xref: /freebsd/share/man/man7/security.7 (revision 6ac7e896c35eacbc40d26ad980c7738fcbcbf38f)

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.\"     @(#)security.1	8.2 (Berkeley) 12/30/93
.\"	$Id: security.7,v 1.2 1998/12/22 19:02:51 dillon Exp $
.\"
.Dd December 30, 1993
.Dt SECURITY 7
.Os
.Sh NAME
.Nm security
.Nd introduction to security under FreeBSD
.Sh DESCRIPTION
.Pp
Security is a function that begins and ends with the system administrator.
While all
.Bx
systems are inherently multi-user capable, the job of building and
maintaining security mechanisms to keep those users 'honest' is probably
one of the single largest undertakings of the sysadmin.  Machines are
only as secure as you make them, and security concerns are ever competing
with the human necessity for convenience.   UNIX systems,
in general, are capable of running a huge number of simultaneous processes
and many of these processes operate as servers - meaning that external entities
can connect and talk to them.  As yesterday's mini-computers and mainframes
become today's desktops, and as computers become networked and internetworked,
security becomes an ever bigger issue.
.Pp
Security concerns can be split up into several categories:
.Bl -enum -offset indent
.It
Denial of service attacks
.It
User account compromises
.It
Root compromise through accessible servers
.It
Root compromise via user accounts
.El
.Pp
A denial of service attack is an action that deprives the machine of needed
resources.  Typically, D.O.S. attacks are brute-force mechanisms that attempt
to crash or otherwise make a machine unusable by overwhelming its servers or
network stack.  Some D.O.S. attacks try to take advantages of bugs in the
networking stack to crash a machine with a single packet.  The latter can
only be fixed by applying a bug fix to the kernel.  Attacks on servers can
often be fixed by properly specifying options to servers to limit the load
they incur on the system under adverse conditions.  Brute-force network
attacks are harder to deal with.  A spoofed-packet attack, for example, is
nearly impossible to stop short of cutting your system off from the internet.
.Pp
A user account compromise is even more common then a D.O.S. attack.  Many
sysadmins still run standard telnetd, rlogind, rshd, and ftpd servers on their
machines.  These servers, by default, do not operate over encrypted
connections.  The result is that if you have any moderate-sized user base,
one or more of your users logging into your system from a remote location
(which is the most common and convenient way to login to a system) will
have his or her password sniffed.  The attentive system admin will analyze
his remote access logs occasionally looking for suspicious source addresses
even for successful logins.
.Pp
One must always assume that once an attacker has access to a user account,
the attacker can break root.  However, the reality is that in a well secured
and maintained system, access to a user account does not necessarily give the
attacker access to root.  The distinction is important because without access
to root the attacker cannot generally hide his tracks and may, at best, be
able to remove that user's files and crash the machine, but not touch anyone
else's files.
.Pp
System administrators must keep in mind that there are several ways to break
root on a machine.  The attacker may know the root password, the attacker
may find a bug in a root-run server and be able to break root over a network
connection to that server, or the attacker may know of a bug in an suid-root
program that allows the attacker to break root once he has broken into a
user's account.
.Pp
Security remedies are always implemented in a multi-layered 'onion peel'
approach and can be categorized as follows:
.Bl -enum -offset indent
.It
Securing root and staff accounts
.It
Securing root - root-run servers and suid/sgid binaries
.It
Securing user accounts
.It
Securing the password file
.It
Securing the kernel core, raw devices, and filesystems
.It
Checking file integrity: binaries, configuration files, and so forth
.It
Paranoia
.El
.Sh SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS
.Pp
Don't bother securing staff accounts if you haven't secured the root
account.  Most systems have a password assigned to the root account.  The
first thing you do is assume that the password is 'always' compromised.
To secure the root account you make sure that it is not possible to login
to the root account using the root password from a random user account or
over the network.  If you haven't already, configure telnetd, rlogind, and
all other servers that handle login operations to refuse root logins, period,
whether the right password is given or not.  Allow direct root logins only
via the system console.  The '/etc/ttys' file comes in handy here and is
secure by default on most systems, but a good sysadmin always checks to make
sure.
.Pp
Of course, as a sysadmin you have to be able to get to root, so we open up
a few holes.  But we make sure these holes require additional password
verification to operate.  One way to make root accessible is to add appropriate
staff accounts to the wheel group (in /etc/group).  The staff members placed
in the wheel group are allowed to 'su' to root.  You should never give staff
members native wheel access via their entry in the password file... put staff
in a 'staff' group or something and only add those that really need root to
the wheel group.  Unfortunately the wheel mechanism still allows an intruder to
break root if the intruder has gotten hold of your password file - he need only
break the root password and the password of one of the staff accounts that
happens to be in the wheel group.  So while the wheel mechanism is usable,
it isn't much safer then not having a wheel group at all.
.Pp
An indirect way to secure the root account is to secure your staff accounts
by using an alternative login access method and *'ing out the crypted password
for the staff accounts.  This way an intruder may be able to steal the password
file but will not be able to break into any staff accounts (or, indirectly,
root, even if root has a crypted password associated with it).  Staff members
get into their staff accounts through a secure login mechanism such as
kerberos(1) or ssh(1) (see /usr/ports/security/ssh) using a private/public
key pair.  When you use something like kerberos you generally must secure
the machines which run the kerberos servers and your desktop workstation.
When you use a public/private key pair with ssh, you must generally secure
the machine you are logging in FROM (typically your workstation), but you can
also add an additional layer of protection to the key pair by password
protecting the key pair when you create it with ssh-keygen(1).  Being able
to *-out the passwords for staff accounts also guarantees that staff members
can only login through secure access methods that you have setup.  You can
thus force all staff members to use secure, encrypted connections for
all their sessions which closes an important hole used by many intruders:  That
of sniffing the network from an unrelated, less secure machine.
.Pp
The more indirect security mechanisms also assume that you are logging in
from a more restrictive server to a less restrictive server.  For example,
if your main box is running all sorts of servers, your workstation shouldn't
 be running any.  In order for your workstation to be reasonably secure
you should run as few servers as possible, up to and including no servers
at all, and you should run a password-protected screen blanker.
 Of course, given physical access to
a workstation an attacker can break any sort of security you put on it.
This is definitely a problem that you should consider but you should also
consider the fact that the vast majority of break-ins occur remotely, over
a network, from people who do not have physical access to your workstation or
servers.
.Pp
Using something like kerberos also gives you the ability to disable or
change the password for a staff account in one place and have it immediately
effect all the machine the staff member may have an account on.  If a staff
member's account gets compromised, the ability to instantly change his
password on all machines should not be underrated.  With discrete passwords,
changing a password on N machines can be a mess.  You can also impose
re-passwording restrictions with kerberos:  not only can a kerberos ticket
be made to timeout after a while, but the kerberos system can require that
the user choose a new password after a certain period of time (say, once a
month).
.Sh SECURING ROOT - ROOT-RUN SERVERS AND SUID/SGID BINARIES
.Pp
The prudent sysadmin only runs the servers he needs to, no more, no less.  Be
aware that third party servers are often the most bug-prone.  For example,
running an old version of imapd or popper is like giving a universal root
ticket out to the entire world.  Never run a server that you have not checked
out carefully.  Many servers do not need to be run as root.  For example,
the ntalk, comsat, and finger daemons can be run in special user 'sandboxes'.
A sandbox isn't perfect unless you go to a large amount of trouble, but the
onion approach to security still stands:  If someone is able to break in
through a server running in a sandbox, they still have to break out of the
sandbox.  The more layers the attacker must break through, the lower the
likelihood of his success.  Root holes have historically been found in
virtually every server ever run as root, including basic system servers.
If you are running a machine through which people only login via sshd and
never login via telnetd or rshd or rlogind, then turn off those services!
.Pp
FreeBSD now defaults to running ntalkd, comsat, and finger in a sandbox.
Another program which may be a candidate for running in a sandbox is
named(8).  The default rc.conf includes the arguments necessary to run
named in a sandbox in a commented-out form.  Depending on whether you
are installing a new system or upgrading an existing system, the special
user accounts used by these sandboxes may not be installed.  The prudent
sysadmin would research and implement sandboxes for servers whenever possible.
.Pp
There are a number of other servers that typically do not run in sandboxes:
sendmail, popper, imapd, ftpd, and others.  There are alternatives to
some of these, but installing them may require more work then you are willing
to put (the convenience factor strikes again).  You may have to run these
servers as root and rely on other mechanisms to detect break-ins that might
occur through them.
.Pp
The other big potential root hole in a system are the suid-root and sgid
binaries installed on the system.  Most of these binaries, such as rlogin,
reside in /bin, /sbin, /usr/bin, or /usr/sbin.  While nothing is 100% safe,
the system-default suid and sgid binaries can be considered reasonably safe.
Still, root holes are occasionally found in these binaries.  A root hole
was found in Xlib in 1998 that made xterm (which is typically suid) vulnerable.
It is better to be safe then sorry and the prudent sysadmin will restrict suid
binaries that only staff should run to a special group that only staff can
access, and get rid of (chmod 000) any suid binaries that nobody uses.  A
server with no display generally does not need an xterm binary.  Sgid binaries
can be almost as dangerous.  If an intruder can break an sgid-kmem binary the
intruder might be able to read /dev/kmem and thus read the crypted password
file, potentially compromising any passworded account.  An intruder that breaks
the tty group can write to almost user's tty.  If a user is running a terminal
program or emulator with a talk-back feature, the intruder can potentially
generate a data stream that causes the user's terminal to echo a command, which
is then run as that user.
.Sh SECURING USER ACCOUNTS
.Pp
User accounts are usually the most difficult to secure.  While you can impose
Draconian access restrictions on your staff and *-out their passwords, you
may not be able to do so with any general user accounts you might have.  If
you do have sufficient control then you may win out and be able to secure the
user accounts properly.  If not, you simply have to be more vigilant in your
monitoring of those accounts.  Use of ssh and kerberos for user accounts is
more problematic, but still a very good solution compared to a crypted
password.
.Sh SECURING THE PASSWORD FILE
.Pp
The only sure fire way is to *-out as many passwords as you can and
use ssh or kerberos for access to those accounts.  Even though the
crypted password file (/etc/spwd.db) can only be read by root, it may
be possible for a intruder to obtain read access to that file even if the
attacker cannot obtain root-write access.
.Pp
Your security scripts should always check for and report changes to
the password file (see 'Checking file integrity' below).
.Sh SECURING THE KERNEL CORE, RAW DEVICES, AND FILESYSTEMS
.Pp
If an attacker breaks root he can do just about anything, but there
are certain conveniences.  For example, most modern kernels have a
packet sniffing device driver built in.  Under FreeBSD it is called
the 'bpf' device.  A intruder will commonly attempt to run a packet sniffer
on a compromised machine.  You do not need to give the intruder the
capability and most systems should not have the bpf device compiled in.
Unfortunately, there is another kernel feature called the Loadable Kernel
Module interface.  An enterprising intruder can use an LKM to install
his own bpf device or other sniffing device on a running kernel.  If you
do not need to use the module loader, turn it off in the kernel configuration
with the NO_LKM option.
.Pp
But even if you turn off the bpf device, and turn off the module loader,
you still have /dev/mem and /dev/kmem to worry about.  For that matter,
the intruder can still write raw devices.  To avoid this you have to run
the kernel at a higher secure level... at least securelevel 1.  The securelevel
can be set with a sysctl on the kern.securelevel variable.  Once you have
set the securelevel to 1, write access to raw devices will be denied and
special chflags flags, such as 'schg', will be enforced.  You must also ensure
that the 'schg' flag is set on critical startup binaries, directories, and
script files - everything that gets run up to the point where the securelevel
is set.  This might be overdoing it, and upgrading the system is much more
difficult when you operate at a higher secure level.  You may compromise and
run the system at a higher secure level but not set the schg flag for every
system file and directory under the sun.
.Sh CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
.Pp
When it comes right down to it, you can only protect your core system
configuration and control files so much before the convenience factor
rears its ugly head.  The last layer of your security onion is perhaps
the most important - detection.
.Pp
The only correct way to check a system's file integrity is via another,
more secure system.  It is fairly easy to setup a 'secure' system: you
simply do not run any services on it.  With a secure system in place you
can then give it access to other system's root spaces via ssh.  This may
seem like a security breech, but you have to put your trust somewhere and
as long as you don't do something stupid like run random servers it really
is possible to build a secure machine.  When I say 'secure' here, I assuming
physical access security as well, of course.  Given a secure machine with
root access on all your other machines, you can then write security scripts
ON the secure machine to check the other machines on the system.  The most
common way of checking is to have the security script scp(1) over a find
and md5 binary and then ssh a shell command to the remote machine to md5
all the files in the system (or, at least, the /, /var, and /usr partitions!).
The security machine copies the results to a file and diff's them against
results from a previous run (or compares the results against its own
binaries), then emails each staff member a daily report of differences.
.Pp
Another way to do this sort of check is to NFS export the major filesystems
from every other machine to the security machine.  This is somewhat more
network intensive but also virtually impossible for an intruder to detect
or spoof.
.Pp
A good security script will also check for changes to user and staff members
access configuration files:  .rhosts, .shosts, .ssh/authorized_keys, and
so forth... files that might fall outside the prevue of the MD5 check.
.Pp
A good security script will check for suid and sgid binaries on all
filesystems and report their absolute existence as well as a diff against
the previous report or some baseline (say, make a baseline once a week).
While you can turn off the ability to run suid and sgid binaries on certain
filesystems through the 'nosuid' option in fstab/mount, you cannot turn this
off on root and anyone who breaks root can just install their binary their.
If you have a huge amount of user disk space, though, it may be useful to
disallow suid binaries and devices ('nodev' option) on the user partitions
so you do not have to scan them for such.  I would scan them anyway, though,
at least once a week, since the object of this onion layer is detection of
a break-in.
.Pp
Process accounting (see accton(1)) is a relatively low-overhead feature of
the operating system which I recommend using as a post-break-in evaluation
mechanism.  It is especially useful in tracking down how an intruder has
actually broken root on a system, assuming the file is still intact after
the break-in occurs.
.Pp
Finally, security scripts should process the log files and the logs themselves
should be generated in as secured a manner as possible - remote syslog can be
very useful.  An intruder tries to cover his tracks, and log files are critical
to the sysadmin trying to track down the time and method of the initial break-in.
.Sh PARANOIA
.Pp
A little paranoia never hurts.  As a rule, a sysadmin can add any number
of security features as long as they do not effect convenience, and
can add security features that do effect convenience with some added
thought.
.Sh SPECIAL SECTION ON D.O.S. ATTACKS
.Pp
This section covers Denial of Service attacks.  A DOS attack is typically
a packet attack.  While there isn't much you can do about modern spoofed
packet attacks that saturate your network, you can generally limit the damage
by ensuring that the attacks cannot take down your servers.
.Bl -enum -offset indent
.It
Limiting server forks
.It
Limiting springboard attacks (ICMP response attacks, ping broadcast, etc...)
.It
Kernel Route Cache
.El
.Pp
A common DOS attack is against a forking server that attempts to cause the
server to eat processes, file descriptors, and memory until the machine
dies.  Inetd (see inetd(8)) has several options to limit this sort of attack.
It should be noted that while it is possible to prevent a machine from going
down it is not generally possible to prevent a service from being disrupted
by the attack.  Read the inetd manual page carefully and pay specific attention
to the -c, -C, and -R options.  Note that spoofed-IP attacks will circumvent
the -C option to inetd, so typically a combination of options must be used.
Some standalone servers have self-fork-limitation parameters.
.Pp
Sendmail has its -OMaxDaemonChildren option which tends to work much
better then trying to use sendmail's load limiting options due to the
load lag.  You should specify a MaxDaemonChildren parameter when you start
sendmail high enough to handle your expected load but no so high that the
computer cannot handle that number of sendmails without falling on its face.
It is also prudent to run sendmail in queued mode (-ODeliveryMode=queued)
and to run the daemon (sendmail -bd) separate from the queue-runs
(sendmail -q15m).   If you still want realtime delivery you can run the queue
at a much lower interval, such as -q1m, but be sure to specify a reasonable
MaxDaemonChildren option for that sendmail to prevent cascade failures.
.Pp
Syslogd can be attacked directly and it is strongly recommended that you use
the -s option whenever possible, and the -a option otherwise.
.Pp
You should also be fairly careful
with connect-back services such as tcpwrapper's reverse-identd, which can
be attacked directly.  You generally do not want to use the reverse-ident
feature of tcpwrappers for this reason.
.Pp
It is a very good idea to protect internal services from external access
by firewalling them off at your border routers.  The idea here is to prevent
saturation attacks from outside your LAN, not so much to protect internal
services from root network-based root compromise.  Always configure an exclusive
firewall, i.e. 'firewall everything *except* ports A, B, C, D, and M-Z'.   This
way you can firewall off all of your low ports except for certain specific
services such as named (if you are primary for a zone), ntalkd, sendmail,
and other internet-accessible services.
If you try to configure the firewall the other
way - as an inclusive or permissive firewall, there is a good chance that you
will forget to 'close' a couple of services or that you will add a new internal
service and forget to update the firewall.  You can still open up the
high-numbered port range on the firewall to allow permissive-like operation
without compromising your low ports.  Also take note that FreeBSD allows you to
control the range of port numbers used for dynamic binding via the various
net.inet.ip.portrange sysctl's (sysctl -a | fgrep portrange), which can also
ease the complexity of your firewall's configuration.  I usually use a normal
first/last range of 4000 to 5000, and a hiport range of 49152 to 65535, then
block everything under 4000 off in my firewall ( except for certain specific
internet-accessible ports, of course ).
.Pp
Another common DOS attack is called a springboard attack - to attack a server
in a manner that causes the server to generate responses which then overload
the server, the local network, or some other machine.  The most common attack
of this nature is the ICMP PING BROADCAST attack.  The attacker spoofed ping
packets sent to your LAN's broadcast address with the source IP address set
to the actual machine they wish to attack.  If your border routers are not
configured to stomp on ping's to broadcast addresses, your LAN winds up
generating sufficient responses to the spoofed source address to saturate the
victim, especially when the attacker uses the same trick on several dozen
broadcast addresses over several dozen different networks at once.  Broadcast
attacks of over a hundred and twenty megabits have been measured.  A second
common springboard attack is against the ICMP error reporting system.  By
constructing packets that generate ICMP error responses, an attacker can
saturate a server's incoming network and cause the server to saturate its
outgoing network with ICMP responses.  This type of attack can also crash the
server by running it out of mbuf's, especially if the server cannot drain the
ICMP responses it generates fast enough.  The FreeBSD kernel has a new kernel
compile option called ICMP_BANDLIM which limits the effectiveness of these
sorts of attacks.  The last major class of springboard attacks is related to
certain internal inetd services such as the udp echo service.  An attacker
simply spoofs a UDP packet with the source address being server A's echo port,
and the destination address being server B's echo port, where server A and B
are both on your LAN.  The two servers then bounce this one packet back and
forth between each other.  The attacker can overload both servers and their
LANs simply by injecting a few packets in this manner.  Similar problems
exist with the internal chargen port.  A competent sysadmin will turn off all
of these inetd-internal test services.
.Pp
Spoofed packet attacks may also be used to overload the kernel route cache.
Refer to the net.inet.ip.rtexpire, rtminexpire, and rtmaxcache sysctl
parameters.  A spoofed packet attack that uses a random source IP will cause
the kernel to generate a temporary cached route in the route table, viewable
with 'netstat -rna | fgrep W3'.  These routes typically timeout in 1600
seconds or so.  If the kernel detects that the cached route table has gotten
too big it will dynamically reduce the rtexpire but will never decrease it to
less then rtminexpire.  There are two problems:  (1) The kernel does not react
quickly enough when a lightly loaded server is suddenly attacked, and (2) The
rtminexpire is not low enough for the kernel to survive a sustained attack.
If your servers are connected to the internet via a T3 or better it may be
prudent to manually override both rtexpire and rtminexpire via sysctl(8).
Never set either parameter to zero (unless you want to crash the machine :-)).
Setting both parameters to 2 seconds should be sufficient to protect the route
table from attack.

.Sh SEE ALSO
.Pp
.Xr accton 1 ,
.Xr chflags 1 ,
.Xr find 1 ,
.Xr kerberos 1 ,
.Xr md5 1 ,
.Xr ssh 1 ,
.Xr sshd 1 ,
.Xr syslogd 1 ,
.Xr xdm 1 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
manual page was originally written by Matthew Dillon and first appeared
in FreeBSD-3.0.1, December 1998.