How to find potentially vulnerable GNU inet-utils telnetd servers on your network

A recently reported authentication-bypass vulnerability in GNU inet-utils telnetd allows a crafted $USER environment variable to bypass authentication and grant remote, unauthenticated access to any user account (including root). Affects inet-utils telnetd versions 1.9.3 and higher; no CVE or patch is available yet, so the guidance is to disable telnet where possible and tighten network access controls.