É possível tornar um telnet seguro usando essas soluções
A) Um túnel ssh, é um absurdo (por que usar o túnel quando você pode usar o ssh?), mas funciona
ssh -L 23:localhost:23 -N -f your host
Claro, feche a porta 23 com firewall e deixe abrir a porta pessoal 22 ou ssh para proibir o acesso direto ao telnet
B) Um stunnel tls ou ssl (eu prefiro o TLS), é muito fácil de fazer, no servidor que você usa
; Sample stunnel configuration file for Unix by Michal Trojnara 2002-2012
; Some options used here may be inadequate for your particular configuration
; This sample file does *not* represent stunnel.conf defaults
; Please consult the manual for detailed description of available options
; **************************************************************************
; * Global options *
; **************************************************************************
; A copy of some devices and system files is needed within the chroot jail
; Chroot conflicts with configuration file reload and many other features
chroot = /var/lib/stunnel/
; Chroot jail can be escaped if setuid option is not used
setuid = nobody
setgid = nogroup
; PID is created inside the chroot jail
pid = /stunnel.pid
; Debugging stuff (may useful for troubleshooting)
;debug = 7
;output = stunnel.log
; **************************************************************************
; * Service defaults may also be specified in individual service sections *
; **************************************************************************
; Certificate/key is needed in server mode and optional in client mode
cert = //etc/ssl/certs/yourserver.crt
key = //etc/ssl/private/yourserver.key
; Authentication stuff needs to be configured to prevent MITM attacks
; It is not enabled by default!
verify = 2
; Don't forget to c_rehash CApath
; CApath is located inside chroot jail
;CApath = /
; It's often easier to use CAfile
CAfile = /yourserver.ca
; Don't forget to c_rehash CRLpath
; CRLpath is located inside chroot jail
;CRLpath = /crls
; Alternatively CRLfile can be used
;CRLfile = /usr/etc/stunnel/crls.pem
; Disable support for insecure SSLv2 protocol
options = NO_SSLv2
; Workaround for Eudora bug
;options = DONT_INSERT_EMPTY_FRAGMENTS
; These options provide additional security at some performance degradation
;options = SINGLE_ECDH_USE
;options = SINGLE_DH_USE
; **************************************************************************
; * Service definitions (remove all services for inetd mode) *
; **************************************************************************
; Example SSL server mode services
[telnet]
accept = 0.0.0.0:5939
connect = 23
no cliente
; Sample stunnel configuration file for Unix by Michal Trojnara 2002-2012
; Some options used here may be inadequate for your particular configuration
; This sample file does *not* represent stunnel.conf defaults
; Please consult the manual for detailed description of available options
client=yes
; **************************************************************************
; * Global options *
; **************************************************************************
; A copy of some devices and system files is needed within the chroot jail
; Chroot conflicts with configuration file reload and many other features
;chroot = /var/lib/stunnel/
; Chroot jail can be escaped if setuid option is not used
;setuid = nobody
;setgid = nogroup
; PID is created inside the chroot jail
;pid = /stunnel.pid
; Debugging stuff (may useful for troubleshooting)
;debug = 7
;output = stunnel.log
; **************************************************************************
; * Service defaults may also be specified in individual service sections *
; **************************************************************************
; Certificate/key is needed in server mode and optional in client mode
cert = /yourclient.crt
key = /yourclient.key
; Authentication stuff needs to be configured to prevent MITM attacks
; It is not enabled by default!
;verify = 2
; Don't forget to c_rehash CApath
; CApath is located inside chroot jail
;CApath = /certs
; It's often easier to use CAfile
CAfile = /yourca.crt
; Don't forget to c_rehash CRLpath
; CRLpath is located inside chroot jail
;CRLpath = /crls
; Alternatively CRLfile can be used
;CRLfile = /usr/etc/stunnel/crls.pem
; Disable support for insecure SSLv2 protocol
options = NO_SSLv2
; Workaround for Eudora bug
;options = DONT_INSERT_EMPTY_FRAGMENTS
; These options provide additional security at some performance degradation
;options = SINGLE_ECDH_USE
;options = SINGLE_DH_USE
; **************************************************************************
; * Service definitions (remove all services for inetd mode) *
; **************************************************************************
; Example SSL server mode services
[telnet]
accept=localhost:23
connect=yourstunnelserver:5939
Claro, use esses arquivos apenas por exemplo e modifique para sua configuração
Você acessará o servidor remoto
telnet localhost 23
usando um túnel tls seguro
O terceiro método é usar telnet com keberos, é legal porque nenhum passwd perguntou e permite um SSO, mas requer cifras fracas, então não é 100% seguro
no inetd.conf edite e adicione ou modifique
telnet stream tcp nowait root /usr/kerberos/sbin/telnetd
no cliente
telnet -x server
Se conectar sem perguntar passe está funcionando, se receber pedir senha ou erro de mensagem, o telnet está claro