KTH Kerberos IV and Kerberos V (Heimdal) for Telnet clients do not encrypt connections if the server does not support the requested encryption, which allows remote attackers to read communications via a man-in-the-middle attack.
https://exchange.xforce.ibmcloud.com/vulnerabilities/10640
https://euvd.enisa.europa.eu/vulnerability/EUVD-2001-1423