ssh - por que posso fazer login com senhas parciais? [duplicado]

Digamos que eu tenha uma máquina (Arago dist) com uma senha de usuário de 12 caracteres alfanuméricos. Quando eu me logo via ssh usando autenticação de senha, notei alguns dias atrás, que posso inserir apenas 8 dos caracteres da senha ou a senha inteira seguida do que eu quiser. O resultado comum em ambas as situações é um login bem-sucedido. Por que isso está acontecendo?

Nesse caso específico, não quero usar a autenticação de chave pública com base em vários motivos.

Como informação adicional, nesta distro os arquivos /etc/shadow e /etc/security/policy.conf estão faltando.

Aqui a configuração ssh do servidor:

[user@machine:~] cat /etc/ssh/sshd_config

#   $OpenBSD: sshd_config,v 1.80 2008/07/02 02:24:18 djm Exp $
# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options change a
# default value.

Banner /etc/ssh/welcome.msg

#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

# Disable legacy (protocol version 1) support in the server for new
# installations. In future the default will change to require explicit
# activation of protocol 1
Protocol 2

# HostKey for protocol version 1
#HostKey /etc/ssh/ssh_host_key
# HostKeys for protocol version 2
#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key

# Lifetime and size of ephemeral version 1 server key
#KeyRegenerationInterval 1h
#ServerKeyBits 1024

# Logging
# obsoletes QuietMode and FascistLogging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
PermitRootLogin no
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#RSAAuthentication yes
#PubkeyAuthentication yes
#AuthorizedKeysFile .ssh/authorized_keys

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#RhostsRSAAuthentication no
# similar for protocol version 2
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes

# Set this to 'yes' to enable PAM authentication, account processing, 
# and session processing. If this is enabled, PAM authentication will 
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
#UsePAM no

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
#X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
UsePrivilegeSeparation no
#PermitUserEnvironment no
Compression no
ClientAliveInterval 15
ClientAliveCountMax 4
#UseDNS yes
#PidFile /var/run/sshd.pid
#MaxStartups 10
#PermitTunnel no
#ChrootDirectory none

# no default banner path
#Banner none

# override default of no subsystems
Subsystem   sftp    /usr/libexec/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#   X11Forwarding no
#   AllowTcpForwarding no
#   ForceCommand cvs server

Aqui a saída do cliente ssh:

myself@ubuntu:~$ ssh -vvv [email protected]
OpenSSH_6.6.1, OpenSSL 1.0.1f 6 Jan 2014
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug2: ssh_connect: needpriv 0
debug1: Connecting to 192.168.1.1 [192.168.1.1] port 22.
debug1: Connection established.
debug3: Incorrect RSA1 identifier
debug3: Could not load "/home/myself/.ssh/id_rsa" as a RSA1 public key
debug1: identity file /home/myself/.ssh/id_rsa type 1
debug1: identity file /home/myself/.ssh/id_rsa-cert type -1
debug1: identity file /home/myself/.ssh/id_dsa type -1
debug1: identity file /home/myself/.ssh/id_dsa-cert type -1
debug1: identity file /home/myself/.ssh/id_ecdsa type -1
debug1: identity file /home/myself/.ssh/id_ecdsa-cert type -1
debug1: identity file /home/myself/.ssh/id_ed25519 type -1
debug1: identity file /home/myself/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.6
debug1: match: OpenSSH_5.6 pat OpenSSH_5* compat 0x0c000000
debug2: fd 3 setting O_NONBLOCK
debug3: load_hostkeys: loading entries for host "192.168.1.1" from file "/home/myself/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/myself/.ssh/known_hosts:26
debug3: load_hostkeys: loaded 1 keys
debug3: order_hostkeyalgs: prefer hostkeyalgs: [email protected],[email protected],ssh-rsa
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: [email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: [email protected],[email protected],ssh-rsa,[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[email protected],[email protected],[email protected],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],[email protected],hmac-md5,hmac-sha1,[email protected],[email protected],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[email protected],zlib
debug2: kex_parse_kexinit: none,[email protected],zlib
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: first_kex_follows 0 
debug2: kex_parse_kexinit: reserved 0 
debug2: kex_parse_kexinit: diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[email protected]
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[email protected],hmac-ripemd160,[email protected],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none
debug2: kex_parse_kexinit: none
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: 
debug2: kex_parse_kexinit: first_kex_follows 0 
debug2: kex_parse_kexinit: reserved 0 
debug2: mac_setup: setup hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 none
debug2: mac_setup: setup hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<3072<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: bits set: 1481/3072
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA 91:66:c0:07:e0:c0:df:b7:8e:49:97:b5:36:12:12:ea
debug3: load_hostkeys: loading entries for host "192.168.1.1" from file "/home/myself/.ssh/known_hosts"
debug3: load_hostkeys: found key type RSA in file /home/myself/.ssh/known_hosts:26
debug3: load_hostkeys: loaded 1 keys
debug1: Host '192.168.1.1' is known and matches the RSA host key.
debug1: Found key in /home/myself/.ssh/known_hosts:26
debug2: bits set: 1551/3072
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/myself/.ssh/id_rsa (0x802b9240),
debug2: key: /home/myself/.ssh/id_dsa ((nil)),
debug2: key: /home/myself/.ssh/id_ecdsa ((nil)),
debug2: key: /home/myself/.ssh/id_ed25519 ((nil)),
debug3: input_userauth_banner
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug3: start over, passed a different list publickey,password,keyboard-interactive
debug3: preferred gssapi-keyex,gssapi-with-mic,publickey,keyboard-interactive,password
debug3: authmethod_lookup publickey
debug3: remaining preferred: keyboard-interactive,password
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /home/myself/.ssh/id_rsa
debug3: send_pubkey_test
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Trying private key: /home/myself/.ssh/id_dsa
debug3: no such identity: /home/myself/.ssh/id_dsa: No such file or directory
debug1: Trying private key: /home/myself/.ssh/id_ecdsa
debug3: no such identity: /home/myself/.ssh/id_ecdsa: No such file or directory
debug1: Trying private key: /home/myself/.ssh/id_ed25519
debug3: no such identity: /home/myself/.ssh/id_ed25519: No such file or directory
debug2: we did not send a packet, disable method
debug3: authmethod_lookup keyboard-interactive
debug3: remaining preferred: password
debug3: authmethod_is_enabled keyboard-interactive
debug1: Next authentication method: keyboard-interactive
debug2: userauth_kbdint
debug2: we sent a keyboard-interactive packet, wait for reply
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug3: userauth_kbdint: disable: no info_req_seen
debug2: we did not send a packet, disable method
debug3: authmethod_lookup password
debug3: remaining preferred: 
debug3: authmethod_is_enabled password
debug1: Next authentication method: password
[email protected]'s password: 
debug3: packet_send2: adding 64 (len 57 padlen 7 extra_pad 64)
debug2: we sent a password packet, wait for reply
debug1: Authentication succeeded (password).
Authenticated to 192.168.1.1 ([192.168.1.1]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Requesting [email protected]
debug1: Entering interactive session.
debug2: callback start
debug2: fd 3 setting TCP_NODELAY
debug3: packet_set_tos: set IP_TOS 0x10
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug1: Sending environment.
debug3: Ignored env XDG_VTNR
debug3: Ignored env MANPATH
debug3: Ignored env XDG_SESSION_ID
debug3: Ignored env CLUTTER_IM_MODULE
debug3: Ignored env SELINUX_INIT
debug3: Ignored env XDG_GREETER_DATA_DIR
debug3: Ignored env COMP_WORDBREAKS
debug3: Ignored env SESSION
debug3: Ignored env NVM_CD_FLAGS
debug3: Ignored env GPG_AGENT_INFO
debug3: Ignored env TERM
debug3: Ignored env SHELL
debug3: Ignored env XDG_MENU_PREFIX
debug3: Ignored env VTE_VERSION
debug3: Ignored env NVM_PATH
debug3: Ignored env GVM_ROOT
debug3: Ignored env WINDOWID
debug3: Ignored env UPSTART_SESSION
debug3: Ignored env GNOME_KEYRING_CONTROL
debug3: Ignored env GTK_MODULES
debug3: Ignored env NVM_DIR
debug3: Ignored env USER
debug3: Ignored env LD_LIBRARY_PATH
debug3: Ignored env LS_COLORS
debug3: Ignored env XDG_SESSION_PATH
debug3: Ignored env XDG_SEAT_PATH
debug3: Ignored env SSH_AUTH_SOCK
debug3: Ignored env SESSION_MANAGER
debug3: Ignored env DEFAULTS_PATH
debug3: Ignored env XDG_CONFIG_DIRS
debug3: Ignored env PATH
debug3: Ignored env DESKTOP_SESSION
debug3: Ignored env QT_IM_MODULE
debug3: Ignored env QT_QPA_PLATFORMTHEME
debug3: Ignored env NVM_NODEJS_ORG_MIRROR
debug3: Ignored env GVM_VERSION
debug3: Ignored env JOB
debug3: Ignored env PWD
debug3: Ignored env XMODIFIERS
debug3: Ignored env GNOME_KEYRING_PID
debug1: Sending env LANG = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug3: Ignored env gvm_pkgset_name
debug3: Ignored env GDM_LANG
debug3: Ignored env MANDATORY_PATH
debug3: Ignored env IM_CONFIG_PHASE
debug3: Ignored env COMPIZ_CONFIG_PROFILE
debug3: Ignored env GDMSESSION
debug3: Ignored env SESSIONTYPE
debug3: Ignored env XDG_SEAT
debug3: Ignored env HOME
debug3: Ignored env SHLVL
debug3: Ignored env GOROOT
debug3: Ignored env LANGUAGE
debug3: Ignored env GNOME_DESKTOP_SESSION_ID
debug3: Ignored env DYLD_LIBRARY_PATH
debug3: Ignored env gvm_go_name
debug3: Ignored env LOGNAME
debug3: Ignored env GVM_OVERLAY_PREFIX
debug3: Ignored env COMPIZ_BIN_PATH
debug3: Ignored env XDG_DATA_DIRS
debug3: Ignored env QT4_IM_MODULE
debug3: Ignored env DBUS_SESSION_BUS_ADDRESS
debug3: Ignored env PrlCompizSessionClose
debug3: Ignored env PKG_CONFIG_PATH
debug3: Ignored env GOPATH
debug3: Ignored env NVM_BIN
debug3: Ignored env LESSOPEN
debug3: Ignored env NVM_IOJS_ORG_MIRROR
debug3: Ignored env INSTANCE
debug3: Ignored env TEXTDOMAIN
debug3: Ignored env XDG_RUNTIME_DIR
debug3: Ignored env DISPLAY
debug3: Ignored env XDG_CURRENT_DESKTOP
debug3: Ignored env GTK_IM_MODULE
debug3: Ignored env LESSCLOSE
debug3: Ignored env TEXTDOMAINDIR
debug3: Ignored env GVM_PATH_BACKUP
debug3: Ignored env COLORTERM
debug3: Ignored env XAUTHORITY
debug3: Ignored env _
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0

Aqui a saída do servidor sshd:

debug1: sshd version OpenSSH_5.6p1
debug1: read PEM private key done: type RSA
debug1: private host key: #0 type 1 RSA
debug1: read PEM private key done: type DSA
debug1: private host key: #1 type 2 DSA
debug1: rexec_argv[0]='/usr/sbin/sshd'
debug1: rexec_argv[1]='-d'
Set /proc/self/oom_adj from 0 to -17
debug1: Bind to port 22 on 0.0.0.0.
Server listening on 0.0.0.0 port 22.
socket: Address family not supported by protocol
debug1: Server will not fork when running in debugging mode.
debug1: rexec start in 4 out 4 newsock 4 pipe -1 sock 7
debug1: inetd sockets after dupping: 3, 3
Connection from 192.168.1.60 port 53445
debug1: Client protocol version 2.0; client software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2.8 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.6
debug1: list_hostkey_types: ssh-rsa,ssh-dss
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST received
debug1: SSH2_MSG_KEX_DH_GEX_GROUP sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_INIT
debug1: SSH2_MSG_KEX_DH_GEX_REPLY sent
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: KEX done
debug1: userauth-request for user user service ssh-connection method none
debug1: attempt 0 failures 0
debug1: userauth_send_banner: sent
Failed none for user from 192.168.1.60 port 53445 ssh2
debug1: userauth-request for user user service ssh-connection method publickey
debug1: attempt 1 failures 0
debug1: test whether pkalg/pkblob are acceptable
debug1: temporarily_use_uid: 0/0 (e=0/0)
debug1: trying public key file //.ssh/authorized_keys
debug1: Could not open authorized keys '//.ssh/authorized_keys': No such file or directory
debug1: restore_uid: 0/0
debug1: temporarily_use_uid: 0/0 (e=0/0)
debug1: trying public key file //.ssh/authorized_keys2
debug1: Could not open authorized keys '//.ssh/authorized_keys2': No such file or directory
debug1: restore_uid: 0/0
Failed publickey for user from 192.168.1.60 port 53445 ssh2
debug1: userauth-request for user user service ssh-connection method keyboard-interactive
debug1: attempt 2 failures 1
debug1: keyboard-interactive devs 
debug1: auth2_challenge: user=user devs=
debug1: kbdint_alloc: devices ''
Failed keyboard-interactive for user from 192.168.1.60 port 53445 ssh2
debug1: Unable to open the btmp file /var/log/btmp: No such file or directory
debug1: userauth-request for user user service ssh-connection method password
debug1: attempt 3 failures 2
Could not get shadow information for user
Accepted password for user from 192.168.1.60 port 53445 ssh2
debug1: Entering interactive session for SSH2.
debug1: server_init_dispatch_20
debug1: server_input_channel_open: ctype session rchan 0 win 1048576 max 16384
debug1: input_session_request
debug1: channel 0: new [server-session]
debug1: session_new: session 0
debug1: session_open: channel 0
debug1: session_open: session 0: link with channel 0
debug1: server_input_channel_open: confirm session
debug1: server_input_global_request: rtype [email protected] want_reply 0
debug1: server_input_channel_req: channel 0 request pty-req reply 1
debug1: session_by_channel: session 0 channel 0
debug1: session_input_channel_req: session 0 req pty-req
debug1: Allocating pty.
debug1: session_pty_req: session 0 alloc /dev/pts/1
debug1: server_input_channel_req: channel 0 request env reply 0
debug1: session_by_channel: session 0 channel 0
debug1: session_input_channel_req: session 0 req env
debug1: server_input_channel_req: channel 0 request shell reply 1
debug1: session_by_channel: session 0 channel 0
debug1: session_input_channel_req: session 0 req shell
debug1: Setting controlling tty using TIOCSCTTY.

/etc/pam.d/sshd:

# PAM configuration for the Secure Shell service

# Read environment variables from /etc/environment and
# /etc/security/pam_env.conf.
auth       required     pam_env.so # [1]

# Standard Un*x authentication.
auth       include      common-auth

# Disallow non-root logins when /etc/nologin exists.
account    required     pam_nologin.so

# Uncomment and edit /etc/security/access.conf if you need to set complex
# access limits that are hard to express in sshd_config.
# account  required     pam_access.so

# Standard Un*x authorization.
account    include      common-accountt

# Standard Un*x session setup and teardown.
session    include      common-session

# Print the message of the day upon successful login.
session    optional     pam_motd.so # [1]

# Print the status of the user's mailbox upon successful login.
session    optional     pam_mail.so standard noenv # [1]

# Set up user limits from /etc/security/limits.conf.
session    required     pam_limits.so

# Standard Un*x password updating.
password   include      common-password