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TUCoPS :: Linux :: Discontinued :: linuxs.txt

Linux Security by D4RKCYDE

-->]OO[::[ Linux System Security ]::[OO--[ by zomba ]------------------------
-->]OO[::::::::::::::::::::::::::::::::::[ ]:::::::::::::::
-->[OO]::::::::::::::::::::::::::::::::::[ ]:::::::::::

************************** D4RKCYDE present (1999) **************************

           --oOo--> Covered in this Article: ]-------------------
           --oOo--> ---------------------------------------------

           --oOo--> Introduction                             ]---
           --oOo--> Thinking up a Security Audit             ]---
           --oOo--> Part 1: The Plan                         ]---
           --oOo--> Part 2: The Tools                        ]---
           --oOo--> Part 3: Knowledge Gathering              ]---
           --oOo--> suid and sgid                            ]---
           --oOo--> How to find suid and sgid files          ]---
           --oOo--> Setting suid and sgid                    ]---
           --oOo--> File and Directory Permissions           ]---
           --oOo--> : Files                                  ]---
           --oOo--> : Directories                            ]---
           --oOo--> How suid and sgid fit into this picture  ]---
           --oOo--> The default mode for a file or directory ]---
           --oOo--> Passwords: A second look                 ]---
           --oOo--> Related WWW sites                        ]---


In this phile you will learn how to protect your box from those nasty hacker-
type people, which more often than not will be your online buddies :] When 
your thinking about your system security you have to remember that your 
system is as secure as its weakest point. Now, this is an old saying but it
has a lot of truth in it, its like locking all your windows to stop intruders
but leaving the back-door unlocked. Read on...

Thinking up a Security Audit

There are three basic parts to a security audit:

  o--> The Plan            - (ie: a set of security apects to be evaluated)
  o--> The Tools           - (ie: what tools are available to you to assist
                             in evaluating the security aspects)
  o--> Knowledge Gathering - (ie: finding out the ways in which your system
                             can be attacked, this includes physical security
                             issues, learning about he system itself and
                             much much more)

Part 1: The Plan

Now the plan doesn't really have to be anything more than a quick scribble on
a bit of paper that details what you are going to do. It should though, 
revolve around two basic questions:

      o--> What types of security problems could I have?

      o--> Which ones can I attempt to fix?

In order to answer these questions, you may have to find out a bit more about
several areas of your system, these include:

      o--> Accountability
      o--> Change control and tracking
      o--> Data integrity, including backups
      o--> Physical security
      o--> Privacy of Data
      o--> System access
      o--> System availability

Okay werd, so now you have a more detailed description of what you want to
achieve you can write up a more complex plan. As always, there will be trade-
offs. For example, privacy of data could mean that only certain people can
log into your box, which affects system access for the users. System 
availibility is always in contention with the change control. For example, 
when do you change that failing hard-drive on a 24/7 system? What i'm trying
to get at here is that the detailed plan that is developed should include a 
set of goals; a way of tracking the progression of the goals, including
changes to the system; and a knowledge base of what types of tools are needed
to do the job.

Part 2: The Tools

Okay, so now you should have a fair idea of what you want to do, now you have 
to think about *how* you are going to do it. A number of tewls are available
on the internet, including tools to check passwords, check system security, 
and protect your system. CERT, CIAC, and the Linux Emergancy Response Team
are often good sources of information for both the beginner and advanced 

The following is a list of tools, all freely available if you look for them,
make sure you look around for some other tools as well though!

--> cops                 [ A set of programs; each checks a different aspect ]
                         [ of security on a *nix system. If any potential    ]
                         [ security holes do exist, the results are either   ]
                         [ mailed or saved to a report file.                 ]

--> crack                [ A program designed to find standard *nix eight-   ]
                         [ character DES-encrypted passwords by standard     ]
                         [ guessing techniques.                              ]

--> deslogin             [ A remote login program that can be used safely    ]
                         [ across insecure networks.                         ]

--> findsuid.tar.Z       [ Finds changes in setuid (set user ID) and setgid  ]
                         [ (set group ID) files.                             ]

--> finger daemon        [ Secure finger daemon for *nix. Should compile out-]
                         [ of-the-box nearly anywhere.                       ]

--> freestone            [ A portable, fully functional firewall             ]
                         [ implementation.                                   ]

--> gabriel              [ A satan detector. gabriel gives the sysadmin an   ]
                         [ early warning of possible network intrusions by   ]
                         [ detecting and identifying satan's network probe.  ]

--> ipfilter             [ A free packet filter that can be incorperated into]
                         [ any of the supported operating systems, providing ]
                         [ IP packet-level filtering per interface.          ]

--> ipfirewall           [ An IP packet filtering tool, similar to the packet]
                         [ filtering facilities provided by most commercial  ]
                         [ routers.                                          ]

--> kerberos             [ A network authentication system for use on        ]
                         [ physically insecure networks. It allows entities  ]
                         [ communicating over a network to prove their       ]
                         [ identities to each other while preventing eves-   ]
                         [ dropping or replay attacks.                       ]

--> merlin               [ Takes a popular secur1ty tewl (such as tiger,     ]
                         [ tripwire, cops, crack, or spi) and provides it    ]
                         [ easy-to-use, consistent graphical interface,      ]
                         [ simplifying and enhancing its capabilities.       ]

--> npasswd              [ passwd replacement with password sanity check.    ]

--> obvious-pw.tar.Z     [ An obvious password detector.                     ]

--> opie                 [ Provides a one-time password system for POSIX-    ]
                         [ compliant UNIX-like operating systems.            ]

--> pcheck.tar.Z         [ Checks formats of /etc/passwd; verifies root      ]
                         [ default shell and passwd fields.                  ]

--> Plugslot Ltd.        [ PCP/PSP UNIX network security and configuration   ]
                         [ monitor.                                          ]

--> rsaeuro              [ A cryptographic tewl-kit providing various        ]
                         [ functions for the use of digital signatures, data ]
                         [ encryption, and supporting areas (PEM encoding,   ]
                         [ random number generation, and so on).             ]

--> rscan                [ Allows sysadmins to execute complex (or simple)   ]
                         [ scanner scripts on one (or many) machines and     ]
                         [ create clean, formatted reports in either ASCII or]
                         [ HTML.                                             ]

--> satan                [ The secur1ty analysis tewl for auditing networks. ]
                         [ In its simplest (and default) mode, it gathers as ]
                         [ much information about remote hosts and networks  ]
                         [ as possible by examining such network services    ]
                         [ such as finger, NFS, NIS, ftp and tftp, rexd, and ]
                         [ many others.                                      ]

--> ssh                  [ Secure shell - a remote login program.            ]
--> tcp wrappers         [ Monitor and control remote access to your local   ]
                         [ tftp, exec, ftp, rsh, telnet, rlogin, finger and  ]
                         [ systat daemon.                                    ]

--> tiger                [ Scans a system for potential secur1ty problems.   ]

--> tis firewall toolkit [ Includes enhancements and bug fixes from V1.2, and]
                         [ new proxies for HTTP/Gopher and X11.              ]

--> tripwire             [ Monitors system for secur1ty break-in attempts.   ]

--> xp-beta              [ An application gateway of X11 protocol. It is     ]
                         [ designed to be used at a site that has a firewall ]
                         [ and uses SOCKS and/or CERN WWW Proxy.             ]

--> xroute               [ Routes X packets from one machine to another.     ]

All of the above tools will be available from my website: when it is finally finished and online.

Part 3: Knowledge Gathering

There is not really that much to say about knowledge gathering other than to
make sure you find out whether or not the system users and the keepers of the
sacred root password (hopefully just yourself) all follow the security 
procedures that you have put into place - and that they gather all the
knowledge necessary to do so. One of the major points of this is that you
don'r use that same passwords for everything, for example, I know someone
whose password is a variation of his name, he uses this password for
*everything*, ISP accounts, web email services, he even used to spell it out
numerically for his VMB pass. If you do this now, then DON'T, it may be safe
to use as the root password because it is hard for someone to find it out but
if they find out your pwd for something less secure that just happens to be
the same pass for root, then you are fux0red.

File secur1ty is another big issue. The use of umask (file creation masks)
should be mandated. It should also be set to the maximum amount possible. It
is easy to change a particular file to give someone else access to it. It is
difficult, if not impossible, to know who is looking at your files. The 
sensitivity of your data, of course, would certainly determine the exact level
of security placed on the file. In extremely sensitive cases, such as all your
h/p related files, it should also be encrypted (make sure you use a lengthy
pass-word as well, mine is a 34 character sentance containing random
upper/lower case letters and numbers, which I have memorised).

It might also be a good idea to occasionally search for programs that have
suid or sgid capability.

suid and sgid

Many people talk about suid (set user ID) and sgid (set group ID) without
really knowing that much about them. The basic concept behind them is that a
program (not a script) is set so that it is run as the owner or group set for
the program, not the person running the program. For example, say you have a
program with suid set, and its owner is root. Anyone running that program
runs the program with the persmissions of the owner instead of his or her own
permissions. The passwd command is a good example of this. The file
/etc/passwd is writable by root, and readable by everyone. The passwd program
has suid turned on. Therefore, anyone can run the program and change their
password. Because the program is running as the user root, not the actual
user, the etc/passwd file can be written to.

The same concept is true of sgid. Instead of the program running with the 
permissions and authority of the group associated with the person calling the 
program, the program is run with the permissions and authority of the group 
that is associated with the program.

How to find suid and sgid files

Using the find command, you can search the entire system looking for programs
with their suid or sgid turned on:

find / -perm -200 -o -perm -400 -print

A good idea is to run the above command when you first load a system, saving
its output to a file readable only by root. Future searches can be performed
and compared to this "clean" list of suid and sgid files. This way you can
insure that only the files that should have these permissions actaully do.

Setting suid and sgid

The set user ID and set group ID can be powerful tools for giving the users
the ability to perform tasks without the other problems that could arise with
the user having the actual permissions of that group or user. However, these
can be dangerous tools too. When considering changing the permissions on a
file to be either suid or sgid, keep in mind these two things:

      o--> Use the lowest permissions needed to accomplish a task.
      o--> Watch for back doors.

Using the lowest permissions means not giving a file an suid of root if at
all possible. Often, a less priveleged person can be configured to do the
task. The same goes for sgid. Many times, setting the group to the
appropriate non-sys group will accomplish the same task while limiting other
potential problems.

Back doors/Trojans come in many forms. A program that allows a shell is a back
door. A program that has multiple entrances and exits are back doors. Keep in
mind that if a user can run an suid program set to root and the program
contains a back door (the user can get out of the program to a prompt without
actually exiting the program), then the system keeps an effective user ID as
what the program is set to (ie: root), and the user now has root permissions.

With that said, how do you set a file to have the effective user be the owner of
the file, or the effective group be the group of the file, instead of running as
the user ID or the users group ID of the person invoking the file? The
permissions are added with the chmod command, as follows:

chmod u+s file(s)
chmod g+s file(s)

The first example sets suid for the file(s) listed. The second example sets
the sgid of the file(s) listed. Remember, suid sets the effective ID of the
process to the owner associated with the file, and sgid sets the effective
groups ID of the process to the group associated with the file. These cannot
be set on non-executables.

File and Directory Permissions

File and directory permissions are the basics for providing security on a
system. These, along with the authentication system, provide the basis for
all security. Unfortunately, many people do not know what permissions on
directories mean, or they assume they mean the same thing they do on files.
The following section describes the permissions on files; after that, the
permissions on directories are described.


The permissions for files are split into three different sections: the owner
of the files, the group associated with the file, and everyone else
(the w0rld). Each section has its own set of file permissions. These
permissions provide the ability to read, write, and execute (or, of course,
to deny the same). These permissions are called a files 'filemode'. Filemodes
are set with the chmod command.

There are two ways to specify the permissions of the object. You can use the 
numeric coding system or the letter coding system. Using the letter coding 
system, the three sections are referred to as 'u' for user, 'g' for group, and 'o'
for other, or 'a' for all three. There are three basic types of permissions:
'r' for read, 'w' for write or 'x' for execute. Combinations of r, w and x
with the three groups provide the permissions for files. In the following
example, the owner of the file (me) has read, write, and execute permissions,
while everyone else has read access only.

shell:/home/zomba$ ls -l 0d4yz
-rwxr--r--   1 zomba   users            10 May 21 48:32 0d4yz

The command ls -l tells the computer to give you a long (-l) listing (ls) of
the file (0d4yz). The resulting line is shown in the second code line, and it
tells you a number of things about the file. First, it tells you the
permissions. Next it tells you how many links the file has. It then tells you
who owns the file (zomba) and what group is associated with the file (users).
Following the ownership section, the date and timestamp for the last time the
file was modefied is given. Finally, the name of the file is listed (0d4yz).
The permissions are actually made up of four sections. The first section is a
single character that identifies the type of object that is listed out, these
can be:

                  -		Plain File
                  b		Block special file
                  c		Character special file
                  d		Directory
                  l		Symbolic link
                  p		Named pipe
                  s		Socket

Following the file type identifier are the three sets of permissions: rwx
(owner), r-- (group), r-- (other).


The permissions on a directory are the same as those used by files: read, write
and execute. The actual permissions, though, mean different things. For a 
directory, read access provides the ability to list the names of the files in
that directory. It does not allow the othet attributes to be seen (owner,
group, size, and so on). Write access provides the ability to alter the
directory contents. This means that the user could create and delete files in
the directory. Finally, execute access lets the user make the directory the
current directory.

As I stated earlier, the permissions can also be manipulated with a numeric
coding system. The basic concept is the same as the letter coding system. As
a matter of fact, the permissions look exactly alike. The difference is that
way the permissions are identified. The numeric system uses binary counting
to determine the value for each permission and sets them. Also, the find
command can accept the permissions as an argument using the -perm option. In
this case, the permissions must be given in their numeric form.

With binary, you count from the right to the left. Therefore, if you look at
a file, you can easily come up with its numeric coding system value. The
following file has full permissions for the owner and read permissions for
the group and the world:

shell:/home/zomba$ ls -la 0clue
-rwxr--r--   1 zomba   users            10 May 22 00:12 0clue

This would be coded as 744, the table below shows how this was formed.

                  Permission        Value

                  Read              4
                  Write             2
                  Execute           1

Permissions use an additive (if thats a word) process. Therefore, a person
with read, write, and execute permissions to a file would have 7 (4+2+1).
Read and execute would have a value of 5. Remember, there are three sets of
values, so each section would have its own value. The following table shows
both the numeric system and the character system for the permissions:

            Permission                    Numeric           Character
            Read-only                     4                 r--
            Write-only                    2                 -w-
            Execute-only                  1                 --x
            Read and write                6                 rw-
            Read and execute              5                 r-x
            Read, write and execute       7                 rwx

Permissions can be changed using the chmod command. With the numeric system,
the chmod command must be given the value of all three fields. Therefore, to
change a file to read, write, and execute by everyone, the following command
would be issued:

$ chmod 777 <filename>

To perform the same task with the character system, the following command
would be issued:

$ chmod a+rwx <filename>

Of course, more than one type of permission can be specified at any one time. The 
following command adds write access for the owner of the file, and adds read
and execute access to the group and everyone else:

$ chmod u+w,og+rx <filename>

The advantage that the character system provides is that you do not have to
know what the previous permissions are. You can selectively add or remove
permissions without worrying about the rest. With the numeric system, each
section of users must always be specified. The downside of the character
system is when complex changes are being made. Looking at the preceding
example (chmod u+w,og+rx <filename>), it might have been easier to use the
numeric system and replace all those letters with three numbers: 755.

How suid and sgid fit into this picture

The special purpose access modes suid and sgid add an extra character to the
picture. Before looking at what a file looks like with the different special
access modes, take a look at the table below for the identifying characters
for each of the modes.

           Code           Name           Meaning

           s              suid           Sets process user ID on execution
           s              sgid           Sets process group ID on execution

suid and sgid are used on executables. Therefore, the code is placed where
the code for the  executable would normally go. The following file has suid

$ ls -la w0rd
-rwsr--r--   1 zomba   users            10 May 22 00:22 w0rd

The difference between the suid being set and the sgid being set is the
placement of the code. The same file with sgid active would look like this:

$ ls -la w0rd
-rwxr-sr--   1 zomba   users            10 May 22 00:22 w0rd

To set the suid with the character system, the following command would be

$ chmod u+s <filename>

To set the sgid with the character system, the following command would be

$ chmod g+s <filename>

To set the suid and the sgid using the numeric system, you will have to use
these two commands:

$ chmod 2### <filename>
$ chmod 4### <filename>

In both instances, the ### is replaced with the rest of the values for the
permiss-ions. The additive process is used to combine permissions; therefore,
the following command would add suid and sgid to a file:

$ chmod 6### <filename>

The default mode for a file or directory

The default mode for a file or directory is set with the umask. The umask
uses the numeric system to define its value. To set the umask, you must first
determine the value that you want the files to have. For example, a common
file permission set is 644. The owner has read and write permissions and the
rest of the world has read permission. After the value is determined, then it
is subtracted from 777. Keeping the same example of 644, the value would then
become 133. This value is the umask value. Typically, this value is placed in
a system file that is read when a user first logs on. After the value is set,
all files created will set their permissions automatically using this value.

Passwords: a second look

The system stores the user's encrypted password in the /etc/passwd file. If
the system is using a shadow password system, the value placed in this field
will be an x. A value of * blocks login access to the account, as * is not a
valid character for and encrypted field. This field should never be edited
(after it is set up) by hand, but a program such as passwd should be used so
that proper encrytpion takes place. If thgis field is changed by hand, the
old password is no longer valid and, more than likely, will have to be
changed by root.

NOTE: if the system is using a shadow password system thena  seperate file
exists called /etc/shadow that contains passwords (encrypted).

A password is a secret set of characters set up by the user that is known
only by the user. The system asks for the password, compares what is input to
the known password, and, if they match, conforms that the user is who they
say they are and lets them access the system. I can't stress enough - do not
write down your password! it might be hard for a remote hax0r to see it but
anyone at your comp will immediatley gain your permissions.

Related WWW sites
--oOo------------  <-- soon all tewls mentioned in this file will be here!  <-- not up yet but be sure to look out for it!

Basically, go to any site that offers exploits/security advisories and read
them, if any are relevant to your system, make sure you install any patches

Greets and shouts

Werd to the darkcyde collective, extra shouts to hybrid, bodie and force. Also 
greetz to [JaSuN], darkflame, xio, PUBLiC NUiSANCE, shadow, gossi, elf, 
downtime, kryptus, and a BIG shout to Oliver Tate....i mean erm...CFiSH..where the
hell did I put his number?...ahh here it is: (+44) 0181 9798895..oops, d1d I s4y 
th4t 0uT l0UD?...heh (c) b4b0 1999.

-----------oOo------------              EOF               ------------oOo-----------

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