security -- introduction to security under FreeBSD
Security is a function that begins and ends with the system administrator.
While all BSD multi-user systems have some inherent security, the
job of building and maintaining additional security mechanisms to keep
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.
Security is best implemented through a layered onion approach. In a nutshell,
what you want to do is to create as many layers of security as are
convenient and then carefully monitor the system for intrusions. You do
not want to overbuild your security or you will interfere with the detection
side, and detection is one of the single most important aspects of
any security mechanism. For example, it makes little sense to set the
schg flags (see chflags(1)) on every system binary because while this may
temporarily protect the binaries, it prevents a hacker who has broken in
from making an easily detectable change that may result in your security
mechanisms not detecting the hacker at all.
System security also pertains to dealing with various forms of attack,
including attacks that attempt to crash or otherwise make a system unusable
but do not attempt to break root. Security concerns can be split up
into several categories:
1. Denial of service attacks
2. User account compromises
3. Root compromise through accessible servers
4. Root compromise via user accounts
5. Backdoor creation
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 limit the load the servers 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. It may not be able to take
your machine down, but it can fill up Internet pipe.
A user account compromise is even more common than 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 looking for suspicious source
addresses even for successful logins.
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 do nothing more than mess with
the user's files or crash the machine. User account compromises are very
common because users tend not to take the precautions that sysadmins
take.
System administrators must keep in mind that there are potentially many
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. If an attacker has found
a way to break root on a machine, the attacker may not have a need to
install a backdoor. Many of the root holes found and closed to date
involve a considerable amount of work by the hacker to cleanup after himself,
so most hackers do install backdoors. This gives you a convenient
way to detect the hacker. Making it impossible for a hacker to install a
backdoor may actually be detrimental to your security because it will not
close off the hole the hacker found to break in the first place.
Security remedies should always be implemented with a multi-layered
`onion peel' approach and can be categorized as follows:
1. Securing root and staff accounts
2. Securing root - root-run servers and suid/sgid binaries
3. Securing user accounts
4. Securing the password file
5. Securing the kernel core, raw devices, and file systems
6. Quick detection of inappropriate changes made to the system
7. Paranoia
SECURING THE ROOT ACCOUNT AND SECURING STAFF ACCOUNTS [Toc] [Back] 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.
This does not mean that you should remove the password. The password is
almost always necessary for console access to the machine. What it does
mean is that you should not make it possible to use the password outside
of the console or possibly even with a su(1) command. For example, make
sure that your pty's are specified as being unsecure in the `/etc/ttys'
file so that direct root logins via telnet or rlogin are disallowed. If
using other login services such as sshd, make sure that direct root
logins are disabled there as well. Consider every access method - services
such as ftp often fall through the cracks. Direct root logins
should only be allowed via the system console.
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 by putting them in
the wheel group in their password entry. Staff accounts should be placed
in a `staff' group, and then added to the wheel group via the
`/etc/group' file. Only those staff members who actually need to have
root access should be placed in the wheel group. It is also possible,
when using an authentication method such as kerberos, to use kerberos's
`.k5login' file in the root account to allow a ksu(1) to root without
having to place anyone at all in the wheel group. This may be the better
solution since the wheel mechanism still allows an intruder to break root
if the intruder has gotten hold of your password file and can break into
a staff account. While having the wheel mechanism is better than having
nothing at all, it isn't necessarily the safest option.
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) 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 keypair 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.
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.
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).
SECURING ROOT - ROOT-RUN SERVERS AND SUID/SGID BINARIES
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!
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.
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 breakins
that might occur through them.
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 than 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. Alternatively an intruder who
breaks group kmem can monitor keystrokes sent through pty's, including
pty's used by users who login through secure methods. An intruder that
breaks the tty group can write to almost any user's tty. If a user is
running a terminal program or emulator with a keyboard-simulation 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.
SECURING USER ACCOUNTS [Toc] [Back] 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 due to the extra administration
and technical support required, but still a very good solution
compared to a crypted password file.
SECURING THE PASSWORD FILE [Toc] [Back] 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 an intruder to obtain read access to that file even if the attacker
cannot obtain root-write access.
Your security scripts should always check for and report changes to the
password file (see `Checking file integrity' below).
SECURING THE KERNEL CORE, RAW DEVICES, AND FILE SYSTEMS
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. An 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.
But even if you turn off the bpf device, you still have /dev/mem and
/dev/kmem to worry about. For that matter, the intruder can still write
to raw disk devices. Also, there is another kernel feature called the
module loader, kldload(8). An enterprising intruder can use a KLD module
to install his own bpf device or other sniffing device on a running kernel.
To avoid these problems 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. Another
possibility is to simply mount / and /usr read-only. It should be noted
that being too draconian in what you attempt to protect may prevent the
all-important detection of an intrusion.
CHECKING FILE INTEGRITY: BINARIES, CONFIG FILES, ETC
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. For example, using chflags to set the schg bit on
most of the files in / and /usr is probably counterproductive because
while it may protect the files, it also closes a detection window. The
last layer of your security onion is perhaps the most important - detection.
The rest of your security is pretty much useless (or, worse,
presents you with a false sense of safety) if you cannot detect potential
incursions. Half the job of the onion is to slow down the attacker
rather than stop him in order to give the detection side of the equation
a chance to catch him in the act.
The best way to detect an incursion is to look for modified, missing, or
unexpected files. The best way to look for modified files is from
another (often centralized) limited-access system. Writing your security
scripts on the extra-secure limited-access system makes them mostly
invisible to potential hackers, and this is important. In order to take
maximum advantage you generally have to give the limited-access box significant
access to the other machines in the business, usually either by
doing a read-only NFS export of the other machines to the limited-access
box, or by setting up ssh keypairs to allow the limit-access box to ssh
to the other machines. Except for its network traffic, NFS is the least
visible method - allowing you to monitor the file systems on each client
box virtually undetected. If your limited-access server is connected to
the client boxes through a switch, the NFS method is often the better
choice. If your limited-access server is connected to the client boxes
through a hub or through several layers of routing, the NFS method may be
too insecure (network-wise) and using ssh may be the better choice even
with the audit-trail tracks that ssh lays.
Once you give a limit-access box at least read access to the client systems
it is supposed to monitor, you must write scripts to do the actual
monitoring. Given an NFS mount, you can write scripts out of simple system
utilities such as find(1) and md5(1) It is best to physically md5 the
client-box files boxes at least once a day, and to test control files
such as those found in /etc and /usr/local/etc even more often. When
mismatches are found relative to the base md5 information the limitedaccess
machine knows is valid, it should scream at a sysadmin to go check
it out. A good security script will also check for inappropriate suid
binaries and for new or deleted files on system partitions such as / and
/usr
When using ssh rather than NFS, writing the security script is much more
difficult. You essentially have to scp the scripts to the client box in
order to run them, making them visible, and for safety you also need to
scp the binaries (such as find) that those scripts use. The ssh daemon
on the client box may already be compromised. All in all, using ssh may
be necessary when running over unsecure links, but it's also a lot harder
to deal with.
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 purview of the MD5
check.
If you have a huge amount of user disk space it may take too long to run
through every file on those partitions. In this case, setting mount
flags to disallow suid binaries and devices on those partitions is a good
idea. The `nodev' and `nosuid' options (see mount(8)) are what you want
to look into. I would scan them anyway at least once a week, since the
object of this layer is to detect a break-in whether or not the breakin
is effective.
Process accounting (see accton(8)) 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 into a system, assuming the file is still
intact after the break-in occurs.
Finally, security scripts should process the log files and the logs themselves
should be generated in as secure 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. One way to keep a permanent record of
the log files is to run the system console to a serial port and collect
the information on a continuing basis through a secure machine monitoring
the consoles.
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.
Even more importantly, a security administrator should mix it up a bit -
if you use recommendations such as those given by this manual page verbatim,
you give away your methodologies to the prospective hacker who also
has access to this manual page.
SPECIAL SECTION ON D.O.S. ATTACKS [Toc] [Back] 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.
1. Limiting server forks
2. Limiting springboard attacks (ICMP response attacks, ping
broadcast, etc...)
3. Kernel Route Cache
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.
Sendmail has its -OMaxDaemonChildren option which tends to work much better
than 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.
Syslogd can be attacked directly and it is strongly recommended that you
use the -s option whenever possible, and the -a option otherwise.
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.
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 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).
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 spoofs 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 inetdinternal
test services.
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 than 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.
ACCESS ISSUES WITH KERBEROS AND SSH [Toc] [Back] There are a few issues with both kerberos and ssh that need to be
addressed if you intend to use them. Kerberos V is an excellent authentication
protocol but the kerberized telnet and rlogin suck rocks. There
are bugs that make them unsuitable for dealing with binary streams.
Also, by default kerberos does not encrypt a session unless you use the
-x option. Ssh encrypts everything by default.
Ssh works quite well in every respect except when it is set up to forward
encryption keys. What this means is that if you have a secure workstation
holding keys that give you access to the rest of the system, and you
ssh to an unsecure machine, your keys become exposed. The actual keys
themselves are not exposed, but ssh installs a forwarding port for the
duration of your login and if a hacker has broken root on the unsecure
machine he can utilize that port to use your keys to gain access to any
other machine that your keys unlock.
We recommend that you use ssh in combination with kerberos whenever possible
for staff logins. Ssh can be compiled with kerberos support. This
reduces your reliance on potentially exposable ssh keys while at the same
time protecting passwords via kerberos. Ssh keys should only be used for
automated tasks from secure machines (something that kerberos is unsuited
to). We also recommend that you either turn off key-forwarding in the
ssh configuration, or that you make use of the from=IP/DOMAIN option that
ssh allows in its authorized_keys file to make the key only usable to
entities logging in from specific machines.
chflags(1), find(1), md5(1), netstat(1), openssl(1), ssh(1), xdm(1),
group(5), ttys(5), accton(8), init(8), kerberos(8), sshd(8), sysctl(8),
syslogd(8), vipw(8)
The security manual page was originally written by Matthew Dillon and
first appeared in FreeBSD 3.1, December 1998.
FreeBSD 5.2.1 September 18, 1999 FreeBSD 5.2.1 [ Back ] |
