||Home : Advisories : Vulnerability in Certain TCP/IP Implementations|
||Vulnerability in Certain TCP/IP Implementations
||21st December 1998
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CERT Advisory CA-98-13-tcp-denial-of-service
Original Issue Date: December 21, 1998
Topic: Vulnerability in Certain TCP/IP Implementations
Some systems with BSD-derived TCP/IP stacks. See Appendix A for a
complete list of affected systems.
Intruders can disrupt service or crash systems with vulnerable TCP/IP
stacks. No special access is required, and intruders can use
source-address spoofing to conceal their true location.
By carefully constructing a sequence of packets with certain
characteristics, an intruder can cause vulnerable systems to crash,
hang, or behave in unpredictable ways. This vulnerability is similar
in its effect to other denial-of-service vulnerabilities, including
the ones described in
Specifically, intruders can use this vulnerability in conjunction with
IP-source-address spoofing to make it difficult or impossible to know
their location. They can also use the vulnerability in conjunction
with broadcast packets to affect a large number of vulnerable machines
with a small number of packets.
Any remote user can crash or hang a vulnerable machine, or cause the
system to behave in unpredictable ways.
A. Install a patch from your vendor.
Appendix A contains input from vendors who have provided information
for this advisory. We will update the appendix as we receive more
information. If you do not see your vendor's name, the CERT/CC did not
hear from that vendor. Please contact your vendor directly.
B. Configure your router or firewall to help prevent source-address spoofing.
We encourage sites to configure their routers or firewalls to reduce
the ability of intruders to use source-address spoofing. Currently,
the best method to reduce the number of IP-spoofed packets exiting
your network is to install filtering on your routers that requires
packets leaving your network to have a source address from your
internal network. This type of filter prevents a source IP-spoofing
attack from your site by filtering all outgoing packets that contain a
source address of a different network.
A detailed description of this type of filtering is available in RFC
2267, "Network Ingress Filtering: Defeating Denial of Service Attacks
which employ IP Source Address Spoofing" by Paul Ferguson of Cisco
Systems, Inc. and Daniel Senie of Blazenet, Inc. We recommend it to
both Internet Service Providers and sites that manage their own
routers. The document is currently available at
Note that this type of filtering does not protect a site from the
attack itself, but it does reduce the ability of intruders to conceal
their location, thereby discouraging attacks.
Appendix A - Vendor Information
Berkeley Software Design, Inc. (BSDI)
BSDI's current release BSD/OS 4.0 is not vulnerable to this problem.
BSD/OS 3.1 is vulnerable and a patch (M310-049) is available from
BSDI's WWW server at http://www.bsdi.com/support/patches or via our
ftp server from the directory
Cisco is not vulnerable.
Compaq Computer Corporation
SOURCE: (c) Copyright 1994, 1995, 1996, 1997, 1998 Compaq Computer
All rights reserved.
SOURCE: Compaq Computer Corporation
Software Security Response Team USA
This reported problem is not present for the as shipped, Compaq's
Digital ULTRIX or Compaq's Digital UNIX Operating Systems Software.
- Compaq Computer Corporation
Data General Corporation
We are investigating. We will provide an update when our investigation
FreeBSD 2.2.8 is not vulnerable.
FreeBSD versions prior to 2.2.8 are vulnerable.
FreeBSD 3.0 is also vulnerable.
FreeBSD 3.0-current as of 1998/11/12 is not vulnerable.
A patch is available at
Regarding this vulnerability, Fujitsu's UXP/V operating system is not
HP is not vulnerable.
AIX is not vulnerable.
IBM and AIX are registered trademarks of International Business
Livingston Enterprises, Inc.
Livingston systems are not vulnerable.
Computer Associates International
CA systems are not vulnerable.
Microsoft is not vulnerable.
NEC Corporation EWS-UX, UP-UX and UX/4800 Unix systems are not
vulnerable to this problem.
Security fixes for this problem are now available for 2.3 and 2.4.
For 2.3, see
For our 2.4 release which is available on CD on Dec 1, see
The bug is fixed in our -current source tree.
Sun Microsystems, Inc.
We have confirmed that SunOS and Solaris are not vulnerable to the DOS
Wind River Systems, Inc.
We've taken a look at our networking code and have determined that
this is not a problem in the currently shipping version of the VxWorks
The vulnerability was originally discovered by Joel Boutros of the
Enterprise Security Services team of Cambridge Technology Partners.
Guido van Rooij of FreeBSD, Inc., provided an analysis of the
vulnerability and information regarding its scope and extent.
This document is available from:
CERT/CC Contact Information
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