I-031a: Malformed UDP Packets in Denial of Service Attacks

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 the "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 
               widespread. 


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:
http://www.microsoft.com/security/netdos.htm

hotfixes for NT 4:
------------------
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/hotfixes-
postSP3/

teardrop2-fix
srv-fix

hotfixes for NT 3.5:
--------------------
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT351/hotfixes-
postSP5/

teardrop2-fix
srv-fix

hotfixes for Windows 95:
------------------------
Windows 95 without Winsock installed (no TCP/IP network) 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 

www.microsoft.com/windows95/info/ws2.htm

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 throughout the world who helped to capture UDP packets and provide
us with valuable information.  We would also like to thank Microsoft Corporation 
for their significant contributions.
_____________________________________________________________________________

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