for the message.
  • @ Offset 0x15 = 8-byte session ID of the message.
  • @ Offset 0x1D = BOT command.

Examples of BOT commands include, but are not limited to, the following:

  • 0x01 = No operation, just contact the C&C server
  • 0x02 = Execute payload via shellcode or [binary file]
  • 0x03 = Retrieve system information (e.g. InternalIP, DomainName, Processes, etc.)
  • 0x04 = Retrieve software installed
  • 0x05 = Retrieve web browser history
  • 0x64 = Execute shellcode
  • 0xDC = Retrieve Windows folder timestamp.


Gatak/Stegoloader may install other modules or malware for stealing sensitive information. Some variants have been found to install the Vundo malware family, which installs adware, ransomware and scareware. The improved digital steganography technique demonstrated by this piece of malware will surely be further adopted and/or enhanced by cybercriminals thanks to its efficiency in hiding code.



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