Port 21 – FTP (File Transfer Protocol)

Why It’s Open

FTP is one of the oldest methods for transferring files over a network. You’ll often find it in legacy systems, embedded devices, or setups that require batch file uploads or remote file access.

Common Risks

Cleartext Authentication: Usernames and passwords are transmitted in plaintext.
Anonymous Access: Misconfigured FTP servers may allow login without credentials.
Directory Traversal: Attackers may access restricted directories using ../.
Brute Force & Default Credentials: Common targets for automated tools.

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Enumeration & Testing

Check if it’s open

nmap -sV -p 21
nmap -p 21 --script=ftp-anon

Connect manually

ftp

ftp
Name: anonymous
Password:

What to Look For

Checkpoint	What it means
Anonymous login enabled	Public access, often a misconfiguration
Banner shows old version	May be vulnerable to known exploits
FTP root access	Can lead to full system compromise

Known Exploits

CVE-2015-3306

The mod_copy module in ProFTPD 1.3.5 allows remote attackers to read and write to arbitrary files via the SITE CPFR and SITE CPTO commands, leading to potential remote code execution.

🔗 NVD Entry

🔗 Exploit-DB Entry

Mitigation

Disable anonymous login.
Use FTPS or switch to SFTP instead.
Restrict access via firewall.
Limit users to specific chroot jails.
Disable write permissions where not needed.

Real-World Example

Publicly exposed FTP servers allowing anonymous access have led to major leaks of personal and financial data in the past. Tools like Shodan still show thousands of such servers online today.

TL;DR

Service: FTP (File Transfer Protocol)
Default Port: 21/TCP
Risks: Unauthorized file access, data leakage
Mitigation: Apply patches, disable anonymous access, use secure alternatives