In this past few days I stumble to some new and old variant of ICEID malware that uses .png steganography to hide and execute its encrypted shellcode. In this article I will share how the structure of the Iceid png payload look like and how to extract its encrypted shellcode.
same as the other malware, IceID Loader changes its Crypter to execute its main module in memory. From old variant where the encrypted code stub and decryption module is place in the RSRC section as a data .rsrc entry (RC4 encrypted) to applib compression like the figure below.
|figure 1: The aplib decompressed module of ICEID|
Before we deal with the ICEID PNG steganography, I think it is better to have some preview what PNG file header format is that will give as clear preview what IceId parse the header and look for its encrypted shellcode.
The PNG file format started with 8-bytes signature header "89 50 4e 47 0d 0a 1a 0a". after this header is a chunk structure or series of chunk structures that contains the information of a "critical chunks" like "IHDR", "IDAT" and etc... or "Ancillary chunks" that may contain some attribute related to the color, pixel or metadata of the png file like "sRGB", "gAMA" and many more.
each PNG chunks layout consist of 4 parts or 4 structure member. the figure below show the 4 parts in IHDR chunk type.
|figure 2: PNG File header Format|
ICEID PNG Module Decryptor:
Now that we have some insight how PNG file format look like, lets dive in to the ICEID PNG decryptor module ("let's call it PNG module") that we already extracted earlier in the memory. This part is really interesting especially in parsing the header. :)
The PNG module start by executing a thread that will do the following:
1. decrypt its data section using same approach how it decrypt the shellcode in PNG payload file.
2. it will check the existence of the PNG file in %appdata%<randomname>/, if not
3. it will try to download it to its C&C server
4. parse and check the png file then extract the shellcode.
The first task is decrypting the C&C server url link that are place in data section that are encrypted in RC4 algorithm. the structure of data it used in decrypting this data section can be seen before call the function that will do the decryption part.
|figure 3: Encrypted Data Structure|
The first 8 bytes of the encrypted data section is the RC4 key and the rest is the encrypted data.
|figure 4: decrypting C&C server URL link|
Next it will create a random name folder in %appdata% using the "username" of the infected machine.
with the use of RDTSC command to generate random character. If the module didn't find the png payload in the said folder it will try to contact the C&C server to download it.
|figure 5: looking for the PNG payload file|
Parsing PNG Header:
after reading the PNG and save it in the memory, it will start the checking in offset 0x8 (skipping the PNG header) which is the "chunk_data_length" of the first chunk type in the header which in our case is "IHDR". The way how it parse the header to look for "IDAT" chunk_type structure is by adding:
next_chunk_type_struct = chunk_data + chunk_data_length (4 bytes) + chunk_type (4byes) + chunk_crc32 (4 bytes)
except for the start chunk_type structure where you need to include or add PNG header size which 8 bytes.
I created a simple python script that will parse this header and give you the basic information about the header.
|figure 6: parsing the header|
Byte Flag and Decryption Key:
as soon as it found the IDAT chunk_type structure it will check first if the chunk_data_length > 5 then it will skip the chunk_data_length , chunk_type and chunk_crc32 by adding 0x0c to the current pointer of the "IDAT" chunk header. the byte in this position looks like a validity flag of the PNG. If this byte is zero the PNG module will check another value parameter to the function that parse the png header which is also zero, which in this case it will exit the flag.
|figure 7: byte flag|
after this byte is the 8 byte Rc4 key followed by the encrypted shellcode using RC4 key. the python script I mentioned earlier will parse the RC4 key, extract the shellcode and check the shellcode header and entrypoint by dis-assemble it using capstone python library.
|figure 8: script tool parser|
In this analysis we learned how PNG file can be used as a weapon to hide the malicious code and how malware keeps on updating their tools to bypassed detection.
Also thanks to the community for sharing the samples :)
Article Link: https://tccontre.blogspot.com/2021/01/extracting-shellcode-in-iceid-png.html