Simple ELF Crypter
ELF Files
There are several techniques that can be used to obsfucate what an executable does at runtime. This post will demonstrate a minimal example of a self-modifying, ELF executable that contains an encrypted section of code that, at runtime, bruteforces its own key and decrypts itself.
ELF Sections
ELF executables are comprised of several “sections”. Code that is executed, such as int main(), is placed inside the .text section.
> readelf -S a.out
There are 30 section headers, starting at offset 0x29d8:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400238 00000238
000000000000001c 0000000000000000 A 0 0 1
...
[13] .text PROGBITS 0000000000400490 00000490
00000000000002b2 0000000000000000 AX 0 0 16
To make things easier for our encryption tool, we will create a new section called .elf where we will put the functions and strings we want to protect. In GCC, thankfully this is as simple as tagging a function with the section attribute: __attribute__((section(".elf"))).
Code Layout
In our program, we are going to protect the function main() so we will “sandwich” it with two functions to mark the start and end so we have pointers to the region we will be decrypting at runtime.
#define CRYPT_SECTION ".elf"
// Align 4k so memprotect works on page aligned data
void section_start() __attribute__((section(CRYPT_SECTION)))
__attribute__((aligned(4096)));
// Protected (encrypted) function
int main() __attribute__((section(CRYPT_SECTION)));
// Mark end
void sentinal() __attribute__((section(CRYPT_SECTION)));
We will define the body of section_start() as 4 NOPs and compile with -fomit-frame-pointer. This will help us to bruteforce the key as the decryption routine will attempt to guess the key until the signature of \x90\x90\x90\x90 (4 NOPs) is found, revealing the guess is correct.
void section_start() {
__asm__ volatile("nop\n\tnop\n\tnop\n\tnop\n\t");
}
Our main() function can contain whatever we want. It is noteworthy to point out that any constant strings placed in here will likely not be encrypted unless we use a couple tricks. This is because the compiler will take the read-only string and place it in the .rodata section of the ELF (not the .elf section we want so that it is encrypted). To circumvent this, we will hack in a string by dynamically computing it as shown below.
int main() { // Encrypted main function
char buf[3];
buf[0] = 'H'; // Build string
buf[1] = 'i';
buf[2] = '\0';
printf("%s\n", buf);
exit(0); // Call exit for clean exit
}
Decryption Routine
When the encrypted program executes, it will need to bruteforce the decryption key so it can decrypt the protected region and jump to it. To accomplish all of this, the protected region needs to have the memory permissions of RWX so we will set it with mprotect(void *addr, size_t len, int prot). With few exceptions, process memory regions are set to not allow writing and execution at the same time (W^X) for security reasons. Because of this, we have to call mprotect.
To calculate the starting address and length of the encrypted region we need to pass to mprotect, we can do some simple pointer math:
unsigned int len = (&sentinal) - (§ion_start);
mprotect(§ion_start, len, PROT_READ|PROT_WRITE|PROT_EXEC);
Now that the memory can be modified, let’s iterate over it and attempt to bruteforce the key by checking if our guess decrypts the first 4 bytes of section_start() to be \0x90\0x90\0x90\0x90:
// Pointer to start of encrypted .elf section & our helper function of NOPs
unsigned char *ptr = (unsigned char *)§ion_start;
unsigned int key = 0;
for (unsigned int i = 0; i < 0xFFFFFFFF; i++) { // Bruteforce 32-bit key
// Check for our 4 NOP opcodes
if (((*(ptr) ^ i) == 0x90) && ((*(ptr + 1) ^ i) == 0x90) &&
((*(ptr + 2) ^ i) == 0x90) && ((*(ptr + 3) ^ i) == 0x90)) {
key = i;
break;
}
}
printf("Found the key! %d\n", key);
Now that the decryption key is found, we just have to decrypt the region and then jump to it:
for (int i = 0; i < len; i++) {
unsigned char v = *(ptr + i);
*(ptr + i) = v ^ key; // Do decryption
}
main(); // Call decrypted function!
Encrypting The ELF File
Since the content we are encrypting is located in the .elf section we created above, we need a simple program to locate the section boundries of .elf and encrypt it.
Pwntools is a great Python library for doing CTFs, pentesting, and shellcoding. We will use their ELF library to make things easier.
#!/usr/bin/python2
from pwn import *
import os
def encryptSection(e):
cryptSection = e.get_section_by_name('.elf')
baseAddr = cryptSection['sh_offset']
sz = cryptSection['sh_size']
print("Entry point @ 0x{:02X}".format(e.address))
print("Crypted Section @ 0x{:02X} - 0x{:02X} Size: {}B".format(baseAddr, baseAddr + sz, sz))
secData = e.read(e.address + baseAddr, sz)
# Encrypt data with XOR cipher (Key = 0xE3)
encryptedData = [chr((ord(x)^0xe3)) for x in list(secData)]
print(hexdump(secData))
print("Encrypted:")
print(hexdump(encryptedData))
e.write(e.address + baseAddr, ''.join(encryptedData))
e = ELF("./a.out")
encryptSection(e)
e.save("encrypted.elf")
os.chmod("encrypted.elf", 0777)
Inspecting the the first 5 instructions of the to-be-protected main() function before encrypting it gives us the following disassembly:
[0x004005c7]> pd 5 @ sym.main
0x00401006 4883ec28 sub rsp, 0x28
0x0040100a 64488b042528. mov rax, qword fs:[0x28]
0x00401013 4889442418 mov qword [var_10h], rax
0x00401018 31c0 xor eax, eax
0x0040101a c6042449 mov byte [rsp], 0x49
Disassembling the same function in the encrypted ELF file gives us:
[0x004005c7]> pd 5 @ sym.main
0x00401006 ab stosd dword [rdi], eax
0x00401007 60 invalid
0x00401008 0fcb bswap ebx
0x0040100a 87ab68e7c6cb xchg dword [rbx - 0x34391898], ebp
0x00401010 e3e3 jrcxz 0x400ff5
As you can see, the opcodes have changed as expected. The first opcode 0x48 XORed with our key 0xe3, gives us the result 0xab we are seeing in the encrypted binary.
By encrypting the region, the opcodes are no longer valid and the disassembler cannot determine what this region of code does. Worse, opcodes will be generated that decode to jump instructions which will result in incorrect control-flow graphs of the code.
Testing & Code
Source Code
Download: simple_elf_crypter.tar.gz
Demo Environment
I created a Docker Container where you can play around with the source and compiled code. If you want to see how the decryption works during runtime, launch the docker container and run the encrypted.elf in GDB.
docker run -ti gfoudree/simple-elf-crypter:latest
Then run make inside the container to build a.out (the executable before being encrypted) and encrypted.elf (encrypted executable).