============================================================================= Security Advisory CERT-NL ============================================================================= Author/Source : CERT-NL (Erik-Jan Bos) Index : S-94-23 Distribution : World Page : 1 Classification: External Version: Final Subject : NFS Vulnerabilities Date :20-Dec-94 ============================================================================= CERT-NL has received information that an increase in the number of reports of root compromises caused by intruders using tools to exploit a number of NFS (Network File System) vulnerabilities is occurring. CERT-NL recommends limiting your exposure to these attacks by implementing the security measures described in Section III below. ----------------------------------------------------------------------------- I. Description There are tools being used by intruders to exploit a number of NFS vulnerabilities. These tools are widely available and widely distributed. II. Impact The impact varies depending on which vulnerabilities are present. In the worst case, intruders gain unauthorized root access from a remote host. III. Security Measures A. Filter packets at your firewall/router. Filter TCP port 111, UDP port 111 (portmapper), TCP port 2049, and UDP port 2049 (nfsd). Consult your vendor or your firewall documentation for detailed instructions on how to configure these ports. This measure will prevent access to NFS at your site from outside your firewall, but it will not protect you from attacks launched from your local network, behind your firewall. B. Use a portmapper that disallows proxy access. Be sure that you do this for every host that runs a portmapper. For Solaris, 2.x, use a version of rpcbind that disallows proxy access. A portmapper that disallows proxy access protects all hosts with the modified portmapper from attacks that originate either inside or outside your firewall. Because this security measure addresses only the portmapper vulnerability, we recommend combining it with measure A above. Wietse Venema has developed a portmapper that disallows proxy access. It is available by anonymous FTP from ftp.win.tue.nl:/pub/security/portmap_3.shar.Z info.cert.org:/pub/tools/nfs_tools/portmap_3.shar.Z MD5 checksum: f6a3ad98772e7a402ddcdac277adc4a6 For Solaris systems, Venema has developed a version of rpcbind that does not allow proxy access. Solaris users should install this program, not the portmapper. Rpcbind is available by anonymous FTP from the same sites as the portmapper: ftp.win.tue.nl:/pub/security/rpcbind_1.1.tar.Z info.cert.org:/pub/tools/nfs_tools/rpcbind_1.1.tar.Z MD5 checksum: 58437adcbea0a55e37d3a3211f72c08b C. Check the configuration of the /etc/exports files on your hosts. In particular: 1. Do *not* self-reference an NFS server in its own exports file. 2. Do not allow the exports file to contain a "localhost" entry. 3. Export file systems only to hosts that require them. 4. Export only to fully qualified hostnames. 5. Ensure that export lists do not exceed 256 characters. If you have aliases, the list should not exceed 256 characters *after* the aliases have been expanded. (See CA-94:02.REVISED.SunOS.rpc.mountd.vulnerability.) 6. Use the showmount(8) utility to check that exports are correct. 7. Wherever possible, mount file systems to be exported read only and export file systems read only. D. Ensure that your systems are current with patches and workarounds available from your vendor and identified in CERT advisories. The following advisories address problems related to NFS: CA-91:21.SunOS.NFS.Jumbo.and.fsirand CA-92:12.REVISED.SunOS.rpc.mountd.vulnerability CA-92:15.Multiple.SunOS.vulnerabilities.patches CA-93:15.SunOS.and.Solaris.vulnerabilities CA-94:02.REVISED.SunOS.rpc.mountd.vulnerability When you ftp to info.cert.org for advisories, also check for README files, which contain updates or clarifications. Vendors may have additional patches not covered by a CERT advisory, so be sure to contact your vendor for further information. --------------------------------------------------------------------------- CERT-NL thanks CERT Coordination Center for bringing this information to our attention and for providing large parts of the text of this Bulletin. The CERT Coordination Center thanks Steve Bellovin, Casper Dik, Leendert van Doorn, and Wietse Venema for their support in responding to this problem. --------------------------------------------------------------------------- ============================================================================== 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://ftp.nic.surfnet.nl/surfnet/net-security/cert-nl/docs/bulletin 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 one (when DST is in effect two) hour(s) ahead of Universal Time Coordinated (i.e. UTC+0100 (UTC+0200)). 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 * 24 hours phone number is available to SURFnet SSC's and FIRST members on request. ============================================================================== --- End of Draft __ Erik-Jan.