![]() So the moment of truth, and to make this work you’ll need to change 0x55555555519b to wherever your compiler assigned the instruction in memory. Also, if it were required to use 00 since this translates to NULL, and code execution stops if it hits a NULL character, you would need to find another way around using the existing instructions. It’s worth noting that the leading zeros don’t matter and should be omitted here. We’re now ready to plug in our memory location 0x000055555555519b. Program received signal SIGSEGV, Segmentation fault. Starting program: /home/marshall/Hack/bof_wt/pwnme <<< $(perl -e 'print "A"x24. The program being debugged has been started already. You’ll need to recalculate the number of A’s as padding to use, it’s usually the number you used - 6.Īddresses in memory will be backward because of the endianness, so to illustrate this let us try: (gdb) r <<< $(perl -e 'print "A"x24. We’ll want to overwrite the return pointer with 0x55555555519b so that we skip past the p conditional. It should look something like this when you’ve found the max overwrite: (gdb) r : lea 0xe65(%rip),%rdi # 0x555555556007īy now you can probably see where this is headed. The next part takes a bit of trial and error, you’ll need to figure out how many A’s (hex 0x41) you can insert past the end of the buffer u until you fully overwrite the RIP address (return instruction pointer). You should also insert a breakpoint at line 11 so it will notify you when you land in the correct spot. (gdb) break 10īreakpoint 1 at 0x1194: file pwnme.c, line 10. We’ll now insert a breakpoint at line 10, the conditional check if (p != 0) that we want to circumvent. Your gdb session should now look something like this: (gdb) list 11 You can get an idea of where the code you want to land at is by typing list 11 which should show you the C source 4 lines before and after line 11 where you want to land, at printf("How you do dat?\n"). Right off the bat, you should see a bunch of locations of various instruction sequences in memory. You can replace main with any function name called from within the code, including libraries used. One of the foremost things I usually do, just to get a feel for the code at hand, is enter disas main (short for disassemble). You should then see some version information, and, assuming you compiled in debugging symbols with -ggdb earlier, you should see: Reading symbols from. You can now fire up gdb and pull in our binary using the command: gdb. This program is vulnerable to a buffer overflow: #include This is perl 5, version 30, subversion 0 (v5.30.0) built for x86_64-linux-gnu-thread-multi You’ll need some patience, a C compiler (I’m using gcc, I recommend you use that to follow along), as well as gdb (the debugger, giddabug as I lovingly call it), and a Linux machine or VM, and perl or python (this walkthrough uses perl). This is done by writing past the end of the buffer and arbitrarily overwriting the stack. Usually, you’ll want to gain privileges, usually by execution of shellcode - or whatever your end goal is, but for the purposes of this tutorial, we’ll just be redirecting program flow to code that would otherwise be unreachable to us (in practice, this can be virtually anything even including the execution of instructions that were not formally there). Luckily for us, manipulation of the stack (stack “smashing”) can allow us to do this. Generally, the idea as we are hacking is to redirect program flow as we see fit. To the outside of this on the stack (which grows downwards on x86 and x86_64, meaning as it gets larger the memory addresses go down), other pieces of the program are stored and manipulated. You have a buffer, a chunk of memory reserved for the purpose of storing data. The basic idea behind a C buffer overflow is pretty simple. We will be debugging a C buffer overflow in gdb to attain higher privileges. Buffer-overflow-gdb exploit vulnerabilities PoC buffer-overflow gdb gcc buffer-overrun stack x86_64 walkthrough stack-based exploitation tutorial primitives stack-overflow
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