CERT Advisory CA-94:15.NFS.Vulnerabilities

CERT Advisory cert-advisory at cert.org
Mon Dec 19 22:41:24 UTC 1994

CA-94:15                         CERT Advisory
                               December 19, 1994
                              NFS Vulnerabilities

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.

As we receive additional information relating to this advisory, we
will place it, along with any clarifications, in a CA-94:15.README
file. CERT advisories and their associated README files are available
by anonymous FTP from info.cert.org. We encourage you to check the
README files regularly for updates on advisories that relate to your

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

        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

        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 

             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: 


            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:

         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.

The CERT Coordination Center thanks Steve Bellovin, Casper Dik, Leendert 
van Doorn, and Wietse Venema for their support in responding to this problem.

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 at 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

Past advisories, CERT bulletins, information about FIRST representatives, 
and other information related to computer security are available for anonymous
FTP from info.cert.org.

CERT is a service mark of Carnegie Mellon University.

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