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TUCoPS :: Windows Net Apps :: ciaci031.txt

Windows Malformed Udp Packets Denial Service Attacks



                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

               Malformed UDP Packets in Denial of Service Attacks

March 6, 1998 23:00 GMT                                          Number I-031a
PROBLEM:       Denial of Service attacks on Windows NT and Windows 95
               platforms are being seen on the Internet.
PLATFORM:      Windows 95, Windows NT
DAMAGE:        Computers freeze or hang, Windows NT machines go to "blue
               screen of death" immediately or soon after the attack, or
               spontaneously reboot.
SOLUTION:      This appears to be the modified teardrop attack (also known as
               bonk/boink/newtear) described in CIAC Bulletin I-19. Apply the
               TCP/IP hotfixes provided by Microsoft for Windows NT and
               Windows 95.
VULNERABILITY  Unprotected systems crash and lose any unsaved data when this
ASSESSMENT:    attack occurs.  This attack seems to be directed at every or at
               least many IP address at a targeted site.  This attack is

Revision a: Add updated Windows 95 patch.

               Malformed UDP Packets in Denial of Service Attacks

CIAC has information that there have been an ongoing series of denial-of-
service attacks directed at whole blocks of IP addresses. The attack uses UDP
fragmentation to exploit a known vulnerability on unpatched Windows NT and
Windows 95 systems.  The attack is a sequence of two UDP packets, the first
being the setup packet, and the second, a malformed UDP packet. Because of the
way Microsoft implements the TCP/IP stack, processing these UDP packets places
the TCP/IP stack in an unstable state. Unprotected Windows NT machines crash
and display the "blue screen of death" during or soon after the attack.
Windows NT boxes with only SP1 applied seem to reboot. Windows 95 machines
hang.  The attack is not intentionally damaging to the machines, but as with
all such issues can do damage if the machine is accessing the hard drive at
the moment the attack occurs.

Microsoft has tested these malformed packets and believes the teardrop2 hotfix
solves this problem. We suggest patching all machines with this hotfix and the
smb/cifs (srv hotfix) which protects against a similar attack.

See CIAC Bulletin I-19 for more information on this type of attack and the
machines that are vulnerable. Note also that Microsoft has updated and
combined the patches for the Teardrop and Land attacks on Windows NT. This
patch is now the teardrop2 fix.  The teardrop2 hotfix should be used instead
of the patches listed in the I-19 Bulletin.

We have noted that Windows NT and Windows95 machines that were located behind
firewalls did not fail during these attacks.  We believe this is due to the
fact that most firewalls automatically drop malformed UDP packets.

To get more information from Microsoft Corporation about these attacks, see
the "Update on Network Denial of Service Attacks" at:

hotfixes for NT 4:
- ------------------


hotfixes for NT 3.5:
- --------------------


hotfixes for Windows 95:
- ------------------------
Windows 95 without Winsock installed is not vulnerable.

All other versions of Windows 95 should update to Winsock 2.
Microsoft has released an update called the "Winsock 2 update" for Windows
95. According to Microsoft, this update contains fixes for all known
vulnerabilities in the Windows 95 TCP/IP stack. Microsoft is updating
their security advisories to recommend that all Windows 95 customers
who are concerned about TCP/IP security and denial of service issues should
upgrade to WinSock 2 using this update. This update works for all existing
Windows 95 systems, and can be installed on top of systems that already have
existing security updates installed.

This update is available from

We have also been advised that Windows 98 RC0 (release candidate 0) contains
all known TCP/IP updates, and is not vulnerable to this attack.

Warning: These are hotfixes from Microsoft that are not as well tested as a
Service Pack. Be sure to follow the directions for installing the patches or
you may make your machine unbootable.  The Service Pack 3 for NT 4 is required
before installing any of the hotfixes noted above.  If you reinstall Service
Pack 3, you must reinstall the hotfixes. We have installed all the hotfixes for Windows NT 4 and Windows 95 and have not experienced any problems.

Windows NT users should turn on the creation of a crash dump file. To do this,
select Settings on the Task Menu from the Start button, select Control Panel,
then the System icon, and in the System Properties window select the "Write
debugging information to" checkbox.  If a machine with the teardrop2 fix
installed is attacked and crashes, please inform your Computer Security
Department and give them information about which patches were applied.  The
crash dump file may be requested by Microsoft.


CIAC wishes to acknowledge the many contributions from Computer Security
Managers who helped to capture UDP packets and provide us with valuable
information.  We would also like to thank Microsoft Corporation for their
significant contributions.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
    Voice:    +1 510-422-8193
    FAX:      +1 510-423-8002
    STU-III:  +1 510-423-2604

For emergencies and off-hour assistance, DOE, DOE contractor sites,
and the NIH may contact CIAC 24-hours a day. During off hours (5PM -
8AM PST), call the CIAC voice number 510-422-8193 and leave a message,
or call 800-759-7243 (800-SKY-PAGE) to send a Sky Page. CIAC has two
Sky Page PIN numbers, the primary PIN number, 8550070, is for the CIAC
duty person, and the secondary PIN number, 8550074 is for the CIAC
Project Leader.

Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.

   World Wide Web:
                        (or -- they're the same machine)
   Anonymous FTP:
                        (or -- they're the same machine)
   Modem access:        +1 (510) 423-4753 (28.8K baud)
                        +1 (510) 423-3331 (28.8K baud)

CIAC has several self-subscribing mailing lists for electronic
1. CIAC-BULLETIN for Advisories, highest priority - time critical
   information and Bulletins, important computer security information;
2. SPI-ANNOUNCE for official news about Security Profile Inspector
   (SPI) software updates, new features, distribution and
3. SPI-NOTES, for discussion of problems and solutions regarding the
   use of SPI products.

Our mailing lists are managed by a public domain software package
called Majordomo, which ignores E-mail header subject lines. To
subscribe (add yourself) to one of our mailing lists, send the
following request as the E-mail message body, substituting
ciac-bulletin, spi-announce OR spi-notes for list-name:

E-mail to or
        subscribe list-name
  e.g., subscribe ciac-bulletin

You will receive an acknowledgment email immediately with a confirmation
that you will need to mail back to the addresses above, as per the
instructions in the email.  This is a partial protection to make sure
you are really the one who asked to be signed up for the list in question.

If you include the word 'help' in the body of an email to the above address,
it will also send back an information file on how to subscribe/unsubscribe,
get past issues of CIAC bulletins via email, etc.

PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins.  If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at

This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products,
process, or service by trade name, trademark, manufacturer, or
otherwise, does not necessarily constitute or imply its endorsement,
recommendation or favoring by the United States Government or the
University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.

LAST 10 CIAC BULLETINS ISSUED (Previous bulletins available from CIAC)

I-021 "smurf" IP Denial-of-Service Attackts
I-022: IBM AIX "routed" daemon Vulnerability
I-023: Macro Virus Update
I-024: CGI Security Hole in EWS1.1 Vulnerability
I-025: Windows NT based Web Servers File Access Vulnerability
I-026: Vulnerability in ssh-agent
I-027: HP-UX Vulnerabilities (CUE, CDE, land)
I-028: Vulnerabilities in CDE
I-029: IBM AIX Telnet Denial-of-Service Vulnerability
I-030: SunOS volrmmount (1) Vulnerability

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