-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 =============================================================================== >> CERT-NL, 01-Mar-2000 << >> All CERT-NL information has been moved to http://cert.surfnet.nl. Links << >> to CERT-NL information contained in this advisory are therefore outdated. << >> << >> CERT-NL also has stopped the CERT-CC-Mirror service. Due to this the << >> links to the CERT-CC mirror are obsolete. Visit the CERT-CC site for the << >> complete CERT-CC advisory texts: http://www.cert.org << =============================================================================== =============================================================================== Security Advisory CERT-NL =============================================================================== Author/Source : Erik-Jan Bos (CERT-NL) Index : S-92-24 Distribution : SURFnet constituency Page : 1 Classification: External Version: Final Subject : Cisco Access List Vulnerability Date : 11-dec-92 =============================================================================== CERT-NL has received information concerning a vulnerability with Cisco routers when access lists are utilized. This vulnerability is present in Cisco software releases 8.2, 8.3, 9.0 and 9.1. Cisco Systems and CERT-NL strongly recommend that sites using Cisco routers for firewalls take immediate action to eliminate this vulnerability from their networks. This vulnerability is fixed in Cisco software releases 8.3 (update 5.10), 9.0 (update 2.5), 9.1 (update 1.1) and in all later releases. Customers who are using software release 8.2 and do not want to upgrade to a later release should contact Cisco's Technical Assistance Center (TAC) at 800-553-2447 (Internet: tac@cisco.com) for more information. The following interim releases are available via anonymous FTP from ftp.cisco.com (131.108.1.111). Note: this FTP server will not allow filenames to be listed or matched with wildcards. You also cannot request the file by its full pathname. You must first cd to the desired directory (beta83_dir, beta90_dir, or beta91_dir) and then request the file desired (gs3-bfx.83-5.10, etc.). Release (Update) Filename Size Checksum 8.3 (5.10) /beta83_dir/gs3-bfx.83-5.10 1234696 02465 1206 9.0 (2.5) /beta90_dir/gs3-bfx.90-2.5 1705364 47092 1666 9.1 (1.1) /beta91_dir/gs3-k.91-1.1 2005548 59407 1959 These releases are also available on Cisco's Customer Information On-Line (CIO) service for those customers having a maintenance contract. Other customers may obtain these releases through Cisco's Technical Assistance Center or by contacting their local Cisco distributor; for the Netherlands that is Geveke Electronics BV in Amsterdam, telephone number 020-5869711, faxnumber 020-5869777. - ----------------------------------------------------------------------------- I. Description A vulnerability in Cisco access lists allows some packets to be erroneously routed which one would expect to be filtered by the access list and vice-versa. This vulnerability can allow unauthorized traffic to pass through the gateway and can block authorized traffic. II. Problem If a Cisco router is configured to use extended IP access lists for traffic filtering on an MCI, SCI, cBus or cBusII interface, and the IP route cache is enabled, and the "established" keyword is used in the access list, then the access list can be improperly evaluated. This can permit packets which should be filtered and filter packets which should be permitted. III. Workaround This vulnerability can be avoided by either rewriting the extended access list to not use the "established" keyword, or by configuring the interface to not use the IP route cache. To disable the IP route cache, use the configuration command "no ip route-cache". Example for a serial interface: router>enable Password: router#configure terminal Enter configuration commands, one per line. Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z interface serial 0 no ip route-cache ^Z router#write memory IV. Solution Obtain and install the appropriate interim release listed above. Sites which are not experienced at this installation process should contact the TAC center at 800-553-2447 for assistance. - --------------------------------------------------------------------------- The CERT Coordination Center wishes to thank Keith Reynolds of the Santa Cruz Operation for his assistance in identifying this problem and Cisco Systems for their assistance in providing technical information for this advisory. - --------------------------------------------------------------------------- CERT-NL wishes to thank the CERT Coordination Center for bringing this information to our attention. ============================================================================== 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: http://cert.surfnet.nl/ 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 ATTENDED REGULARLY ALL DAYS Phone: +31 302 305 305 BUSINESS HOURS ONLY Fax: +31 302 305 329 BUSINESS HOURS ONLY Snailmail: SURFnet bv Attn. CERT-NL P.O. Box 19035 NL - 3501 DA UTRECHT The Netherlands NOODGEVALLEN: 06 22 92 35 64 ALTIJD BEREIKBAAR EMERGENCIES : +31 6 22 92 35 64 ATTENDED AT ALL TIMES CERT-NL'S EMERGENCY PHONENUMBER IS ONLY TO BE USED IN CASE OF EMERGENCIES: THE SURFNET HELPDESK OPERATING THE EMERGENCY NUMBER HAS A *FIXED* PROCEDURE FOR DEALING WITH YOUR ALERT AND WILL IN REGULAR CASES RELAY IT TO CERT-NL IN AN APPROPRIATE MANNER. CERT-NL WILL THEN CONTACT YOU. =============================================================================== -----BEGIN PGP SIGNATURE----- Version: PGP 6.5.1i iQA/AwUBOL6V/jSYjBqwfc9jEQIT+ACeNIeoL9UAETRh8ig8b8JYETrOkGcAoJ9E E8emiWFROZvpb+/fa6Eup+MB =loA7 -----END PGP SIGNATURE-----