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CERT Advisory 087
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CERT(sm) Advisory CA-94:15
Original issue date: December 19, 1994
Last revised: August 30, 1996
Information previously in the README was inserted
into the advisory.
A complete revision history is at the end of this file.
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.
Copyright 1994, 1995, 1996 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is
included.
CERT is a service mark of Carnegie Mellon University.
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Revision history
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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