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FedCIRC Advisory FA-98-90

Original issue date: October 12, 1998
Last Revised: November 9, 1998
   Added vendor information for IBM Corporation and Silicon Graphics I nc.
   Updated information for Data General

A complete revision history is at the end of this file.

Topic: Remotely Exploitable Buffer Overflow Vulnerability in mountd

Affected systems:

NFS servers running certain implementations of mountd, primarily Linux systems. On some systems, the vulnerable NFS server is enabled by default. This vulnerability can be exploited even if the NFS server does not share any file systems.

See Appendix A for information from vendors. If your vendor's name does not appear, we did not hear from that vendor.


NFS is a distributed file system in which clients make use of file systems provided by servers. There is a vulnerability in some implementations of the software that NFS servers use to log requests to use file systems.

When a client makes a request to use a file system and subsequently makes that file system available as a local resource, the client is said to "mount" the file system. The vulnerability lies in the software on the NFS server that handles requests to mount file systems. This software is usually called "mountd" or "rpc.mountd."

Intruders who exploit the vulnerability are able to gain administrative access to the vulnerable NFS file server. That is, they can do anything the system administrator can do. This vulnerability can be exploited remotely and does not require an account on the target machine.

On some vulnerable systems, the mountd software is installed and enabled by default. See Appendix A for more information.

We will update this advisory as we receive additional information. Please check our advisory files regularly for updates that relate to your site.

I. Description

NFS is used to share files among different computers over the network using a client/server paradigm. When an NFS client computer wishes to access files on an NFS server, the client must first make a request to mount the file system. There is a vulnerability in some implementations of the software that handles NFS mount requests (the mountd program). Specifically, it is possible for an intruder to overflow a buffer in the area of code responsible for logging NFS activity.

We have received reports indicating that intruders are actively using this vulnerability to compromise systems and are engaging in large-scale scans to locate vulnerable systems.

On some systems, the vulnerable NFS server is enabled by default. See the vendor information in Appendix A.

II. Impact

After causing a buffer overflow, a remote intruder can use the resulting condition to execute arbitrary code with root privileges.

III. Solution

A. Install a patch from your vendor.

Appendix A contains input from vendors who have provided information for this advisory. We will update the appendix as we receive more information. If you do not see your vendor's name, the CERT/CC did not hear from that vendor. Please contact your vendor directly.

B. Until you install a patch, use the following workaround.

Consider disabling NFS until you are able to install the patch. In particular, since some systems have vulnerable versions of mountd installed and enabled by default, we recommend you disable mountd on those systems unless you are actively using those systems as NFS servers. 

Appendix A - Vendor Information

Below is a list of the vendors who have provided information for this advisory. We will update this appendix as we receive additional information. If you do not see your vendor's name, the CERT/CC did not hear from that vendor. Please contact the vendor directly.

Berkeley Software Design, Inc. (BSDI)

BSDI systems are not vulnerable to this attack.


Caldera provided a fixed version as nfs-server-2.2beta35-2 on Aug 28. It is available from


10fdb82ed8fd1b88c73fd962d8980bb4 RPMS/nfs-server-2.2beta35-2.i386.rpm
59e275b1ed6b98a39a38406f0415a226 RPMS/nfs-server-clients-2.2beta35-2.i386.rpm
6b075faf1d424e099c6932d95e76fd6b SRPMS/nfs-server-2.2beta35-2.src.rpm

Compaq Computer Corporation

SOURCE: (c) Copyright 1994, 1995, 1996, 1997, 1998 Compaq Computer Corporation. All rights reserved.
SOURCE: Compaq Computer Corporation Compaq Services Software Security Response Team USA
x-ref: SSRT0574U mountd

This reported problem is not present for the as shipped, Compaq's Digital ULTRIX or Compaq's Digital UNIX Operating Systems Software.

- Compaq Computer Corporation

Data General Corporation

DG/UX is not vulnerable to this problem.

FreeBSD, Inc.

FreeBSD 2.2.6 and above seem not be vulnerable to this exploit.

Fujitsu Limited

Fujitsu's UXP/V operating system is not vulnerable.

Hewlett-Packard Company

Not vulnerable.

IBM Corporation

The version of rpc.mountd shipped with AIX is not vulnerable.

IBM and AIX are registered trademarks of International Business Machines Corporation.


NCR is not vulnerable. We do not do any of the specified logging, nor do we have mountd (or normally anything else) hanging on port 635.

The NetBSD Project

NetBSD is not vulnerable to this attack in any configuration. Neither the NFS server or mount daemon are enabled by default.

The OpenBSD Project

OpenBSD is not affected.

Red Hat Software, Inc.

All versions of Red Hat Linux are vulnerable, and we have provided fixed packages for all our users. Updated nfs-server packages are available from our site at http://www.redhat.com/support/docs/errata.html

The Santa Cruz Operation, Inc.

No SCO platforms are vulnerable.

Silicon Graphics Inc.

Please refer to Silicon Graphics Inc. Security Advisory, "mountd Buffer Overflow Vulnerability", Number: 19981006-01-I, distributed October 26, 1998 for additional information about this vulnerability.

Silicon Graphics provides a comprehensive customer World Wide Web site. This site is located at http://www.sgi.com/Support/security/security.html

Sun Microsystems, Inc.

Sun's mountd is not affected.

Our thanks to Olaf Kirch and Wolfgang Ley for their input and assistance in constructing this advisory.

If you believe that your system has been compromised, contact FedCIRC. We strongly urge you to encrypt sensitive information sent by email. We can support a shared DES key or PGP. Contact us for more information.


FedCIRC Contact Information


+1 888-282-0870 (toll-free 24-hour hotline)

+1 412-268-6989

Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890

FedCIRC (Federal Computer Incident Response Capability) is operated by the CERT/CC for the U.S. General Services Administration. FedCIRC provides security services to U. S. Federal civilian agencies.

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This file: http://www2.fedcirc.gov/alerts/advisories/1998/FA-98-90.html

Revision History
Nov.  9, 1998  Added vendor information for IBM and SGI
               Updated information for Data General

Oct. 21, 1998  Added vendor information for Berkeley Software Design, Inc.