-----BEGIN PGP SIGNED MESSAGE----- =============================================================================== Author/Source : Gert Meijerink Index : S-97-37 Distribution : World Page : 1 Classification: External Version: 1 Subject : Windows NT/95 Out of Band Data Exploit Date : 21-Mei-97 =============================================================================== By courtesy of CIAC we received the following information. Windows systems are vulnerable to denial-of-service attack. CERT-NL recommends to apply the countermeasures decribed in the information bulletin. ============================================================================== ____________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Windows NT/95 Out of Band Data Exploit May 14, 1997 18:00 GMT Number H-57 ______________________________________________________________________________ PROBLEM: Windows systems are vulnerable to denial-of-service attack PLATFORM: Windows NT (server and workstation versions up through 4.0) Windows 95, Windows for Workgroups 3.11. DAMAGE: Windows NT: Causes a system crash. Unsaved data is lost. Windows 95, Windows for Workgroups: Causes most open applications to crash, unsaved data is lost. SOLUTION: Apply the countermeasures described below for firewall configuration and local Microsoft patch. ******Make sure correct service packs are installed! ****** Installing the Microsoft patches without correct service packs will render your system unbootable!!! ______________________________________________________________________________ VULNERABILITY Code and exploit are widely available, and can be run remotely ASSESSMENT: over a network. ______________________________________________________________________________ Introduction: An executable code which exploits a Windows' Out-of-Band data assumption has been released on the Internet. When a Windows system receives a packet with the "URGENT" flag set, it expects data will follow that flag. The exploit consists of setting the URGENT flag, but not following it with data. The port most susceptible is TCP Port 139, the Netbios Session Service port. Although port 139 is the most commonly attacked port, there is potential for successful attacks on other ports as well. This attack is effective remotely or locally (it also works on the machine it's executing from). Windows NT: When Windows NT is successfully attacked, it crashes. The system displays the "blue screen of death", and is not respondent. Except for losing the contents of unsaved documents and files, there are no long-lasting effects from this attack. Windows 95, Windows for Workgroups 3.11: When Windows for Workgroups or Windows 95 is successfully attacked, an application exception screen will be displayed. This is a blue screen alerting the user that an application is not responding. Any unsaved data will be lost, however there are no other long-lasting effects from this attack. Solution: If your Windows NT system has been successfully attacked, it must be rebooted. The system operates normally once the system is rebooted. If your Windows for Workgroups or Windows 95 system has been successfully attacked, follow the instructions on the Application exception screen to close the applications not responding and return to the desktop. The system will operate normally once you have returned to the desktop. A reboot is not required, but recommended. Prevention: The best and most conservative measure for preventing this and similar attacks is to have a strong firewall in place. The firewall can be configured to choose and authorize trusted hosts to enter through the firewall. Additionally, this specific exploit can be prevented by disabling Netbios services through firewalls/routers. (Be aware that some versions of the exploit have the potential to choose ports, and therefore may successfully attack other ports). Since changing the firewall configuration is not always feasible in a network environment, there is a "local" patch provided by Microsoft for Windows NT systems. This patch updates the Tcpip.sys file, which contains the TCP/IP Driver. Microsoft plans to include the patch in the next Service Pack and therefore does not recommend applying the patch unless you are severely impacted by this problem. If you still need to apply this patch, update your Emergency Repair Disk first, as the patches have not been regression tested and therefore may not work as described. ********************************************************************* ***** MAKE SURE THE CORRECT SERVICE PACKS ARE INSTALLED!!!!! ***** ***** INSTALLING THIS FIX WITHOUT THE CORRECT SERVICE PACKS ***** ***** WILL RENDER YOUR SYSTEM UNBOOTABLE!!!!!! ***** ********************************************************************* Microsoft's patch locations: Windows NT 4.0: ****SERVICE PACK 2 MUST BE INSTALLED**** ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/nt40/hotfixes- postSP2/oob-fix Read the text files, Q143478.txt and Readme.txt, in the directory and follow instructions. Windows NT 3.51: *****SERVICE PACK 5 MUST BE INSTALLED**** ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/NT351/hotfixes- postSP5/oob-fix Read the text files, Q143478.txt and Readme.txt, in the directory and follow instructions. ============================================================================== CERT-NL is the Computer Emergency Response Team for SURFnet customers. SURFnet is the Dutch network for educational, research and related institutes. CERT-NL is a member of the Forum of Incident Response and Security Teams (FIRST). All CERT-NL material is available under: In case of computer or network security problems please contact your local CERT/security-team or CERT-NL (if your institute is NOT a SURFnet customer please address the appropriate (local) CERT/security-team). CERT-NL is one/two hour(s) ahead of UTC (GMT) in winter/summer, i.e. UTC+0100 in winter and UTC+0200 in summer (DST). Email: cert-nl@surfnet.nl Phone: +31 302 305 305 Fax: +31 302 305 329 Snailmail: SURFnet bv Attn. CERT-NL P.O. Box 19035 NL - 3501 DA UTRECHT The Netherlands A 7 * 24 hours phone number is available to SURFnet SSC's and FIRST members on request. ============================================================================== -----BEGIN PGP SIGNATURE----- Version: 2.6.3i Charset: cp850 iQCVAwUBM4NOe0U5nQkWIq1FAQEESgP/cnKiW/zZKP4oNqMrhUUgb+eadOvBVwNS VLv9ni4BVy3gSflpB4sOIs4C2vc1ZaCfUpnisBIIQ+ZKHDlWKqho3PFvOpuk1N8P t22zHBWbGWFDAwOXXYxNgHW8u+pubHTMNnkkg5/Itgaq5qIuwjrI7s6tO+FHmh6M uR+R2IH0LQ0= =miW0 -----END PGP SIGNATURE-----