Telnet is a clear text remote connect and command protocol and suffers from the following vulnerabilities:
- Vulnerable FTP server application code
- Hardcoded credentials
- Sniffing attack
- Replay attack
Telnet has a long-lasting reputation for being an insecure protocol, but it is still getting implemented on many ICS networks nonetheless. The misconception that ICS networks are hidden away and shielded from networks is probably the cause of this.