Squid Proxy Pentesting

A skill for testing and exploiting Squid HTTP proxy services (default port 3128) during penetration testing engagements.

What this skill does

This skill helps you:

• Enumerate Squid proxy capabilities and configuration
• Test proxy authentication and access controls
• Pivot through Squid to scan internal networks
• Configure proxychains for transparent HTTP interaction
• Chain browser/Burp tools through Squid for interception

When to use this skill

Use this skill when:

• You discover a Squid proxy service (port 3128/tcp, http-proxy)
• You need to test proxy authentication or access controls
• You want to scan internal networks through a discovered proxy
• You need to configure proxychains for HTTP proxy interaction
• You're pivoting through a proxy to reach internal services

Quick Start

1. Basic Proxy Enumeration

Test if the proxy is accessible and check for authentication:

# Test proxy connectivity
curl --proxy http://TARGET_IP:3128 http://TARGET_IP

# Check proxy banner
curl -v --proxy http://TARGET_IP:3128 http://example.com 2>&1 | head -20

If authentication is required, you'll see a 407 Proxy Authentication Required response.

2. Scan Internal Networks Through Proxy

Use the bundled SPOSE scanner to enumerate ports reachable from the proxy:

# Scan all TCP ports through Squid
./scripts/squid-pivot-scan.sh TARGET_IP

# Or with uv (if available)
uv run spose.py --proxy http://TARGET_IP:3128 --target localhost --allports

3. Configure Proxychains

Set up proxychains for transparent HTTP interaction:

# Configure proxychains with Squid
./scripts/configure-proxychains.sh TARGET_IP 3128

# Test with curl
proxychains curl http://127.0.0.1:9191 -v

# Scan internal hosts
proxychains nmap -sT -n -p- localhost

4. Chain Browser/Burp Through Squid

Configure Burp Suite to route through Squid:

1. Open Burp → Proxy → Settings → Network → Connections → Upstream proxy servers
2. Add:

   http://TARGET_IP:3128

3. Requests will flow: Browser → Burp → Squid → Internal Target

This enables interception of services bound to 127.0.0.1 or internal networks.

Detailed Techniques

Proxy Authentication Testing

If the proxy requires authentication:

# Test with credentials
curl --proxy http://user:pass@TARGET_IP:3128 http://TARGET_IP

# Try common credentials (if authorized)
for user in admin root guest; do
  for pass in admin password 123456; do
    curl -s --proxy http://$user:$pass@TARGET_IP:3128 http://TARGET_IP | grep -q "200" && echo "Found: $user:$pass"
  done
done

Nmap Through Proxy

Scan internal networks using proxychains:

# Configure proxychains (see script above)
# Then scan
proxychains nmap -sT -n -p- 127.0.0.1
proxychains nmap -sT -n -p- 10.0.0.0/24

SPOSE Scanner

SPOSE (Squid Pivoting Open Port Scanner) is optimized for proxy pivoting:

# Basic scan
python spose.py --proxy http://TARGET_IP:3128 --target TARGET_IP

# Scan all ports
python spose.py --proxy http://TARGET_IP:3128 --target localhost --allports

# With uv package manager
uv add --script spose.py -r requirements.txt
uv run spose.py --proxy http://TARGET_IP:3128 --target localhost --allports

Common Squid Configurations

Open Proxy (No Auth)

# Will return 200 OK or redirect
curl --proxy http://TARGET_IP:3128 http://example.com

Authenticated Proxy

# Returns 407 Proxy Authentication Required
curl -v --proxy http://TARGET_IP:3128 http://example.com

ACL-Restricted Proxy

May allow only specific destinations or methods. Test with:

# Test different methods
curl -X CONNECT --proxy http://TARGET_IP:3128 example.com:443
curl -X GET --proxy http://TARGET_IP:3128 http://example.com

Security Considerations

• Authorization: Only test proxies you have permission to assess
• Rate limiting: Be mindful of scan rates to avoid triggering alerts
• Logging: Proxy access is typically logged; document your testing
• Legal: Ensure you have written authorization before pivoting through proxies

References

• SPOSE – Squid Pivoting Open Port Scanner
• Squid Documentation
• Proxychains-ng

Bundled Scripts

• scripts/squid-enumerate.sh

  - Basic proxy enumeration and testing

• scripts/squid-pivot-scan.sh

  - SPOSE-based internal network scanning

• scripts/configure-proxychains.sh

  - Proxychains configuration helper