SECURITY(7) FreeBSD Miscellaneous Information Manual SECURITY(7)
NAME
security – introduction to security under FreeBSD
DESCRIPTION
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.
System security also pertains to dealing with various forms of attacks,
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 (DoS)
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, DoS attacks are brute-force mechanisms that
attempt to crash or otherwise make a machine unusable by overwhelming its
servers or network stack. Some DoS 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 your Internet pipe.
A user account compromise is even more common than a DoS attack. Many
sysadmins still run standard telnetd(8), rlogind(8), rshd(8), and ftpd(8)
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 log in
to a system) will have his or her password sniffed. The attentive system
administrator 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 attacker to clean up after
himself, so most attackers do install backdoors. This gives you a
convenient way to detect the attacker. Making it impossible for an
attacker to install a backdoor may actually be detrimental to your
security because it will not close off the hole the attacker used to
break in originally.
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
Do not bother securing staff accounts if you have not 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) utility. For example, make
sure that your PTYs are specified as being “insecure” in the /etc/ttys
file so that direct root logins via telnet(1) or rlogin(1) are
disallowed. If using other login services such as sshd(8), make sure
that direct root logins are disabled there as well. Consider every
access method — services such as ftp(1) 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(1) 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 is not 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 root, even if root has a crypted password associated
with it (assuming, of course, that you have limited root access to the
console). Staff members get into their staff accounts through a secure
login mechanism such as kerberos(8) 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 log in through secure access methods that you have
set up. 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 should not 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 affect all the machines 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(8) or popper(8)
(ports/mail/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
talkd(8), comsat(8), and fingerd(8) daemons can be run in special user
“sandboxes”. A sandbox is not 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 log in via sshd(8) and never log in via telnetd(8),
rshd(8), or rlogind(8), then turn off those services!
FreeBSD now defaults to running talkd(8), comsat(8), and fingerd(8) in a
sandbox. 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(8), popper(8), imapd(8), ftpd(8), and others. There
are alternatives to some of these, but installing them may require more
work than 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.
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(1), 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(1)
(ports/x11/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(1) 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 PTYs, including PTYs used by users who log in
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
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
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(4)
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(4) device compiled in.
But even if you turn off the bpf(4) 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(4) device or other sniffing device on a running
kernel. To avoid these problems you have to run the kernel at a higher
security level, at least level 1. The security level can be set with a
sysctl(8) on the kern.securelevel variable. Once you have set the
security level to 1, write access to raw devices will be denied and
special chflags(1) 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 security level is set. This might be overdoing it, and
upgrading the system is much more difficult when you operate at a higher
security level. You may compromise and run the system at a higher
security 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.
The kernel runs with five different security levels. Any super-user
process can raise the level, but no process can lower it. The security
levels are:
-1 Permanently insecure mode - always run the system in insecure mode.
This is the default initial value.
0 Insecure mode - immutable and append-only flags may be turned off.
All devices may be read or written subject to their permissions.
1 Secure mode - the system immutable and system append-only flags may
not be turned off; disks for mounted file systems, /dev/mem and
/dev/kmem may not be opened for writing; /dev/io (if your platform
has it) may not be opened at all; kernel modules (see kld(4)) may
not be loaded or unloaded. The kernel debugger may not be entered
using the debug.kdb.enter sysctl. A panic or trap cannot be forced
using the debug.kdb.panic and other sysctl's.
2 Highly secure mode - same as secure mode, plus disks may not be
opened for writing (except by mount(2)) whether mounted or not.
This level precludes tampering with file systems by unmounting
them, but also inhibits running newfs(8) while the system is multi-
user.
In addition, kernel time changes are restricted to less than or
equal to one second. Attempts to change the time by more than this
will log the message “Time adjustment clamped to +1 second”.
3 Network secure mode - same as highly secure mode, plus IP packet
filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be
changed and dummynet(4) or pf(4) configuration cannot be adjusted.
The security level can be configured with variables documented in
rc.conf(5).
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(1) 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 layer 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 attackers, 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(1) 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 limited-access 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(1) the scripts to the client box
in order to run them, making them visible, and for safety you also need
to scp(1) the binaries (such as find(1)) that those scripts use. The
sshd(8) daemon on the client box may already be compromised. All in all,
using SSH may be necessary when running over unsecure links, but it is
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 on those partitions is a good idea. The
nosuid option (see mount(8)) is 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 break-in 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.
PARANOIA
A little paranoia never hurts. As a rule, a sysadmin can add any number
of security features as long as they do not affect convenience, and can
add security features that do affect 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 attacker
who also has access to this manual page.
SPECIAL SECTION ON DoS ATTACKS
This section covers Denial of Service attacks. A DoS attack is typically
a packet attack. While there is not 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.
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. The inetd(8) server 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(8) 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(8), so
typically a combination of options must be used. Some standalone servers
have self-fork-limitation parameters.
The sendmail(8) daemon has its -OMaxDaemonChildren option which tends to
work much better than trying to use sendmail(8)'s load limiting options
due to the load lag. You should specify a MaxDaemonChildren parameter
when you start sendmail(8) high enough to handle your expected load but
not so high that the computer cannot handle that number of sendmail's
without falling on its face. It is also prudent to run sendmail(8) in
“queued” mode (-ODeliveryMode=queued) and to run the daemon (“sendmail
-bd”) separate from the queue-runs (“sendmail -q15m”). If you still want
real-time 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(8) to prevent cascade failures.
The syslogd(8) daemon 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 talkd(8), sendmail(8), 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 net.inet.ip.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(8) 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(8)-internal test services.
ACCESS ISSUES WITH KERBEROS AND SSH
There are a few issues with both Kerberos and SSH that need to be
addressed if you intend to use them. Kerberos5 is an excellent
authentication protocol but the kerberized telnet(1) and rlogin(1) 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(1) to an unsecure machine, your keys become exposed. The
actual keys themselves are not exposed, but ssh(1) installs a forwarding
port for the duration of your login and if an attacker 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.
SEE ALSO
chflags(1), find(1), md5(1), netstat(1), openssl(1), ssh(1), xdm(1)
(ports/x11/xorg-clients), group(5), ttys(5), accton(8), init(8), sshd(8),
sysctl(8), syslogd(8), vipw(8)
HISTORY
The security manual page was originally written by Matthew Dillon and
first appeared in FreeBSD 3.1, December 1998.
FreeBSD 11.1-RELEASE-p4 December 25, 2013 FreeBSD 11.1-RELEASE-p4
