TUCoPS :: Unix :: General :: cert0126.txt

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CERT(sm) Advisory CA-96.21
Original issue date: September 19, 1996
Last revised: May 8, 1997
 Updates - updated vendor information for Hewlett-Packard.
 A complete revision history is at the end of this file.
Topic: TCP SYN Flooding and IP Spoofing Attacks
- -----------------------------------------------------------------------------
 *** This advisory supersedes the IP spoofing
 portion of CA-95:01. ***
Two "underground magazines" have recently published code to conduct
denial-of-service attacks by creating TCP "half-open" connections. This code
is actively being used to attack sites connected to the Internet. There is,
as yet, no complete solution for this problem, but there are steps that can be
taken to lessen its impact. Although discovering the origin of the attack is
difficult, it is possible to do; we have received reports of attack origins
being identified.
Any system connected to the Internet and providing TCP-based network services
(such as a Web server, FTP server, or mail server) is potentially subject to
this attack. Note that in addition to attacks launched at specific hosts,
these attacks could also be launched against your routers or other network
server systems if these hosts enable (or turn on) other TCP services (e.g.,
echo). The consequences of the attack may vary depending on the system;
however, the attack itself is fundamental to the TCP protocol used by all
systems.
If you are an Internet service provider, please pay particular attention to
Section III and Appendix A, which describes step we urge you to take to
lessen the effects of these attacks. If you are the customer of an Internet
service provider, please encourage your provider to take these steps.
This advisory provides a brief outline of the problem and a partial solution.
We will update this advisory as we receive new information. If the change in
information warrants, we may post an updated advisory on comp.security.announce
and redistribute an update to our cert-advisory mailing list. As always, the
latest information is available at the URLs listed at the end of this advisory.
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I. Description
 When a system (called the client) attempts to establish a TCP connection
 to a system providing a service (the server), the client and server
 exchange a set sequence of messages. This connection technique applies
 to all TCP connections--telnet, Web, email, etc.
 The client system begins by sending a SYN message to the server. The
 server then acknowledges the SYN message by sending SYN-ACK message to
 the client. The client then finishes establishing the connection by
 responding with an ACK message. The connection between the client and
 the server is then open, and the service-specific data can be exchanged
 between the client and the server. Here is a view of this message flow:
 Client Server
 ------ ------
 SYN-------------------->
 <--------------------SYN-ACK
 ACK-------------------->
 Client and server can now
 send service-specific data
 The potential for abuse arises at the point where the server system has
 sent an acknowledgment (SYN-ACK) back to client but has not yet received
 the ACK message. This is what we mean by half-open connection. The
 server has built in its system memory a data structure describing all
 pending connections. This data structure is of finite size, and it can be
 made to overflow by intentionally creating too many partially-open
 connections.
 Creating half-open connections is easily accomplished with IP
 spoofing. The attacking system sends SYN messages to the victim server
 system; these appear to be legitimate but in fact reference a client
 system that is unable to respond to the SYN-ACK messages. This means that
 the final ACK message will never be sent to the victim server system.
 The half-open connections data structure on the victim server system
 will eventually fill; then the system will be unable to accept any new
 incoming connections until the table is emptied out. Normally there is a
 timeout associated with a pending connection, so the half-open
 connections will eventually expire and the victim server system will
 recover. However, the attacking system can simply continue sending
 IP-spoofed packets requesting new connections faster than the victim
 system can expire the pending connections.
 In most cases, the victim of such an attack will have difficulty in
 accepting any new incoming network connection. In these cases, the
 attack does not affect existing incoming connections nor the ability to
 originate outgoing network connections.
 However, in some cases, the system may exhaust memory, crash, or be
 rendered otherwise inoperative.
 The location of the attacking system is obscured because the source
 addresses in the SYN packets are often implausible. When the packet
 arrives at the victim server system, there is no way to determine its
 true source. Since the network forwards packets based on destination
 address, the only way to validate the source of a packet is to use input
 source filtering (see Appendix A).
II. Impact
 Systems providing TCP-based services to the Internet community may
 be unable to provide those services while under attack and for some
 time after the attack ceases. The service itself is not harmed by the
 attack; usually only the ability to provide the service is impaired.
 In some cases, the system may exhaust memory, crash, or be rendered
 otherwise inoperative.
III. Solution
 There is, as yet, no generally accepted solution to this problem with
 the current IP protocol technology. However, proper router configuration
 can reduce the likelihood that your site will be the source of one of
 these attacks.
 Appendix A contains details about how to filter packets to reduce the
 number of IP-spoofed packets entering and exiting your network. It also
 contains a list of vendors that have reported support for this type of
 filtering.
 NOTE to Internet Service Providers:
 We STRONGLY urge you to install these filters in your routers to
 protect your customers against this type of an attack. Although these
 filters do not directly protect your customers from attack, the
 filters do prevent attacks from originating at the sites of any of your
 customers. We are aware of the ramifications of these filters on some
 current Mobile IP schemes and are seeking a position statement from
 the appropriate organizations.
 NOTE to customers of Internet service providers:
 We STRONGLY recommend that you contact your service provider to verify
 that the necessary filters are in place to protect your network.
 Many networking experts are working together to devise improvements to
 existing IP implementations to "harden" kernels to this type of attack.
 When these improvements become available, we suggest that you install
 them on all your systems as soon as possible. This advisory will be
 updated to reflect changes made by the vendor community.
IV. Detecting an Attack
 Users of the attacked server system may notice nothing unusual since the
 IP-spoofed connection requests may not load the system noticeably. The
 system is still able to establish outgoing connections. The problem will
 most likely be noticed by client systems attempting to access one of the
 services on the victim system.
 To verify that this attack is occurring, check the state of the server
 system's network traffic. For example, on SunOS this may be done by the
 command:
 netstat -a -f inet
 Note that use of the above command depends on the OS version, for
 example for a FreeBSD system use
 netstat -s |grep "listenqueue overflows"
 Too many connections in the state "SYN_RECEIVED" could indicate that the
 system is being attacked.
^L
...........................................................................
Appendix A - Reducing IP Spoofed Packets
1. Filtering Information
- ------------------------
With the current IP protocol technology, it is impossible to eliminate
IP-spoofed packets. However, you can take steps to reduce the number of
IP-spoofed packets entering and exiting your network.
Currently, the best method is to install a filtering router that restricts
the input to your external interface (known as an input filter) by not
allowing a packet through if it has a source address from your internal
network. In addition, you should filter outgoing packets that have a source
address different from your internal network to prevent a source IP spoofing
attack from originating from your site.
The combination of these two filters would prevent outside attackers from
sending you packets pretending to be from your internal network. It would also
prevent packets originating within your network from pretending to be from
outside your network. These filters will *not* stop all TCP SYN attacks, since
outside attackers can spoof packets from *any* outside network, and internal
attackers can still send attacks spoofing internal addresses.
We STRONGLY urge Internet service providers to install these filters in your
routers.
In addition, we STRONGLY recommend customers of Internet service providers to
contact your service provider to verify that the necessary filters are in
place to protect your network.
2. Vendor Information
- ---------------------
The following vendor(s) have reported support for the type of filtering we
recommend and provided pointers to additional information that describes how
to configure your router. If you need more information about your router or
about firewalls, please contact your vendor directly.
 Cisco
 -----
 Refer to the section entitled "ISP Security Advisory"
 on http://www.cisco.com for an up-to-date explanation of
 how to address TCP SYN flooding on a Cisco router.
NOTE to vendors:
If you are a router vendor who has information on router capabilities and
configuration examples and you are not represented in this list, please
contact the CERT Coordination Center at the addresses given in the Contact
Information section below. We will update the advisory after we hear from you.
3. Alternative for routers that do not support filtering on the inbound side
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If your vendor's router does not support filtering on the inbound side of the
interface or if there will be a delay in incorporating the feature into your
system, you may filter the spoofed IP packets by using a second router
between your external interface and your outside connection. Configure this
router to block, on the outgoing interface connected to your original router,
all packets that have a source address in your internal network. For this
purpose, you can use a filtering router or a UNIX system with two interfaces
that supports packet filtering.
Note: Disabling source routing at the router does not protect you from this
attack, but it is still good security practice to follow.
On the input to your external interface, that is coming from the Internet to
your network, you should block packets with the following addresses:
* Broadcast Networks: The addresses to block here are network 0 (the all zeros
 broadcast address) and network 255.255.255.255 (the all ones broadcast
 network).
* Your local network(s): These are your network addresses
* Reserved private network numbers: The following networks are defined
 as reserved private networks, and no traffic should ever be received
 from or transmitted to these networks through a router:
 10.0.0.0 - 10.255.255.255 10/8 (reserved)
 127.0.0.0 - 127.255.255.255 127/8 (loopback)
 172.16.0.0 - 172.31.255.255 172.16/12 (reserved)
 192.168.0.0 - 192.168.255.255 192.168/16 (reserved)
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The CERT Coordination Center staff thanks the team members of NASIRC
for contributing much of the text for this advisory and thanks the many
experts who are devoting time to addressing the problem and who provided input
to this advisory.
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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see ftp://info.cert.org/pub/FIRST/first-contacts).
CERT/CC Contact Information
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Email cert@cert.org
Phone +1 412-268-7090 (24-hour hotline)
 CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
 and are on call for emergencies during other hours.
Fax +1 412-268-6989
Postal address
 CERT Coordination Center
 Software Engineering Institute
 Carnegie Mellon University
 Pittsburgh PA 15213-3890
 USA
Using encryption
 We strongly urge you to encrypt sensitive information sent by email. We can
 support a shared DES key or PGP. Contact the CERT/CC for more information.
 Location of CERT PGP key
 ftp://info.cert.org/pub/CERT_PGP.key
Getting security information
 CERT publications and other security information are available from
 http://www.cert.org/
 ftp://info.cert.org/pub/
 CERT advisories and bulletins are also posted on the USENET newsgroup
 comp.security.announce
 To be added to our mailing list for advisories and bulletins, send your
 email address to
 cert-advisory-request@cert.org
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Copyright 1996 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is
included.
CERT is a service mark of Carnegie Mellon University.
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This file: ftp://info.cert.org/pub/cert_advisories/CA-96.21.tcp_syn_flooding
 http://www.cert.org
 click on "CERT Advisories"
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UPDATES
3COM
====
 Please refer to the "Network Security Advisory" link on
 http://www.3com.com/ for a thorough discussion of
 how to address TCP SYN flooding attacks on a 3Com router.
Berkeley Software Design, Inc.
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BSDI has patches available.
PATCH:
 K210-021 (ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/K210-021)
 md5 checksum: c386e72f41d0e409d91b493631e364dd K210-021
 This patch adds two networking features that can help defeat
 and detect some types of denial of service attacks.
 This patch requires U210-025 which provides new copies of
 sysctl(8) and netstat(1) for configuration and monitoring of
 these new features.
PATCH:
 K210-022 (ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/K210-22)
 md5 checksum: 9ec62b5e9cc424b9b42089504256d926 K210-022
 This patch adds a TCP SYN cache which reduces and/or
 eliminates the effects of SYN-type denial of service attacks
 such as those discussed in CERT advisory CA 96.21.
PATCH:
 U210-025 (ftp://ftp.bsdi.com/bsdi/patches/patches-2.1/U210-025)
 md5 checksum: d2ee01238ab6040e9b7a1bd2c3bf1016 U210-025
 This patch should be installed in conjunction with IP source
 address check and IP fragmentation queue limit patch
 (K210-021) and SYN flooding patch (K210-022).
Additional details about these patches are available from
 http://www.bsdi.com
 ftp://ftp.bsdi.com
Hewlett-Packard Company
=======================
 HPSBUX9704-060
 Description: SYN Flooding Security Vulnerability in HP-UX
 HEWLETT-PACKARD SECURITY BULLETIN: #00060
 Security Bulletins are available from the HP Electronic
 Support Center via electronic mail.
 User your browser to get to the HP Electronic Support
 Center page at:
 http://us-support.external.hp.com
 (for US, Canada, Asia-Pacific, & Latin-America)
 http://europe-support.external.hp.com
 (for Europe)
IBM Corporation
===============
Any system that is connected to a TCP/IP-based network (Internet or
intranet) and offers TCP-based services is vulnerable to the SYN flood
attack. The attack does not distinguish between operating systems,
software version levels, or hardware platforms; all systems are
vulnerable. IBM has released AIX operating system fixes for the SYN
flood vulnerability.
NOTE: If you are using the IBM Internet Connection Secured Network Gateway
 (SNG) firewall software, you must also apply the fixes listed in
 the next section.
The following Automated Program Analysis Reports (APARs) for IBM AIX
are now available to address the SYN flood attack:
 AIX 3.2.5
 ---------
 No APAR available; upgrade to AIX 4.x recommended
 AIX 4.1.x
 ---------
 APAR - IX62476
 AIX 4.2.x
 ---------
 APAR - IX62428
Fixes for IBM SNG Firewall
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The following Automated Program Analysis Reports (APARs) for the IBM
Internet Connection Secured Network Gateway firewall product are now
available to address the SYN flood and "Ping o' Death" attacks:
NOTE: The fixes in this section should ONLY be applied to systems running
 the IBM Internet Connection Secured Network Gateway (SNG)
 firewall software. They should be applied IN ADDITION TO the
 IBM AIX fixes listed in the previous section.
 IBM SNG V2.1
 ------------
 APAR - IR33376 PTF UR46673
 IBM SNG V2.2
 ------------
 APAR - IR33484 PTF UR46641
Obtaining Fixes
- ---------------
IBM AIX APARs may be ordered using Electronic Fix Distribution (via the
FixDist program), or from the IBM Support Center. For more information on
FixDist, and to obtain fixes via the Internet, please reference
 http://service.software.ibm.com/aixsupport/
or send electronic mail to "aixserv@austin.ibm.com" with the word "FixDist"
in the "Subject:" line.
Livingston Enterprises, Inc.
============================
Refer to the following Applications Note for more information on
configuring a Livingston IRX or PortMaster to help block outgoing SYN
attacks from an ISP's users:
 ftp://ftp.livingston.com/pub/le/doc/notes/filters.syn-attack
Silicon Graphics, Inc.
=====================
Updated Silicon Graphics information concerning SYN attacks
can be found in SGI advisory number 19961202-01-PX.
SGI advisories are available from
 ftp://sgigate.sgi.com/security/
Sun Microsystems, Inc.
======================
Sun published a bulletin on October 9, 1996--Sun security bulletin number
00136. Sun Security Bulletins are available via the security-alert@sun.com
alias and on SunSolve.
Note: Advisories from vendors listed in this section can also be found at
 ftp://info.cert.org/pub/vendors/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Revision history
May 8, 1997 Updates - updated vendor information for Hewlett-Packard.
Jan. 2, 1997 Updates - added or modified vendor information for SGI,
 Livingston, HP, 3COM.
Dec. 19, 1996 Updates - corrected Sun Microsystems security-alert email
 address.
Dec. 10, 1996 Appendix A, #3 - corrected next to last reserved private
 network number entry.
Dec. 09, 1996 Updates - added IBM patch information.
Nov. 12, 1996 Introduction, paragraph 2 - added some clarification.
Oct. 10, 1996 Updates - added a pointer to Sun Microsystems advisory.
 added a pointer to the CERT /pub/vendors directory.
Oct. 08, 1996 Appendix A, #3 - revised the last item, reserved private
 network numbers
 Updates - added BSDI patch information.
Oct. 07, 1996 Updates - added a pointer to Silicon Graphics advisory.
Sep. 24, 1996 Modified the supersession statement.
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