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Basic Binary Exploitation Methodology

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ELF Basic Info

Before start exploiting anything it's interesting to understand part of the structure of an ELF binary:

{% content-ref url="elf-tricks.md" %} elf-tricks.md {% endcontent-ref %}

Exploiting Tools

{% content-ref url="tools/" %} tools {% endcontent-ref %}

Stack Overflow Methodology

With so many techniques it's good to have a scheme when each technique will be useful. Note that the same protections will affect different techniques. You can find ways to bypass the protections on each protection section but not in this methodology.

Controlling the Flow

There are different was you could end controlling the flow of a program:

  • Stack Overflows overwriting the return pointer from the stack or the EBP -> ESP -> EIP.
  • Or via Arbitrary Writes + Write What Where to Execution
    • Format strings: Abuse printf to write arbitrary content in arbitrary addresses.
    • Array Indexing: Abuse a poorly designed indexing to be able to control some arrays and get an arbitrary write.
    • bof to WWW via ROP: Abuse a buffer overflow to construct a ROP and be able to get a WWW.

You can find the Write What Where to Execution techniques in:

{% content-ref url="../arbitrary-write-2-exec/" %} arbitrary-write-2-exec {% endcontent-ref %}

Eternal Loops

Something to take into account is that usually just one exploitation of a vulnerability might not be enough to execute a successful exploit, specially some protections need to be bypassed. Therefore, it's interesting discuss some options to make a single vulnerability exploitable several times in the same execution of the binary:

  • Write in a ROP chain the address of the main function or to the address where the vulnerability is occurring.
    • Controlling a proper ROP chain you might be able to perform all the actions in that chain
  • Write in the exit address in GOT (or any other function used by the binary before ending) the address to go back to the vulnerability
  • As explained in .fini_array, store 2 functions here, one to call the vuln again and another to call**__libc_csu_fini** which will call again the function from .fini_array.

Exploitation Goals

Goal: Call an Existing function

  • ret2win: There is a function in the code you need to call (maybe with some specific params) in order to get the flag.
    • In a regular bof without PIE and canary you just need to write the address in the return address stored in the stack.
    • In a bof with PIE, you will need to bypass it
    • In a bof with canary, you will need to bypass it
    • If you need to set several parameter to correctly call the ret2win function you can use:
      • A ROP chain if there are enough gadgets to prepare all the params
      • SROP (in case you can call this syscall) to control a lot of registers
      • Gadgets from ret2csu and ret2vdso to control several registers
    • Via a Write What Where you could abuse other vulns (not bof) to call the win function.
  • Pointers Redirecting: In case the stack contains pointers to a function that is going to be called or to a string that is going to be used by an interesting function (system or printf), it's possible to overwrite that address.
    • ASLR or PIE might affect the addresses.
  • Uninitialized vatiables: You never know.

Goal: RCE

Via shellcode, if nx disabled or mixing shellcode with ROP:

  • (Stack) Shellcode: This is useful to store a shellcode in the stack before of after overwriting the return pointer and then jump to it to execute it:
    • In any case, if there is a canary, in a regular bof you will need to bypass (leak) it
    • Without ASLR and nx it's possible to jump to the address of the stack as it won't never change
    • With ASLR you will need techniques such as ret2esp/ret2reg to jump to it
    • With nx, you will need to use some ROP to call memprotect and make some page rwx, in order to then store the shellcode in there (calling read for example) and then jump there.
      • This will mix shellcode with a ROP chain.

Via syscalls

  • Ret2syscall: Useful to call execve to run arbitrary commands. You need to be able to find the gadgets to call the specific syscall with the parameters.
    • If ASLR or PIE are enabled you'll need to defeat them in order to use ROP gadgets from the binary or libraries.
    • SROP can be useful to prepare the ret2execve
    • Gadgets from ret2csu and ret2vdso to control several registers

Via libc

  • Ret2lib: Useful to call a function from a library (usually from libc) like system with some prepared arguments (e.g. '/bin/sh'). You need the binary to load the library with the function you would like to call (libc usually).
    • If statically compiled and no PIE, the address of system and /bin/sh are not going to change, so it's possible to use them statically.
    • Without ASLR and knowing the libc version loaded, the address of system and /bin/sh are not going to change, so it's possible to use them statically.
    • With ASLR but no PIE, knowing the libc and with the binary using the system function it's possible to ret to the address of system in the GOT with the address of '/bin/sh' in the param (you will need to figure this out).
    • With ASLR but no PIE, knowing the libc and without the binary using the system :
      • Use ret2dlresolve to resolve the address of system and call it
      • Bypass ASLR and calculate the address of system and '/bin/sh' in memory.
    • With ASLR and PIE and not knowing the libc: You need to:
      • Bypass PIE
      • Find the libc version used (leak a couple of function addresses)
      • Check the previous scenarios with ASLR to continue.

Via EBP/RBP

  • Stack Pivoting / EBP2Ret / EBP Chaining: Control the ESP to control RET through the stored EBP in the stack.
    • Useful for off-by-one stack overflows
    • Useful as an alternate way to end controlling EIP while abusing EIP to construct the payload in memory and then jumping to it via EBP

Misc

  • Pointers Redirecting: In case the stack contains pointers to a function that is going to be called or to a string that is going to be used by an interesting function (system or printf), it's possible to overwrite that address.
    • ASLR or PIE might affect the addresses.
  • Uninitialized variables: You never know

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