Insecure network protocols such as HTTP or FTP which do not make use of TLS/SSL can allow Man in the Middle (MitM) attacks to occur.

Use secure protocols whenever possible.

Clear-text protocols lack both encryption and verification features, and as such can allow attackers to easily intercept and/or manipulate data sent over them.

The risks of using such protocols are numerous, including but not limited to:

• Sensitive data leakage - any authentication data sent through an insecure protocol can be read by the attacker.
• Phishing attacks - The attacker could pose as the server the client intended to talk to, or could impersonate a client in their communications to the server.
• Client side attacks - The attacker could send the client malicious data or code to be executed.
• Server side attacks - The attacker could modify requests from the client and attach malicious data or code that would be executed by the server.
• Data loss- The attacker could corrupt the data in the request, leading to data loss or server side data corruption.

Additionally, HTTP as a protocol is deprecated by all major browsers.

Bad Practice

// These are from the Apache Commons Net library:

TelnetClient telnet = new TelnetClient();

FTPClient ftpClient = new FTPClient();

SMTPClient smtpClient = new SMTPClient();

Recommended

Use SSH instead of Telnet. The JSch library is a good option to use here:

JSch jsch = new JSch();

Instead of FTP, use SFTP, SCP or FTPS. JSch supports both SFTP and SCP protocols as well as SSH.

Apache provides a client implementation for FTPS:

FTPSClient client = new FTPSClient(implicit); // the connection can be implicit or explicit.
client.connect(...);
if (!implicit && client.execTLS()) {
    // ... Explicit mode.
} else {
    // ... Implicit mode.
}

Note that implicit FTP is deprecated. Prefer explicit mode unless the connection is required to be implicit (here's an explanation of this).

Use Apache's SMTPSClient to make secure SMTP connections:

SMTPSClient client = new SMTPSClient(true);
client.connect(...);
if (!implicit && client.execTLS()) {
    // ... Explicit mode.
} else {
    // ... Implicit mode.
}

Use proper encryption when creating HTTPS connections.

Exceptions

This issue will not be raised if a loopback connection (to 127.0.0.1 or localhost) is found to be made after creating the client.

Additionally, connections which are designed to operate within a private and secure environment such as a VPN may use unencrypted protocols.

While this is not ideal you may ignore this issue in such cases at your discretion.

References

• OWASP Top Ten (2021) - Category A02 - Cryptographic Failures
• CWE-200 - Exposure of Sensitive Information to an Unauthorized Actor
• CWE-319 - Cleartext Transmission of Sensitive Information
• Apache Commons Net - FTPSClient and SMTPSClient
• FTP Today - Implicit mode vs Explicit mode