Exploit Development : Payloads
As you develop exploits for newer versions of the Windows operation systems, you will find that they now have Data Execution Prevention (DEP) enabled by default. DEP prevents shellcode from being executed on the stack and has forced exploit developers to find a way around this mitigation and the so-called Return Oriented Programming (ROP) was developed.
A ROP payload in created by using pre-existing sets of instructions from non-ASLR enabled binaries to make your shellcode executable. Each set of instructions needs to end in a RETN instruction to carry on the ROP-chain with each set of instructions commonly referred to as a gadget.
The “msfrop” tool in Metasploit will search a given binary and return the usable gadgets.
root@kali:# msfrop -h Options: -d, --depth [size] Number of maximum bytes to backwards disassemble from return instructions -s, --search [regex] Search for gadgets matching a regex, match intel syntax or raw bytes -n, --nocolor Disable color. Useful for piping to other tools like the less and more commands -x, --export [filename] Export gadgets to CSV format -i, --import [filename] Import gadgets from previous collections -v, --verbose Output very verbosely -h, --help Show this message
Running msfrop with the -v switch will return all of the found gadgets directly to the console:
root@kali:/tmp# msfrop -v metsrv.dll Collecting gadgets from metsrv.dll Found 4829 gadgets metsrv.dll gadget: 0x10001057 0x10001057: leave 0x10001058: ret metsrv.dll gadget: 0x10001241 0x10001241: leave 0x10001242: ret metsrv.dll gadget: 0x1000132e 0x1000132e: leave 0x1000132f: ret metsrv.dll gadget: 0x1000138c 0x1000138c: leave 0x1000138d: ret ...snip...
The verbose msfrop output is not particularly helpful when a binary contains thousands of gadgets, so a far more useful switch is ‘-x‘ which allows you to output the gadgets into a CSV file that you can then search later.
root@kali:/tmp# msfrop -x metsrv_gadgets metsrv.dll Collecting gadgets from metsrv.dll Found 4829 gadgets Found 4829 gadgets total Exporting 4829 gadgets to metsrv_gadgets Success! gadgets exported to metsrv_gadgets root@kali:/tmp# head -n 10 metsrv_gadgets Address,Raw,Disassembly "0x10001098","5ec20c00","0x10001098: pop esi | 0x10001099: ret 0ch | " "0x100010f7","5ec20800","0x100010f7: pop esi | 0x100010f8: ret 8 | " "0x1000113d","5dc21800","0x1000113d: pop ebp | 0x1000113e: ret 18h | " "0x1000117a","5dc21c00","0x1000117a: pop ebp | 0x1000117b: ret 1ch | " "0x100011c3","5dc22800","0x100011c3: pop ebp | 0x100011c4: ret 28h | " "0x100018b5","5dc20c00","0x100018b5: pop ebp | 0x100018b6: ret 0ch | " "0x10002cb4","c00f9fc28d54","0x10002cb4: ror byte ptr [edi], 9fh | 0x10002cb7: ret 548dh | " "0x10002df8","0483c20483","0x10002df8: add al, -7dh | 0x10002dfa: ret 8304h | " "0x10002e6e","080bc20fb6","0x10002e6e: or [ebx], cl | 0x10002e70: ret 0b60fh | " root@kali:/tmp#