利用系统特性伪装成一个免密登陆后门

#

# The PAM configuration file for the Shadow `su' service

#

# This allows root to su without passwords (normal operation)

auth       sufficient pam_rootok.so

# Uncomment this to force users to be a member of group root

# before they can use `su'. You can also add "group=foo"

# to the end of this line if you want to use a group other

# than the default "root" (but this may have side effect of

# denying "root" user, unless she's a member of "foo" or explicitly

# permitted earlier by e.g. "sufficient pam_rootok.so").

# (Replaces the `SU_WHEEL_ONLY' option from login.defs)

# auth       required   pam_wheel.so

# Uncomment this if you want wheel members to be able to

# su without a password.

# auth       sufficient pam_wheel.so trust

# Uncomment this if you want members of a specific group to not

# be allowed to use su at all.

# auth       required   pam_wheel.so deny group=nosu

# Uncomment and edit /etc/security/time.conf if you need to set

# time restrainst on su usage.

# (Replaces the `PORTTIME_CHECKS_ENAB' option from login.defs

# as well as /etc/porttime)

# account    requisite  pam_time.so

# This module parses environment configuration file(s)

# and also allows you to use an extended config

# file /etc/security/pam_env.conf.

# 

# parsing /etc/environment needs "readenv=1"

session       required   pam_env.so readenv=1

# locale variables are also kept into /etc/default/locale in etch

# reading this file *in addition to /etc/environment* does not hurt

session       required   pam_env.so readenv=1 envfile=/etc/default/locale

# Defines the MAIL environment variable

# However, userdel also needs MAIL_DIR and MAIL_FILE variables

# in /etc/login.defs to make sure that removing a user 

# also removes the user's mail spool file.

# See comments in /etc/login.defs

#

# "nopen" stands to avoid reporting new mail when su'ing to another user

session    optional   pam_mail.so nopen

# Sets up user limits according to /etc/security/limits.conf

# (Replaces the use of /etc/limits in old login)

session    required   pam_limits.so

# The standard Unix authentication modules, used with

# NIS (man nsswitch) as well as normal /etc/passwd and

# /etc/shadow entries.

@include common-auth

@include common-account

@include common-session

重点是这行：
    auth       sufficient pam_rootok.so