telnet in GNU inetutils through 2.7 allows servers to read arbitrary environment variables from clients via NEW_ENVIRON SEND USERVAR.
Exploitability
AV:NAC:HPR:NUI:RScope
S:CImpact
C:LI:NA:N3.4/CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:N/A:NOther