Plink.exe Windows Pivoting - HTB Academy Page 8

📋 Module Overview

Purpose: Windows-based SSH tunneling and pivoting using Plink.exe
Tool: PuTTY Link (plink.exe) - Windows command-line SSH client
Scenario: Windows attack host or compromised Windows pivot
Technique: Dynamic port forwarding with SOCKS proxy
Integration: Proxifier for Windows application tunneling

1. Introduction to Plink.exe

What is Plink?

Full Name: PuTTY Link
Type: Windows command-line SSH tool
Package: Part of PuTTY suite
Capability: SSH tunneling, port forwarding, SOCKS proxy
Era: Pre-Windows 10 standard (before native OpenSSH)

Why Use Plink?

Living off the Land - often pre-installed on Windows systems
Windows Native - no need to transfer additional tools
Stealth - uses legitimate administrative tool
Compatibility - works on older Windows versions
Integration - pairs well with Windows tools like Proxifier

Common Scenarios

Windows-based attack host instead of Linux
Compromised Windows system as pivot point
Locked down environment where uploading tools is risky
Legacy systems with PuTTY already installed
File share access to plink.exe without installation

2. Plink vs SSH Comparison

Aspect	SSH (Linux)	Plink (Windows)
Platform	Linux/Unix	Windows
Syntax	`ssh -D 9050 user@host`	`plink -ssh -D 9050 user@host`
Authentication	Key/password	Key/password
Integration	Native Linux tools	Proxifier, Windows apps
Stealth	Standard on Linux	Legitimate Windows tool
Availability	Always present	Depends on PuTTY install

3. Basic Plink Dynamic Port Forwarding

Network Topology

[Windows Attack Host] → [Ubuntu Pivot] → [Internal Network]
    10.10.15.5           10.129.15.50      172.16.5.0/24
    Plink Client         SSH Server        Target Systems
    SOCKS :9050

Command Syntax

# Basic dynamic port forward with Plink
plink -ssh -D 9050 ubuntu@10.129.15.50

# Command breakdown:
# -ssh        - Use SSH protocol
# -D 9050     - Dynamic port forward on local port 9050
# ubuntu      - Username on pivot host
# @10.129.15.50 - Pivot host IP address

Expected Output

Using username "ubuntu".
ubuntu@10.129.15.50's password:
Welcome to Ubuntu 20.04.3 LTS (GNU/Linux 5.4.0-88-generic x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

Last login: Mon Mar  7 15:30:45 2022 from 10.10.15.5
ubuntu@pivot:~$

Verification

# Check if SOCKS proxy is listening (Windows Command Prompt)
netstat -an | findstr :9050

# Expected output:
TCP    127.0.0.1:9050         0.0.0.0:0              LISTENING

4. Proxifier Integration

What is Proxifier?

Purpose: Windows SOCKS/HTTP proxy client
Function: Routes application traffic through proxies
Capability: Proxy chaining, application-specific routing
Target: Desktop applications (RDP, browsers, etc.)

Proxifier Configuration Steps

Step 1: Add SOCKS Server

Proxifier → Profile Menu → Proxy Servers → Add

Server Configuration:
- Address: 127.0.0.1
- Port: 9050
- Protocol: SOCKS Version 4
- Authentication: None (for basic setup)

Step 2: Create Proxification Rules

Proxifier → Profile Menu → Proxification Rules → Add

Rule Configuration:
- Name: "RDP through Plink"
- Applications: mstsc.exe
- Target hosts: 172.16.5.*
- Action: Proxy SOCKS 127.0.0.1:9050

Step 3: Enable Proxification

Proxifier → Profile Menu → Proxification Rules → Enable Rules
Check: "Process all connections through proxy"

5. RDP Through Plink SOCKS Tunnel

Complete Workflow

Step 1: Start Plink SOCKS Tunnel

# Windows Command Prompt
plink -ssh -D 9050 ubuntu@10.129.15.50

# Keep this session active for tunneling

Step 2: Configure Proxifier

1. Open Proxifier
2. Add SOCKS proxy: 127.0.0.1:9050
3. Create rule for mstsc.exe
4. Enable proxification

Step 3: Launch RDP Session

# Start Remote Desktop Connection
mstsc.exe

# Connect to internal target:
Computer: 172.16.5.19
Username: victor
Password: pass@123

Traffic Flow Analysis

[mstsc.exe] → [Proxifier] → [Plink SOCKS] → [SSH Tunnel] → [Ubuntu Pivot] → [Windows Target RDP]
Windows RDP     Proxy        Local :9050     Encrypted      SSH Server      172.16.5.19:3389
Client          Client                       Connection

6. Advanced Plink Techniques

Authentication Methods

Password Authentication

# Interactive password prompt
plink -ssh -D 9050 ubuntu@10.129.15.50

# Scripted password (less secure)
echo password | plink -ssh -D 9050 ubuntu@10.129.15.50 -pw

Key-based Authentication

# Using PuTTY private key format (.ppk)
plink -ssh -D 9050 -i C:\keys\ubuntu.ppk ubuntu@10.129.15.50

# Convert OpenSSH key to PuTTY format with PuTTYgen if needed

Multiple Port Forwards

# Dynamic + Local port forwards
plink -ssh -D 9050 -L 8080:172.16.5.19:80 ubuntu@10.129.15.50

# Multiple local forwards
plink -ssh -L 3389:172.16.5.19:3389 -L 445:172.16.5.19:445 ubuntu@10.129.15.50

Background Process

# Run Plink in background (Windows)
start /B plink -ssh -D 9050 ubuntu@10.129.15.50

# Check running processes
tasklist | findstr plink

7. Windows Application Integration

Applications That Work with SOCKS Proxies

Native SOCKS Support

✅ Web Browsers (Firefox, Chrome with proxy)
✅ FTP Clients (WinSCP, FileZilla)
✅ SSH Clients (PuTTY, KiTTY)
✅ Tor Browser (built-in SOCKS)

Proxifier-Required Applications

⚙️ mstsc.exe (Remote Desktop)
⚙️ Windows Explorer (SMB shares)
⚙️ Command line tools (ping, telnet)
⚙️ Custom applications

Browser Configuration Example

Firefox → Settings → Network Settings → Manual Proxy Configuration
SOCKS Host: 127.0.0.1
Port: 9050
SOCKS v4

8. Operational Security with Plink

Stealth Considerations

Legitimate Tool - Plink is standard administrative software
Network Noise - SSH traffic appears normal
Process Name - plink.exe is not suspicious
Registry Traces - Minimal system footprint

Detection Risks

Network Monitoring - SSH connections to pivot hosts
Process Monitoring - Unusual plink.exe usage patterns
Proxy Detection - SOCKS traffic analysis
Authentication Logs - SSH login records

Mitigation Strategies

# Use legitimate-looking SSH sessions
plink -ssh -D 9050 admin@server.company.com

# Vary timing and ports
plink -ssh -D 8080 ubuntu@10.129.15.50

# Clean up processes when done
taskkill /F /IM plink.exe

9. Troubleshooting Plink Issues

Common Problems and Solutions

Authentication Failures

# Problem: Access denied
plink: Access denied

# Solutions:
1. Verify username/password
2. Check SSH key permissions
3. Confirm SSH service is running
4. Test with PuTTY GUI first

Connection Refused

# Problem: Network unreachable
plink: Network error: Connection refused

# Solutions:
1. Verify pivot host IP
2. Check SSH port (default 22)
3. Confirm firewall rules
4. Test with telnet

SOCKS Proxy Not Working

# Problem: Applications can't connect through proxy
# Solutions:
1. Verify port 9050 is listening
   netstat -an | findstr :9050

2. Check Proxifier configuration
3. Test with SOCKS-aware application
4. Restart Plink session

Proxifier Issues

# Problem: Proxifier not routing traffic
# Solutions:
1. Check proxy server settings (127.0.0.1:9050)
2. Verify proxification rules
3. Enable debug logging
4. Restart Proxifier service

10. Alternative Windows SSH Tools

Built-in Windows SSH (Windows 10+)

# Modern Windows has native SSH client
ssh -D 9050 ubuntu@10.129.15.50

# Check if available:
where ssh

Other Windows SSH Clients

# KiTTY (PuTTY fork)
kitty -ssh -D 9050 ubuntu@10.129.15.50

# Bitvise SSH Client
BvSsh -host=10.129.15.50 -user=ubuntu -localFwd=9050:127.0.0.1:9050

# MobaXterm
MobaXterm with SSH tunneling

11. Lab Exercise Recreation

HTB Academy Optional Exercise

Task: "Attempt to use Plink from a Windows-based attack host. Set up a proxy connection and RDP to the Windows target (172.16.5.19) with 'victor:pass@123'"

Complete Solution Steps

Step 1: Environment Setup

# Requirements:
- Windows attack host
- Plink.exe available
- Network access to 10.129.202.64 (pivot)
- Target: 172.16.5.19 (internal Windows)

Step 2: Establish Plink Tunnel

# Create SOCKS tunnel through Ubuntu pivot
plink -ssh -D 9050 ubuntu@10.129.202.64

# Enter password when prompted
ubuntu@10.129.202.64's password: HTB_@cademy_stdnt!

Step 3: Configure Proxifier

1. Open Proxifier
2. Profile → Proxy Servers → Add
   - Address: 127.0.0.1
   - Port: 9050
   - Type: SOCKS4
3. Profile → Proxification Rules → Add
   - Applications: mstsc.exe
   - Target Hosts: 172.16.5.19
   - Action: Proxy 127.0.0.1:9050

Step 4: RDP Connection

# Launch Remote Desktop
mstsc.exe

# Connection details:
Computer: 172.16.5.19
User name: victor
Password: pass@123

Step 5: Submit Answer

Answer: "I tried Plink"

12. Comparison with Linux SSH Methods

Functionality Comparison

Feature	Linux SSH	Windows Plink
Dynamic Forward	`ssh -D 9050`	`plink -ssh -D 9050`
Local Forward	`ssh -L 8080:target:80`	`plink -ssh -L 8080:target:80`
Remote Forward	`ssh -R 8080:localhost:80`	`plink -ssh -R 8080:localhost:80`
Background	`ssh -fN -D 9050`	`start /B plink -ssh -D 9050`
Key Auth	`ssh -i key`	`plink -i key.ppk`

Integration Differences

Linux Integration

# Direct proxychains support
proxychains nmap -sT 172.16.5.19

# Built-in SOCKS applications
curl --socks5 127.0.0.1:9050 http://172.16.5.19

Windows Integration

# Requires Proxifier for most applications
Proxifier → mstsc.exe → 172.16.5.19

# Some native SOCKS support
firefox → proxy settings → SOCKS 127.0.0.1:9050

13. Real-World Scenarios

Scenario 1: Corporate Windows Environment

Situation: Pentesting corporate network
Environment: Windows workstations with PuTTY installed
Goal: Pivot through DMZ host to internal network
Solution: Use Plink for SOCKS tunneling + Proxifier for RDP

Scenario 2: Legacy System Compromise

Situation: Compromised older Windows server
Limitation: Cannot upload new tools
Available: PuTTY suite installed for administration
Solution: Leverage existing Plink for tunneling

Scenario 3: Windows Red Team Operation

Situation: Windows-based red team infrastructure
Challenge: Need to blend in with Windows environment
Approach: Use Windows-native tools (Plink, Proxifier, mstsc)
Benefit: Reduced detection, natural tool usage

14. Best Practices

Operational Guidelines

Test Locally First - Verify Plink works before deployment
Multiple Tunnels - Create redundant paths when possible
Authentication Security - Use keys when possible
Clean Exit - Properly terminate sessions
Documentation - Record tunnel configurations

Security Recommendations

Timing Variation - Don't establish tunnels at predictable times
Port Diversity - Use different SOCKS ports
Session Management - Monitor and limit session duration
Log Cleanup - Clear relevant Windows event logs
Process Hiding - Consider process migration techniques

Performance Optimization

Compression - Use SSH compression for slow links
Keep-Alive - Maintain persistent connections
Concurrent Sessions - Balance load across multiple tunnels
Bandwidth Monitoring - Track usage patterns

15. Integration with Other Tools

Metasploit Integration

# Metasploit with SOCKS proxy (requires Proxychains4Windows)
msf6 > setg Proxies socks4:127.0.0.1:9050
msf6 > use auxiliary/scanner/portscan/tcp
msf6 auxiliary(scanner/portscan/tcp) > set RHOSTS 172.16.5.19
msf6 auxiliary(scanner/portscan/tcp) > run

PowerShell Integration

# PowerShell with proxy settings
$proxy = New-Object System.Net.WebProxy("socks://127.0.0.1:9050")
$webClient = New-Object System.Net.WebClient
$webClient.Proxy = $proxy
$webClient.DownloadString("http://172.16.5.19")

Nmap through Proxy

# Using ProxyChains4Windows (if available)
proxychains4 nmap -sT -Pn 172.16.5.19

# Alternative: nmap with HTTP proxy (if SOCKS-to-HTTP converter used)
nmap --proxy socks4://127.0.0.1:9050 172.16.5.19

References

HTB Academy: Pivoting, Tunneling & Port Forwarding - Page 8
PuTTY Documentation: Official PuTTY Manual
Proxifier Manual: Proxifier Documentation
SANS: SSH Tunneling with Windows
Microsoft: Windows SSH Client

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