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ångstromCTF 2023: gaga2

70 points, 220 solves

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TL;DR

Leak puts address, and find the correct LIBC version from Bulkat database. Use the LIBC base address to perform a ret2libc attack.

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Basic File Checks

In this 3 part challenge, gaga0 & gaga1 were solved using ret2win. However, gaga2 suggests that there is no win function available, so we're left with shellcode injection or ret2libc attack.

┌──(kali💀JesusCries)-[~/Desktop/CTF/angStrom2023/gaga]
└─$ ./gaga2 
Awesome! Now there's no system(), so what will you do?!
Your input: test

Performing binary protection check shows that NX is enabled, so shellcode injection is not feasible. Additionally, NX enabled is hinting that we need to perform a ROP attack.

┌──(kali💀JesusCries)-[~/Desktop/CTF/angStrom2023/gaga]
└─$ checksec --file=gaga2
[*] '/home/kali/Desktop/CTF/angStrom2023/gaga/gaga2'
    Arch:     amd64-64-little
    RELRO:    Partial RELRO
    Stack:    No canary found
    NX:       NX enabled
    PIE:      No PIE (0x400000)

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Static Code Analysis

gets function is used without boundary check, leading to a pwnable scenario for Buffer Overflow attack.

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Finding Offset

Using de Brujin Sequence and pwndbg to locate the offset.

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Attack Strategy

Since there is no win function available, we can instead create our own system function /bin/sh from LIBC, and direct program execution to that function address, this is known as a ret2libc or ret2system attack. To do so, we will need the base address of LIBC library.

Technically, you can manually carve for the address of system(/bin/sh) from LIBC.

However, this is only the relative offset, not the actual address due to ASLR. In order for this attack to work, we need to calculate LIBC base address + offset address to get the actual address of system(/bin/sh).

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Leaking GOT Address

Using the BoF vulnerability, we can leak any function address from the Global Offset Table (GOT).

There are 2 methods to enumerate LIBC functions from the GOT, using either objdump or Ghidra's Program Tree.

The exploit ROP chain consists of 5 parts:

  • asm('nop') * padding: Filling up the buffer to reach the offset.

  • pop_rdi: Prepare the rdi register by popping the value off stack (emptying out the register).

  • elf.got.puts: The address we want to leak. This can be any exportable function from gaga2 binary. In this case, we will leak the address of puts function.

  • elf.plt.puts: The function of choice invoked to leak got.puts address. The reason why puts is used again here, is because puts only requires 1 arguments, so we only need to prepare 1 register rdi. Technically speaking, we can also use write or printf here, but it requires more arguments, hence more ROPGadgets to prepare.

  • elf.symbols.main: Finally, we want to redirect program execution back to main to re-exploit the BoF vulnerability.

Exploit the BoF vulnerability once to leak puts address. Note that this is the actual address, not the relative offset.

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Calculating LIBC Base Address

Since the challenge author did not provide the LIBC file used by the remote instance, we can perform a lookup from Blukat Database using the leaked puts address. Turns out to be libc6_2.31-0ubuntu9.9_amd64.

Now that we know which version of LIBC is used, we can carve the relative offset of puts using libc.symbols['puts'], and compare it with the leaked address of got.puts to calculate the base address of LIBC.

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ret2libc

Using the base address of LIBC and relative offsets, we can calculate the actual address of system(/bin/sh) to pwn the remote server. The ROP chain for 2nd round of exploitation is essentially the same. We need to pop rdi again since system() accepts 1 argument, and insert ret in the middle for 64-bit stack alignment.

Flag: actf{b4by's_f1rst_pwn!_3857ffd6bfdf775e}

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