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Date: Mon, 19 Dec 1994 17:41:24 -0500
Message-Id: <199412192241.RAA28528@delphi.cert.org>
Subject: CERT Advisory CA-94:15.NFS.Vulnerabilities
To: cert-advisory-request@cert.org
From: cert-advisory@cert.org (CERT Advisory)
Reply-To: cert-advisory@cert.org
Organization: CERT Coordination Center
Address: Software Engineering Institute
Carnegie Mellon University
Pittsburgh, Pennsylvania 15213-3890
Phone: +1 412 268-7090
Fax: +1 412 268-6989
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CA-94:15 CERT Advisory
December 19, 1994
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.
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 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). 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.
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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.