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CERT(*) Advisory CA-94:15

Original issue date:  December 19, 1994

Last revised:  Septmeber 23, 1997

                Updated copyright statement





Topic: NFS Vulnerabilities

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The CERT Coordination Center is experiencing an increase in reports of root

compromises caused by intruders using tools to exploit a number of NFS

(Network File System) vulnerabilities.



CERT recommends limiting your exposure to these attacks by implementing

the security measures described in Section III below.



We will update this advisory as we receive additional information.

Please check advisory files regularly for updates that relate to your site.



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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).



        Note: Some sites may run NFS on a port other than 2049. To determine

              which port is running NFS, enter the following command on the

              machine in question:

                         rpcinfo -p

              If NFS is on a different port, then that is the port number to

              block at the firewall.



        Consult your vendor or your firewall documentation for detailed

        instructions on how to configure the 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



         Vendors may have additional patches not covered by a CERT

         advisory, so be sure to contact your vendor for further information.



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The CERT Coordination Center thanks Steve Bellovin, Casper Dik, Leendert

van Doorn, and Wietse Venema for their support in responding to this problem.

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If you believe that your system has been compromised, contact the CERT

Coordination Center or your representative in Forum of Incident

Response and Security Teams (FIRST).



If you wish to send sensitive incident or vulnerability information to

CERT via electronic mail, CERT strongly advises that the e-mail be

encrypted.  CERT can support a shared DES key, PGP (public key

available via anonymous FTP on info.cert.org), or PEM (contact CERT

for details).



Internet E-mail: cert@cert.org

Telephone: 412-268-7090 (24-hour hotline)

           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),

           and are on call for emergencies during other hours.



CERT Coordination Center

Software Engineering Institute

Carnegie Mellon University

Pittsburgh, PA 15213-3890

USA



Past advisories, CERT bulletins, information about FIRST representatives,

and other information related to computer security are available for anonymous

FTP from info.cert.org.



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Copyright 1994, 1995, 1996 Carnegie Mellon University. Conditions for use,

disclaimers, and sponsorship information can be found

ihttp://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.



CERT is registered in the U.S. Patent and Trademark Office.







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Revision history



Sep. 23, 1997  Updated copyrigth statement

Aug. 30, 1996  Information previously in the README was inserted

               into the advisory.

Feb. 02, 1995  Sec. III - Added a note about checking port numbers.



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