|
.0. FOREWORD
.A. INTRODUCTION
.A.1. WHAT IS A DENIAL OF SERVICE ATTACK?
.A.2. WHY WOULD SOMEONE CRASH A SYSTEM?
.A.2.1. INTRODUCTION
.A.2.2. SUB-CULTURAL STATUS
.A.2.3. TO GAIN ACCESS
.A.2.4. REVENGE
.A.2.5. POLITICAL REASONS
.A.2.6. ECONOMICAL REASONS
.A.2.7. NASTINESS
.A.3. ARE SOME OPERATING SYSTEMS MORE SECURE?
.B. SOME BASIC TARGETS FOR AN ATTACK
.B.1. SWAP SPACE
.B.2. BANDWIDTH
.B.3. KERNEL TABLES
.B.4. RAM
.B.5. DISKS
.B.6. CACHES
.B.7. INETD
.C. ATTACKING FROM THE OUTSIDE
.C.1. TAKING ADVANTAGE OF FINGER
.C.2. UDP AND SUNOS 4.1.3.
.C.3. FREEZING UP X-WINDOWS
.C.4. MALICIOUS USE OF UDP SERVICES
.C.5. ATTACKING WITH LYNX CLIENTS
.C.6. MALICIOUS USE OF telnet
.C.7. MALICIOUS USE OF telnet UNDER SOLARIS 2.4
.C.8. HOW TO DISABLE ACCOUNTS
.C.9. LINUX AND TCP TIME, DAYTIME
.C.10. HOW TO DISABLE SERVICES
.C.11. PARAGON OS BETA R1.4
.C.12. NOVELLS NETWARE FTP
.C.13. ICMP REDIRECT ATTACKS
.C.14. BROADCAST STORMS
.C.15. EMAIL BOMBING AND SPAMMING
.C.16. TIME AND KERBEROS
.C.17. THE DOT DOT BUG
.C.18. SUNOS KERNEL PANIC
.C.19. HOSTILE APPLETS
.C.20. VIRUS
.C.21. ANONYMOUS FTP ABUSE
.C.22. SYN FLOODING
.C.23. PING FLOODING
.C.24. CRASHING SYSTEMS WITH PING FROM WINDOWS 95 MACHINES
.C.25. MALICIOUS USE OF SUBNET MASK REPLY MESSAGE
.C.26. FLEXlm
.C.27. BOOTING WITH TRIVIAL FTP
.D. ATTACKING FROM THE INSIDE
.D.1. KERNEL PANIC UNDER SOLARIS 2.3
.D.2. CRASHING THE X-SERVER
.D.3. FILLING UP THE HARD DISK
.D.4. MALICIOUS USE OF eval
.D.5. MALICIOUS USE OF fork()
.D.6. CREATING FILES THAT IS HARD TO REMOVE
.D.7. DIRECTORY NAME LOOKUPCACHE
.D.8. CSH ATTACK
.D.9. CREATING FILES IN /tmp
.D.10. USING RESOLV_HOST_CONF
.D.11. SUN 4.X AND BACKGROUND JOBS
.D.12. CRASHING DG/UX WITH ULIMIT
.D.13. NETTUNE AND HP-UX
.D.14. SOLARIS 2.X AND NFS
.D.15. SYSTEM STABILITY COMPROMISE VIA MOUNT_UNION
.D.16. trap_mon CAUSES KERNEL PANIC UNDER SUNOS 4.1.X
.E. DUMPING CORE
.E.1. SHORT COMMENT
.E.2. MALICIOUS USE OF NETSCAPE
.E.3. CORE DUMPED UNDER WUFTPD
.E.4. ld UNDER SOLARIS/X86
.F. HOW DO I PROTECT A SYSTEM AGAINST DENIAL OF SERVICE ATTACKS?
.F.1. BASIC SECURITY PROTECTION
.F.1.1. INTRODUCTION
.F.1.2. PORT SCANNING
.F.1.3. CHECK THE OUTSIDE ATTACKS DESCRIBED IN THIS PAPER
.F.1.4. CHECK THE INSIDE ATTACKS DESCRIBED IN THIS PAPER
.F.1.5. EXTRA SECURITY SYSTEMS
.F.1.6. MONITORING SECURITY
.F.1.7. KEEPING UP TO DATE
.F.1.8. READ SOMETHING BETTER
.F.2. MONITORING PERFORMANCE
.F.2.1. INTRODUCTION
.F.2.2. COMMANDS AND SERVICES
.F.2.3. PROGRAMS
.F.2.4. ACCOUNTING
.G. SUGGESTED READING
.G.1. INFORMATION FOR DEEPER KNOWLEDGE
.G.2. KEEPING UP TO DATE INFORMATION
.G.3. BASIC INFORMATION
.H. COPYRIGHT
.I. DISCLAIMER
.0. FOREWORD
------------
In this paper I have tried to answer the following questions:
- What is a denial of service attack?
- Why would someone crash a system?
- How can someone crash a system.
- How do I protect a system against denial of service attacks?
I also have a section called SUGGESTED READING were you can find
information about good free information that can give you a deeper
understanding about something.
Note that I have a very limited experience with Macintosh, OS/2
and Windows and most of the material are therefore for Unix use.
You can always find the latest version at the following address:
http://www.student.tdb.uu.se/~t95hhu/secure/denial/DENIAL.TXT
Feel free to send comments, tips and so on to address: t95hhu@student.tdb.uu.se
~~~~~~~~~~~~~~~~
.A.1. WHAT IS A DENIAL OF SERVICE ATTACK?
-----------------------------------------
Denial of service is about without permission knocking off services,
for example through crashing the whole system. This kind of attacks
are easy to launch and it is hard to protect a system against them.
The basic problem is that Unix assumes that users on the system
or on other systems will be well behaved.
.A.2. WHY WOULD SOMEONE CRASH A SYSTEM?
---------------------------------------
.A.2.1. INTRODUCTION
--------------------
Why would someone crash a system? I can think of several reasons
that I have presentated more precisely in a section for each reason,
but for short:
.1. Sub-cultural status.
.2. To gain access.
.3. Revenge.
.4. Political reasons.
.5. Economical reasons.
.6. Nastiness.
I think that number one and six are the more common today, but
that number four and five will be the more common ones in the future.
.A.2.2. SUB-CULTURAL STATUS
---------------------------
After all information about syn flooding a bunch of such attacks
were launched around Sweden. The very most of these attacks were
not a part of a IP-spoof attack, it was "only" a denial
of service attack. Why?
I think that hackers attack systems as a sub-cultural pseudo career
and I think that many denial of service attacks, and here in the
example syn flooding, were performed for these reasons. I also think
that many hackers begin their carrer with denial of service attacks.
.A.2.3. TO GAIN ACCESS
----------------------
Sometimes could a denial of service attack be a part of an attack
to gain access at a system. At the moment I can think of these reasons
and specific holes:
.1. Some older X-lock versions could be crashed with a method
from the denial of service family leaving the system open. Physical
access was needed to use the work space after.
.2. Syn flooding could be a part of a IP-spoof attack method.
.3. Some program systems could have holes under the startup, that
could be used to gain root, for example SSH (secure shell).
.4. Under an attack it could be usable to crash other machines
in the network or to deny certain persons the ability to access
the system.
.5. Also could a system being booted sometimes be subverted, especially
rarp-boots. If we know which port the machine listen to (69 could
be a good guess) under the boot we can send false packets to it
and almost totally control the boot.
.A.2.4. REVENGE
---------------
A denial of service attack could be a part of a revenge against
a user or an administrator.
.A.2.5. POLITICAL REASONS
-------------------------
Sooner or later will new or old organizations understand the potential
of destroying computer systems and find tools to do it.
For example imaginate the Bank A loaning company B money to build
a factory threating the environment. The organization C therefor
crash A:s computer system, maybe with help from an employee. The
attack could cost A a great deal of money if the timing is right.
.A.2.6. ECONOMICAL REASONS
--------------------------
Imaginate the small company A moving into a business totally dominated
by company B. A and B customers make the orders by computers and
depends heavily on that the order is done in a specific time (A
and B could be stock trading companies). If A and B can't perform
the order the customers lose money and change company.
As a part of a business strategy A pays a computer expert a sum
of money to get him to crash B:s computer systems a number of times.
A year later A is the dominating company.
.A.2.7. NASTINESS
-----------------
I know a person that found a workstation where the user had forgotten
to logout. He sat down and wrote a program that made a kill -9 -1
at a random time at least 30 minutes after the login time and placed
a call to the program from the profile file. That is nastiness.
.A.3. ARE SOME OPERATING SYSTEMS MORE SECURE?
---------------------------------------------
This is a hard question to answer and I don't think that it will
give anything to compare different Unix platforms. You can't say
that one Unix is more secure against denial of service, it is all
up to the administrator.
A comparison between Windows 95 and NT on one side and Unix on
the other could however be interesting.
Unix systems are much more complex and have hundreds of built in
programs, services... This always open up many ways to crash the
system from the inside.
In the normal Windows NT and 95 network were is few ways to crash
the system. Although were is methods that always will work.
That gives us that no big different between Microsoft and Unix
can be seen regardning the inside attacks. But there is a couple
of points left:
- Unix have much more tools and programs to discover an
attack and monitoring the users. To watch what another user is up
to under windows is very hard.
- The average Unix administrator probably also have much more
experience than the average Microsoft administrator.
The two last points gives that Unix is more secure against inside
denial of service attacks.
A comparison between Microsoft and Unix regarding outside attacks
are much more difficult. However I would like to say that the average
Microsoft system on the Internet are more secure against outside
attacks, because they normally have much less services.
| .B.
SOME BASIC TARGETS FOR AN ATTACK |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.B.1. SWAP SPACE
----------------
Most systems have several hundred Mbytes of swap space to service
client requests. The swap space is typical used for forked child
processes which have a short life time. The swap space will therefore
almost never in a normal cause be used heavily. A denial of service
could be based on a method that tries to fill up the swap space.
.B.2. BANDWIDTH
---------------
If the bandwidth is to high the network will be useless. Most denial
of service attack influence the bandwidth in some way.
.B.3. KERNEL TABLES
-------------------
It is trivial to overflow the kernel tables which will cause serious
problems on the system. Systems with write through caches and small
write buffers is especially sensitive.
Kernel memory allocation is also a target that is sensitive. The
kernel have a kernelmap limit, if the system reach this limit it
can not allocate more kernel memory and must be rebooted. The kernel
memory is not only used for RAM, CPU:s, screens and so on, it it
also used for ordinaries processes. Meaning that any system can
be crashed and with a mean (or in some sense good) algorithm pretty
fast.
For Solaris 2.X it is measured and reported with the sar command
how much kernel memory the system is using, but for SunOS 4.X there
is no such command. Meaning that under SunOS 4.X you don't even
can get a warning. If you do use Solaris you should write sar -k
1 to get the information. netstat -k can also be used and shows
how much memory the kernel have allocated in the subpaging.
.B.4. RAM
---------
A denial of service attack that allocates a large amount of RAM
can make a great deal of problems. NFS and mail servers are actually
extremely sensitive because they do not need much RAM and therefore
often don't have much RAM. An attack at a NFS server is trivial.
The normal NFS client will do a great deal of caching, but a NFS
client can be anything including the program you wrote yourself...
.B.5. DISKS
-----------
A classic attack is to fill up the hard disk, but an attack at
the disks can be so much more. For example can an overloaded disk
be misused in many ways.
.B.6. CACHES
-------------
A denial of service attack involving caches can be based on a method
to block the cache or to avoid the cache.
These caches are found on Solaris 2.X:
Directory name lookup cache: Associates the name of a file with
a vnode.
Inode cache: Cache information read from disk in case it is needed
again.
Rnode cache: Holds information about the NFS filesystem.
Buffer cache: Cache inode indirect blocks and cylinders to realed
disk
I/O.
.B.7. INETD
-----------
Well once inetd crashed all other services running through inetd
no
longer will work.
| .C.
ATTACKING FROM THE OUTSIDE |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.C.1. TAKING ADVANTAGE OF FINGER
--------------------------------
Most fingerd installations support redirections to an other host.
Ex:
$finger @system.two.com@system.one.com
finger will in the example go through system.one.com and on to
system.two.com. As far as system.two.com knows it is system.one.com
who is fingering. So this method can be used for hiding, but also
for a very dirty denial of service attack. Lock at this:
$ finger @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@host.we.attack
All those @ signs will get finger to finger host.we.attack again
and again and again... The effect on host.we.attack is powerful
and the result is high bandwidth, short free memory and a hard disk
with less free space, due to all child processes (compare with .D.5.).
The solution is to install a fingerd which don't support redirections,
for example GNU finger. You could also turn the finger service off,
but I think that is just a bit to much.
.C.2. UDP AND SUNOS 4.1.3.
--------------------------
SunOS 4.1.3. is known to boot if a packet with incorrect information
in the header is sent to it. This is the cause if the ip_options
indicate a wrong size of the packet.
The solution is to install the proper patch.
.C.3. FREEZING UP X-WINDOWS
---------------------------
If a host accepts a telnet session to the X-Windows port (generally
somewhere between 6000 and 6025. In most cases 6000) could that
be used to freeze up the X-Windows system. This can be made with
multiple telnet connections to the port or with a program which
sends multiple XOpenDisplay() to the port.
The same thing can happen to Motif or Open Windows.
The solution is to deny connections to the X-Windows port.
.C.4. MALICIOUS USE OF UDP SERVICES
-----------------------------------
It is simple to get UDP services (echo, time, daytime, chargen)
to loop, due to trivial IP-spoofing. The effect can be high bandwidth
that causes the network to become useless. In the example the header
claim that the packet came from 127.0.0.1 (loopback) and the target
is the echo port at system.we.attack. As far as system.we.attack
knows is 127.0.0.1 system.we.attack and the loop has been establish.
Ex:
from-IP=127.0.0.1
to-IP=system.we.attack
Packet type:UDP
from UDP port 7
to UDP port 7
Note that the name system.we.attack looks like a DNS-name, but
the target should always be represented by the IP-number.
Quoted from proberts@clark.net (Paul D. Robertson) comment on comp.security.firewalls
on matter of "Introduction to denial of service"
" A great deal of systems don't put loopback on the wire, and
simply emulate it. Therefore, this attack will only effect that
machine in some cases. It's much better to use the address of a
different machine on the same network. Again, the default services
should be disabled in inetd.conf. Other than some hacks for mainframe
IP stacks that don't support ICMP, the echo service isn't used by
many legitimate programs, and TCP echo should be used instead of
UDP where it is necessary. "
.C.5. ATTACKING WITH LYNX CLIENTS
---------------------------------
A World Wide Web server will fork an httpd process as a respond
to a request from a client, typical Netscape or Mosaic. The process
lasts for less than one second and the load will therefore never
show up if someone uses ps. In most causes it is therefore very
safe to launch a denial of service attack that makes use of multiple
W3 clients, typical lynx clients. But note that the netstat command
could be used to detect the attack (thanks to Paul D. Robertson).
Some httpd:s (for example http-gw) will have problems besides the
normal high bandwidth, low memory... And the attack can in those
causes get the server to loop (compare with .C.6.)
.C.6. MALICIOUS USE OF telnet
-----------------------------
Study this little script:
Ex:
while : ; do
telnet system.we.attack &
done
An attack using this script might eat some bandwidth, but it is
nothing compared to the finger method or most other methods. Well
the point is that some pretty common firewalls and httpd:s thinks
that the attack is a loop and turn them self down, until the administrator
sends kill -HUP.
This is a simple high risk vulnerability that should be checked
and if present fixed.
.C.7. MALICIOUS USE OF telnet UNDER SOLARIS 2.4
-----------------------------------------------
If the attacker makes a telnet connections to the Solaris 2.4 host
and quits using:
Ex:
Control-}
quit
then will inetd keep going "forever". Well a couple of
hundred...
The solution is to install the proper patch.
.C.8. HOW TO DISABLE ACCOUNTS
-----------------------------
Some systems disable an account after N number of bad logins, or
waits N seconds. You can use this feature to lock out specific users
from the system.
.C.9. LINUX AND TCP TIME, DAYTIME
----------------------------------
Inetd under Linux is known to crash if to many SYN packets sends
to
daytime (port 13) and/or time (port 37).
The solution is to install the proper patch.
.C.10. HOW TO DISABLE SERVICES
------------------------------
Most Unix systems disable a service after N sessions have been
open in a given time. Well most systems have a reasonable default
(lets say 800 - 1000), but not some SunOS systems that have the
default set to 48...
The solutions is to set the number to something reasonable.
.C.11. PARAGON OS BETA R1.4
---------------------------
If someone redirects an ICMP (Internet Control Message Protocol)
packet to a paragon OS beta R1.4 will the machine freeze up and
must be rebooted. An ICMP redirect tells the system to override
routing tables. Routers use this to tell the host that it is sending
to the wrong router.
The solution is to install the proper patch.
.C.12. NOVELLS NETWARE FTP
--------------------------
Novells Netware FTP server is known to get short of memory if multiple
ftp sessions connects to it.
.C.13. ICMP REDIRECT ATTACKS
----------------------------
Gateways uses ICMP redirect to tell the system to override routing
tables, that is telling the system to take a better way. To be able
to misuse ICMP redirection we must know an existing connection (well
we could make one for ourself, but there is not much use for that).
If we have found a connection we can send a route that loses it
connectivity or we could send false messages to the host if the
connection we have found don't use cryptation.
Ex: (false messages to send)
DESTINATION UNREACHABLE
TIME TO LIVE EXCEEDED
PARAMETER PROBLEM
PACKET TOO BIG
The effect of such messages is a reset of the connection.
The solution could be to turn ICMP redirects off, not much proper
use of the service.
.C.14. BROADCAST STORMS
-----------------------
This is a very popular method in networks there all of the hosts
are acting as gateways.
There are many versions of the attack, but the basic method is
to send a lot of packets to all hosts in the network with a destination
that don't exist. Each host will try to forward each packet so the
packets will bounce around for a long time. And if new packets keep
coming the network will soon be in trouble.
Services that can be misused as tools in this kind of attack is
for example ping, finger and sendmail. But most services can be
misused in some way or another.
.C.15. EMAIL BOMBING AND SPAMMING
---------------------------------
In a email bombing attack the attacker will repeatedly send identical
email messages to an address. The effect on the target is high bandwidth,
a hard disk with less space and so on... Email spamming is about
sending mail to all (or rather many) of the users of a system. The
point of using spamming instead of bombing is that some users will
try to send a replay and if the address is false will the mail bounce
back. In that cause have one mail transformed to three mails. The
effect on the bandwidth is obvious.
There is no way to prevent email bombing or spamming. However have
a look at CERT:s paper "Email bombing and spamming".
.C.16. TIME AND KERBEROS
------------------------
If not the the source and target machine is closely aligned will
the ticket be rejected, that means that if not the protocol that
set the time is protected it will be possible to set a kerberos
server of function.
.C.17. THE DOT DOT BUG
----------------------
Windows NT file sharing system is vulnerable to the under Windows
95 famous dot dot bug (dot dot like ..). Meaning that anyone can
crash the system. If someone sends a "DIR ..\" to the
workstation will a STOP messages appear on the screen on the Windows
NT computer. Note that it applies to version 3.50 and 3.51 for both
workstation and server version.
The solution is to install the proper patch.
.C.18. SUNOS KERNEL PANIC
-------------------------
Some SunOS systems (running TIS?) will get a kernel panic if a
getsockopt() is done after that a connection has been reset.
The solution could be to install Sun patch 100804.
.C.19. HOSTILE APPLETS
----------------------
A hostile applet is any applet that attempts to use your system
in an inappropriate manner. The problems in the java language could
be sorted in two main groups:
1) Problems due to bugs.
2) Problems due to features in the language.
In group one we have for example the java bytecode verifier bug,
which makes is possible for an applet to execute any command that
the user can execute. Meaning that all the attack methods described
in .D.X. could be executed through an applet. The java bytecode
verifier bug was discovered in late March 1996 and no patch have
yet been available (correct me if I'am wrong!!!).
Note that two other bugs could be found in group one, but they
are both fixed in Netscape 2.01 and JDK 1.0.1.
Group two are more interesting and one large problem found is the
fact that java can connect to the ports. Meaning that all the methods
described in .C.X. can be performed by an applet. More information
and examples could be found at address:
http://www.math.gatech.edu/~mladue/HostileArticle.html
If you need a high level of security you should use some sort of
firewall for protection against java. As a user you could have java
disable.
.C.20. VIRUS
------------
Computer virus is written for the purpose of spreading and destroying
systems. Virus is still the most common and famous denial of service
attack method.
It is a misunderstanding that virus writing is hard. If you know
assembly language and have source code for a couple of virus it
is easy. Several automatic toolkits for virus construction could
also be found, for example:
* Genvir.
* VCS (Virus Construction Set).
* VCL (Virus Construction Laboratory).
* PS-MPC (Phalcon/Skism - Mass Produced Code Generator).
* IVP (Instant Virus Production Kit).
* G2 (G Squared).
PS-MPC and VCL is known to be the best and can help the novice
programmer to learn how to write virus.
An automatic tool called MtE could also be found. MtE will transform
virus to a polymorphic virus. The polymorphic engine of MtE is well
known and should easily be catch by any scanner.
.C.21. ANONYMOUS FTP ABUSE
--------------------------
If an anonymous FTP archive have a writable area it could be misused
for a denial of service attack similar with with .D.3. That is we
can fill up the hard disk.
Also can a host get temporarily unusable by massive numbers of
FTP requests.
For more information on how to protect an anonymous FTP site could
CERT:s "Anonymous FTP Abuses" be a good start.
.C.22. SYN FLOODING
-------------------
Both 2600 and Phrack have posted information about the syn flooding
attack. 2600 have also posted exploit code for the attack.
As we know the syn packet is used in the 3-way handshake. The syn
flooding attack is based on an incomplete handshake. That is the
attacker host will send a flood of syn packet but will not respond
with an ACK packet. The TCP/IP stack will wait a certain amount
of time before dropping the connection, a syn flooding attack will
therefore keep the syn_received connection queue of the target machine
filled.
The syn flooding attack is very hot and it is easy to find more
information about it, for example:
[.1.] http://www.eecs.nwu.edu/~jmyers/bugtraq/1354.html Article
by Christopher Klaus, including a "solution".
[.2.] http://jya.com/floodd.txt 2600, Summer, 1996, pp. 6-11. FLOOD
WARNING by Jason Fairlane
[.3.] http://www.fc.net/phrack/files/p48/p48-14.html IP-spoofing
Demystified by daemon9 / route / infinity for Phrack Magazine
.C.23. PING FLOODING
--------------------
I haven't tested how big the impact of a ping flooding attack is,
but it might be quite big.
Under Unix we could try something like: ping -s host to send 64
bytes packets.
If you have Windows 95, click the start button, select RUN, then
type in: PING -T -L 256 xxx.xxx.xxx.xx. Start about 15 sessions.
.C.24. CRASHING SYSTEMS WITH PING FROM WINDOWS 95 MACHINES
----------------------------------------------------------
If someone can ping your machine from a Windows 95 machine he or
she might reboot or freeze your machine. The attacker simply writes:
ping -l 65510 address.to.the.machine
And the machine will freeze or reboot.
Works for kernel 2.0.7 up to version 2.0.20. and 2.1.1. for Linux
(crash).
AIX4, OSF, HPUX 10.1, DUnix 4.0 (crash).
OSF/1, 3.2C, Solaris 2.4 x86 (reboot).
.C.25. MALICIOUS USE OF SUBNET MASK REPLY MESSAGE
--------------------------------------------------
The subnet mask reply message is used under the reboot, but some
hosts are known to accept the message any time without any check.
If so all communication to or from the host us turned off, it's
dead.
The host should not accept the message any time but under the reboot.
.C.26. FLEXlm
-------------
Any host running FLEXlm can get the FLEXlm license manager daemon
on any network to shutdown using the FLEXlm lmdown command.
# lmdown -c /etc/licence.dat
lmdown - Copyright (C) 1989, 1991 Highland Software, Inc.
Shutting down FLEXlm on nodes: xxx
Are you sure? [y/n]: y
Shut down node xxx
#
.C.27. BOOTING WITH TRIVIAL FTP
-------------------------------
To boot diskless workstations one often use trivial ftp with rarp
or bootp. If not protected an attacker can use tftp to boot the
host.
| .D.
ATTACKING FROM THE INSIDE |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.D.1. KERNEL PANIC UNDER SOLARIS 2.3
------------------------------------
Solaris 2.3 will get a kernel panic if this
is executed:
EX:
$ndd /dev/udp udp_status
The solution is to install the proper patch.
.D.2. CRASHING THE X-SERVER
---------------------------
If stickybit is not set in /tmp then can the file /tmp/.x11-unix/x0
be removed and the x-server will crash.
Ex:
$ rm /tmp/.x11-unix/x0
.D.3. FILLING UP THE HARD DISK
-----------------------------
If your hard disk space is not limited by a quota or if you can
use /tmp then it`s possible for you to fill up the file system.
Ex:
while : ;
mkdir .xxx
cd .xxx
done
.D.4. MALICIOUS USE OF eval
---------------------------
Some older systems will crash if eval '\!\!' is executed in the
C-shell.
Ex:
% eval '\!\!'
.D.5. MALICIOUS USE OF fork()
-----------------------------
If someone executes this C++ program the result will result in
a crash on most systems.
Ex:
#include <sys/types.h>
#include <unistd.h>
#include <iostream.h>
main()
{
int x;
while(x=0;x<1000000;x++)
{
system("uptime");
fork();
}
}
You can use any command you want, but uptime is nice because it
shows the workload.
To get a bigger and very ugly attack you should however replace
uptime (or fork them both) with sync. This is very bad.
If you are real mean you could also fork a child process for every
child process and we will get an exponential increase of workload.
There is no good way to stop this attack and similar attacks. A
solution could be to place a limit on time of execution and size
of processes.
.D.6. CREATING FILES THAT IS HARD TO REMOVE
-------------------------------------------
Well all files can be removed, but here is some ideas:
Ex.I.
$ cat > -xxx
^C
$ ls
-xxx
$ rm -xxx
rm: illegal option -- x
rm: illegal option -- x
rm: illegal option -- x
usage: rm [-fiRr] file ...
$
Ex.II.
$ touch xxx!
$ rm xxx!
rm: remove xxx! (yes/no)? y
$ touch xxxxxxxxx!
$ rm xxxxxxxxx!
bash: !": event not found
$
(You see the size do count!)
Other well know methods is files with odd characters or spaces
in the name.
These methods could be used in combination with ".D.3 FILLING
UP THE HARDDISK". If you do want to remove these files you
must use some sort of script or a graphical interface like OpenWindow:s
File Manager. You can also try to use: rm ./<filename>. It
should work for the first example if you have a shell.
.D.7. DIRECTORY NAME LOOKUPCACHE
--------------------------------
Directory name lookupcache (DNLC) is used whenever a file is opened.
DNLC associates the name of the file to a vnode. But DNLC can only
operate on files with names that has less than N characters (for
SunOS 4.x up to 14 character, for Solaris 2.x up 30 characters).
This means that it's dead easy to launch a pretty discreet denial
of service attack.
Create lets say 20 directories (for a start) and put 10 empty files
in every directory. Let every name have over 30 characters and execute
a script that makes a lot of ls -al on the directories.
If the impact is not big enough you should create more files or
launch more processes.
.D.8. CSH ATTACK
----------------
Just start this under /bin/csh (after proper modification) and
the load level will get very high (that is 100% of the cpu time)
in a very short time.
Ex:
|I /bin/csh
nodename : **************b
.D.9. CREATING FILES IN /tmp
----------------------------
Many programs creates files in /tmp, but are unable to deal with
the problem if the file already exist. In some cases this could
be used for a denial of service attack.
.D.10. USING RESOLV_HOST_CONF
-----------------------------
Some systems have a little security hole in the way they use the
RESOLV_HOST_CONF variable. That is we can put things in it and through
ping access confidential data like /etc/shadow or crash the system.
Most systems will crash if /proc/kcore is read in the variable and
access through ping.
Ex:
$ export RESOLV_HOST_CONF="/proc/kcore" ; ping asdf
.D.11. SUN 4.X AND BACKGROUND JOBS
----------------------------------
Thanks to Mr David Honig <honig@amada.net> for the following:
" Put the string "a&" in a file called "a"
and perform "chmod +x a". Running "a" will quickly
disable a Sun 4.x machine, even disallowing (counter to specs) root
login as the kernel process table fills."
" The cute thing is the size of the script, and how few keystrokes
it takes to bring down a Sun as a regular user."
.D.12. CRASHING DG/UX WITH ULIMIT
---------------------------------
ulimit is used to set a limit on the system resources available
to the shell. If ulimit 0 is called before /etc/passwd, under DG/UX,
will the passwd file be set to zero.
.D.13. NETTUNE AND HP-UX
------------------------
/usr/contrib/bin/nettune is SETUID root on HP-UX meaning that any
user can reset all ICMP, IP and TCP kernel parameters, for example
the following parameters:
- arp_killcomplete
- arp_killincomplete
- arp_unicast
- arp_rebroadcast
- icmp_mask_agent
- ip_defaultttl
- ip_forwarding
- ip_intrqmax
- pmtu_defaulttime
- tcp_localsubnets
- tcp_receive
- tcp_send
- tcp_defaultttl
- tcp_keepstart
- tcp_keepfreq
- tcp_keepstop
- tcp_maxretrans
- tcp_urgent_data_ptr
- udp_cksum
- udp_defaultttl
- udp_newbcastenable
- udp_pmtu
- tcp_pmtu
- tcp_random_seq
The solution could be to set the proper permission on /sbin/mount_union:
#chmod u-s /sbin/mount_union
.D.14. SOLARIS 2.X AND NFS
--------------------------
If a process is writing over NFS and the user goes over the disk
quota will the process go into an infinite loop.
.D.15. SYSTEM STABILITY COMPROMISE VIA MOUNT_UNION
--------------------------------------------------
By executing a sequence of mount_union commands any user can cause
a system reload on all FreeBSD version 2.X before 1996-05-18.
$ mkdir a
$ mkdir b
$ mount_union ~/a ~/b
$ mount_union -b ~/a ~/b
The solution could be to set the proper permission on
/sbin/mount_union:
#chmod u-s /sbin/mount_union
.D.16. trap_mon CAUSES KERNEL PANIC UNDER SUNOS 4.1.X
----------------------------------------------------
Executing the trap_mon instruction from user mode can cause a kernel
panic or a window underflow watchdog reset under SunOS 4.1.x, sun4c
architecture.
~~~~~~~~~~~~~~~~
.E.1. SHORT COMMENT
-------------------
The core dumps things don't really belongs in this paper but I
have put them here anyway.
.E.2. MALICIOUS USE OF NETSCAPE
-------------------------------
Under Netscape 1.1N this link will result in a segmentation fault
and a core dump.
Ex:
<a name="http://xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.
xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxxxxx.xxx.xxx.
xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxxxxx.xxx.xxx.xxx.xxx.xxx.
xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxxxxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.xxx.
xxx.xxx.xxx.xxx.xxxxxx.xxx.xxx.xxx.xxx.xxx...>
.E.3. CORE DUMPED UNDER WUFTPD
------------------------------
A core dumped could be created under wuftp with two different methods:
(1) Then pasv is given (user not logged in (ftp -n)). Almost all
versions of BSD:s ftpd.
(2) More than 100 arguments is given with any executable command.
Presents in all versions of BSD:sd ftpd.
.E.4. ld UNDER SOLARIS/X86
--------------------------
Under Solaris 2.4/X86 ld dumps core if given with the -s option.
| .F.
HOW DO I PROTECT A SYSTEM AGAINST DENIAL OF SERVICE ATTACKS?
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.F.1. BASIC SECURITY PROTECTION
-------------------------------
.F.1.1. INTRODUCTION
--------------------
You can not make your system totally secured against denial of
service attacks but for attacks from the outside you can do a lot.
I put this work list together and hope that it can be of some use.
.F.1.2. SECURITY PATCHES
------------------------
Always install the proper security patches. As for patch numbers
I don't want to put them out, but that doesn't matter because you
anyway want to check that you have all security patches installed,
so get a list and check! Also note that patches change over time
and that a solution suggested in security bulletins (i.e. CERT)
often is somewhat temporary.
.F.1.3. PORT SCANNING
---------------------
Check which services you have. Don't check with the manual or some
configuration file, instead scan the ports with sprobe or some other
port scanner. Actual you should do this regualy to see that anyone
don't have installed a service that you don't want on the system
(could for example be service used for a pirate site).
Disable every service that you don't need, could for example be
rexd, fingerd, systat, netstat, rusersd, sprayd, pop3, uucpd, echo,
chargen, tftp, exec, ufs, daytime, time... Any combination of echo,
time, daytime and chargen is possible to get to loop. There is however
no need to turn discard off. The discard service will just read
a packet and discard it, so if you turn off it you will get more
sensitive to denial of service and not the opposite.
Actual can services be found on many systems that can be used for
denial of service and brute force hacking without any logging. For
example Stock rexec never logs anything. Most popd:s also don't
log anything
.F.1.4. CHECK THE OUTSIDE ATTACKS DESCRIBED IN THIS PAPER
---------------------------------------------------------
Check that attacks described in this paper and look at the solution.
Some attacks you should perform yourself to see if they apply to
your system, for example:
- Freezing up X-Windows.
- Malicious use of telnet.
- How to disable services.
- SunOS kernel panic.
- Attacking with lynx clients.
- Crashing systems with ping from Windows 95 machines.
That is stress test your system with several services and look at
the effect.
Note that Solaris 2.4 and later have a limit on the number of ICMP
error messages (1 per 500 ms I think) that can cause problems then
you test your system for some of the holes described in this paper.
But you can easy solve this problem by executing this line:
$ /usr/sbin/ndd -set /dev/ip ip_icmp_err_interval 0
.F.1.5. CHECK THE INSIDE ATTACKS DESCRIBED IN THIS PAPER
--------------------------------------------------------
Check the inside attacks, although it is always possibly to crash
the system from the inside you don't want it to be to easy. Also
have several of the attacks applications besides denial of service,
for example:
- Crashing the X-Server: If stickybit is not set in /tmp a number
of attacks to gain access can be performed.
- Using resolv_host_conf: Could be used to expose confidential
data like /etc/shadow.
- Core dumped under wuftpd: Could be used to extract password-strings.
If I don't have put out a solution I might have recommended son
other paper. If not I don't know of a paper with a solution I feel
that I can recommend. You should in these causes check with your
company.
.F.1.6. EXTRA SECURITY SYSTEMS
------------------------------
Also think about if you should install some extra security systems.
The basic that you always should install is a logdaemon and a wrapper.
A firewall could also be very good, but expensive. Free tools that
can be found on the Internet is for example:
TYPE: NAME: URL:
LOGDAEMON NETLOG ftp://net.tamu.edu/pub/security/TAMU
WRAPPER TCP WRAPPERS ftp://cert.org/pub/tools/tcp_wrappers
FIREWALL TIS ftp://ftp.tis.com/pub/firewalls/toolkit
Note that you should be very careful if building your own firewall
with TIS or you might open up new and very bad security holes, but
it is a very good security packer if you have some basic knowledge.
It is also very good to replace services that you need, for example
telnet, rlogin, rsh or whatever, with a tool like ssh. Ssh is free
and can be
found at URL:
ftp://ftp.cs.hut.fi/pub/ssh
The addresses I have put out are the central sites for distributing
and I don't think that you should use any other except for CERT.
For a long list on free general security tools I recommend: "FAQ:
Computer Security Frequently Asked Questions".
.F.1.7. MONITORING SECURITY
---------------------------
Also monitor security regular, for example through examining system
log files, history files... Even in a system without any extra security
systems could several tools be found for monitoring, for example:
- uptime
- showmount
- ps
- netstat
- finger
(see the man text for more information).
.F.1.8. KEEPING UP TO DATE
--------------------------
It is very important to keep up to date with security problems.
Also understand that then, for example CERT, warns for something
it has often been dark-side public for sometime, so don't wait.
The following resources that helps you keeping up to date can for
example be found on the Internet:
- CERT mailing list. Send an e-mail to cert@cert.org to be placed
on the list.
- Bugtraq mailing list. Send an e-mail to bugtraq-request@fc.net.
- WWW-security mailing list. Send an e-mail to
www-security@ns2.rutgers.edu.
.F.1.9. READ SOMETHING BIGGER AND BETTER
----------------------------------------
Let's start with papers on the Internet. I am sorry to say that
it is not very many good free papers that can be found, but here
is a small collection and I am sorry if have have over looked a
paper.
(1) The Rainbow books is a long series of free books on computer
security. US citizens can get the books from:
INFOSEC AWARENESS OFFICE
National Computer Security Center
9800 Savage Road
Fort George G. Meader, MD 20755-600
We other just have to read the papers on the World Wide Web. Every
paper can not however be found on the Internet.
(2) "Improving the security of your Unix system" by Curry
is also very nice if you need the very basic things. If you don't
now anything about computer security you can't find a better start.
(3) "The WWW security FAQ" by Stein is although it deal
with W3-security the very best better on the Internet about computer
security.
(4) CERT have aklso published several good papers, for example:
- Anonymous FTP Abuses.
- Email Bombing and Spamming.
- Spoofed/Forged Email.
- Protecting yourself from password file attacks.
I think however that the last paper have overlooked several things.
(5) For a long list on papers I can recommend: "FAQ: Computer
Security Frequently Asked Questions".
(6) Also see section ".G. SUGGESTED READING"
You should also get some big good commercial book, but I don't
want to recommend any.
.F.2. MONITORING PERFORMANCE
----------------------------
.F.2.1. INTRODUCTION
--------------------
There is several commands and services that can be used for monitoring
performance. And at least two good free programs can be found on
Internet.
.F.2.2. COMMANDS AND SERVICES
-----------------------------
For more information read the man text.
netstat Show network status.
nfsstat Show NFS statistics.
sar System activity reporter.
vmstat Report virtual memory statistics.
timex Time a command, report process data and system
activity.
time Time a simple command.
truss Trace system calls and signals.
uptime Show how long the system has been up.
Note that if a public netstat server can be found you might be
able to use netstat from the outside. netstat can also give information
like tcp sequence numbers and much more.
.F.2.3. PROGRAMS
----------------
Proctool: Proctool is a freely available tool for Solaris that
monitors and controls processes. ftp://opcom.sun.ca/pub/binaries/
Top: Top might be a more simple program than Proctool, but is good
enough.
.F.2.4. ACCOUNTING
------------------
To monitor performance you have to collect information over a long
period of time. All Unix systems have some sort of accounting logs
to identify how much CPU time, memory each program uses. You should
check your manual to see how to set this up.
You could also invent your own account system by using crontab
and a script with the commands you want to run. Let crontab run
the script every day and compare the information once a week. You
could for example let the script run the following commands:
- netstat
- iostat -D
- vmstat
~~~~~~~~~~~~~~~~~~~~~
.F.1. INFORMATION FOR DEEPER KNOWLEDGE
-------------------------------------
(1) Hedrick, C. Routing Information Protocol. RFC 1058, 1988.
(2) Mills, D.L. Exterior Gateway Protocol Formal Specification.
RFC 904, 1984.
(3) Postel, J. Internet Control Message Protocol. RFC 792, 1981.
(4) Harrenstien, K. NAME/FINGER Protocol, RFC 742, 1977.
(5) Sollins, K.R. The TFTP Protocol, RFC 783, 1981.
(6) Croft, W.J. Bootstrap Protocol, RFC 951, 1985.
Many of the papers in this category was RFC-papers. A RFC-paper
is a paper that describes a protocol. The letters RCS stands for
Request For Comment. Hosts on the Internet are expected to understand
at least the common ones. If you want to learn more about a protocol
it is always good to read the proper RFC. You can find a nice sRFC
index search form at URL:
http://pubweb.nexor.co.uk/public/rfc/index/rfc.html
.F.2. KEEPING UP TO DATE INFORMATION
------------------------------------
(1) CERT mailing list. Send an e-mail to cert@cert.org to be placed
on the list.
(2) Bugtraq mailinglist. Send an e-mail to bugtraq-request@fc.net.
(3) WWW-security mailinglist. Send an e-mail to www-security@ns2.rutgers.edu.
(4) Sun Microsystems Security Bulletins.
(5) Various articles from: - comp.security.announce
- comp.security.unix
- comp.security.firewalls
(6) Varius 40Hex Issues.
.F.3. BASIC INFORMATION
-----------------------
(1) Husman, H. INTRODUKTION TILL DATASÄKERHET UNDER X-WINDOWS,
1995.
(2) Husman, H. INTRODUKTION TILL IP-SPOOFING, 1995.
(3) The following rainbow books: - Teal Green Book (Glossary of
Computer Security Terms).
- Bright Orange Book( A Guide
to Understanding Security Testing
and Test Documentation in Trusted
Systems).
- C1 Technical Report-001
(Computer Viruses: Preventation,
Detection, and Treatment).
(4) Ranum, Marcus. Firewalls, 1993.
(5) Sun Microsystems, OpenWindows V3.0.1. User Commands, 1992.
(6) Husman, H. ATT SPÅRA ODOKUMENTERADE SÄKERHETSLUCKOR,
1996.
(7) Dark OverLord, Unix Cracking Tips, 1989.
(8) Shooting Shark, Unix Nasties, 1988.
(9) LaDue, Mark.D. Hostile Applets on the Horizone, 1996.
(10) Curry, D.A. Improving the security of your unix system, 1990.
(11) Stein, L.D. The World Wide Web security FAQ, 1995.
(12) Bellovin, S.M. Security Problems in the TCP/IP Protocol, 1989.
------------
This paper is Copyright (c) 1996 by Hans Husman.
Permission is hereby granted to give away free copies electronically.
You may distribute, transfer, or spread this paper electronically.
You may not pretend that you wrote it. This copyright notice must
be maintained in any copy made. If you wish to reprint the whole
or any part of this paper in any other medium excluding electronic
medium, please ask the author for permission.
--------------
The information within this paper may change without notice. Use
of this information constitutes acceptance for use in an AS IS condition.
There are NO warranties with regard to this information. In no event
shall the author be liable for any damages whatsoever arising out
of or in connection with the use or spread of this information.
Any use of this information is at the user's own risk.
|