Runtime Function Decryption

Created the Friday 22 March 2024. Updated 1 month ago.

This technique is used to store the function body in an encrypted form. They will only be decrypted just before the execution of that code and will be re-encrypted after the code has been executed.

This technique is used by SmokeLoader to evade anti-virus and EDRs, since the function body is in encrypted form except at the time of execution of the function. It also makes the static analysis of SmokeLoader harder.

Technique Identifier

U0523

Technique Tags

encryption smokeloader

Code Snippets

Description

This Ghidra script decrypts shellcode by XORing each byte with a given key and writes the decrypted bytes back to a specified address in the program.

def decryptShellcode(size, xor_key, rva):
    va = rva + 0x400000
    va = hex(va)[2:]
    addr = toAddr(va)
    addr2 = addr
    enc = get_bytes(toAddr(va), size)
    for i in range(size):
            clearListing(addr2)
            addr2 = addr2.add(1)
    size2 = size
    for i in range(0,size):
        enc[i] = enc[i]^xor_key
        
            
    for i in enc:
       i = i & 0xFF
       setByte(addr, i)
       addr = addr.add(1)

Contributors

irfan_eternal

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.

Understanding Internals of SmokeLoader - irfan_eternal