=============================================================================== 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, located in The Netherlands. CERT-NL is a Full Member of the Forum of Incident Response and Security Teams (FIRST). The constituency of CERT-NL are the SURFnet connected institutions. Past CERT-NL Security Bulletins and other CERT-NL related material can be found on the anonymous FTP server of SURFnet bv: "ftp.nic.surfnet.nl" [192.87.46.3], in the directory "netman/cert-nl". This information is also available using email. Send an email saying "help" to "mailserv@nic.surfnet.nl". In case of computer or network security problems please contact CERT-NL or the CERT of your own constituency. Please be aware of the fact that we are are one hour ahead of Universal Time Coordinated (i.e. UTC+0100). Email: cert-nl@surfnet.nl Phone: +31 30 310290 Fax: +31 30 340903 Snailmail: SURFnet bv Attn. CERT-NL P.O. Box 19035 NL - 3501 DA UTRECHT The Netherlands A 7*24h phonenumber is available to SURFnet SSC's and FIRST members on request ==============================================================================