Vulnerabilities in PCNFSD

Original issue date: April 18, 1996
Last revised: December 5, 1997
Updated information for NCR Corporation.

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

The CERT Coordination Center has received reports of two vulnerabilities in the pcnfsd program (pcnfsd is also known as rpc.pcnfsd); we have also received reports that these problems are being exploited. These vulnerabilities are present in some vendor-provided versions of pcnfsd and in some publicly available versions.

These two vulnerabilities were reported by Avalon Security Research in reports entitled "pcnfsd."

If you are using a vendor-supplied version of pcnfsd, please see the vendor information in Section III.A and Appendix A. Until you can install a patch from your vendor for these vulnerabilities, consider using the publicly available version described in Section III.B.

If you already use or plan to switch to a public version, we urge you to use the version cited in Section III.B or install the patch described in Section III.C. This patch has already been incorporated into the pcnfsd version described in III.B. There are many different public domain versions of pcnfsd, and we have not analyzed the vulnerability of those versions. We have analyzed and fixed the problems noted in this advisory only in the version described in III.B.

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


I. Description

The pcnfsd program (also called rpc.pcnfsd) is an authentication and printing program that runs on a UNIX server. There are many publicly available versions, and several vendors supply their own version.

pcnfsd supports a printing model that uses NFS to transfer files from a client to the pcnfsd server. (Note: pcnfsd does *not* provide NFS services.) When a client wants to print a file, it requests the path to a spool directory from the server. The client then writes the necessary files for printing using NFS, and informs the pcnfsd server that the files are ready for printing.

pcnfsd creates a subdirectory for each of its clients using the client's hostname, then returns this path name to the client. The returned path name must be exported via to its clients by the NFS server. The NFS server and the pcnfsd server may be two separate machines.

The first vulnerability is that pcnfsd, which runs as root, creates the aforementioned directories with mkdir(2) and then changes their mode with chmod(2) to mode 777. If the target directory is replaced with a symbolic link pointing to a restricted file or directory, the mkdir(2) will fail but the chmod(2) will succeed. This means that the target of the symbolic link will be mode 777.

Note that pcnfsd must run as root when servicing print requests so that it can assume the identity of the PC user when interacting with UNIX print commands. On some systems, pcnfsd may also have to run as root so it can read restricted files when carrying out authentication tasks.

The second vulnerability is that pcnfsd calls the system(3) subroutine as root, and the string passed to system(3) can be influenced by the arguments given in the remote procedure call. Remote users can execute arbitrary commands on the machine where pcnfsd runs.

II. Impact

For the first vulnerability, local users can change the permissions on any file accessible to the local system that the root user can change. For the second vulnerability, remote users can execute arbitrary commands as root on the machine where pcnfsd runs.

The impact is that directories can become world writable (mode 777). What this can lead to is bounded by the creativity of the intruder. For example, once the mode of /etc were changed to mode 777, one could then replace the password file, and then go on from there.

Exploitation of these vulnerabilities is only part of a larger attack scenario. Once exploited, there are many pathologies that could follow.

III. Solution

If you are using pcnfsd from a vendor, consult the vendor list in Section A. If your vendor is not listed, we recommend that you contact your vendor directly.

Until a vendor patch is available, we recommend that you obtain the publicly available version of pcnfsd as described in Section B. This version already has the patch described in Section C.

If you are presently using a public version of pcnfsd, we recommend that you either change to the version listed in Section B or apply the patch described in Section C. (The version in Section B already contains this patch.)

A. Obtain and install the appropriate patch according to the instructions included with the patch.

Below is a list of the vendors who have reported to us as of the date of this advisory. More complete information, is provided in the appendix. We will update the appendix as we receive more information.
If your vendor's name is not on this list, please contact the vendor directly.
Vendor or Source Status
BSDI BSD/OS Vulnerable. Patch available.
Hewlett Packard Vulnerable. Patch under development.
IBM AIX 3.2 Vulnerable. Patches available.
IBM AIX 4.1 Vulnerable. Patches available.
NCR Corporation Vulnerable. Patches available.
NEXTSTEP Vulnerable. Will be fixed in version 4.0.
SCO OpenServer 5 Vulnerable. Patch under development.
SC UnixWare 2.1 Vulnerable. Patch under development.
SCO UnixWare 2.1 Vulnerable. Patch under development.
SGI IRIX 5.3 Vulnerable. Patch under development.
SGI IRIX 6.2 Not vulnerable.

B. Until you are able to install the appropriate patch, we recommend that you obtain a version of pcnfsd from one of the following locations.

This version already has the patch mentioned in Section III.C.

ftp://ftp.cert.org/pub/tools/pcnfsd/pcnfsd.93.02.16-cert-dist.tar.Z
ftp://ftp.cert.dfn.de/pub/tools/net/pcnfsd/pcnfsd.93.02.16-cert-dist.tar.Z

MD5 (pcnfsd.93.02.16-cert-dist.tar.Z) = b7af99a07dfcf24b3da3446d073f8649

Build, install, and restart rpc.pcnfsd.

Ensure that the mode of the top-level pcnfsd spool directory is 755. In this version of pcnfsd, the top level spool directory is /usr/spool/pcnfs. To change this to mode 755, do the following as root:

chmod 755 /usr/spool/pcnfs

C. A patch is available for the two vulnerabilities described in this advisory.

Apply the patch using the GNU patch utility or by hand as necessary. Rebuild, reinstall, and restart rpc.pcnfsd. Set the mode of the top-level pcnfsd spool directory to 755.

For example, in the version of pcnfsd cited in Section B, the top level spool directory is /usr/spool/pcnfs. To change this to mode 755, do the following as root:

chmod 755 /usr/spool/pcnfs
Below is the location of a version of the patch that is an improvement over the patch originally cited in the advisory. The modifications are in the suspicious() function in pcnfsd_misc.c., courtesy of Sun Microsystems, Inc.

To prevent any confusion concerning the checksums, please see the file README.pcnfsd.93.02.16-cert. Checksums are also included below:

ftp://ftp.cert.org/pub/tools/pcnfsd/README.pcnfsd.93.02.16-cert
MD5 (README.pcnfsd.93.02.16-cert) = 07c64cd714bfaab3eb3849439a615b79

ftp://ftp.cert.org/pub/tools/pcnfsd/pcnfsd.93.02.16-cert-dist.tar.Z
MD5 (pcnfsd.93.02.16-cert-dist.tar.Z) = dc9b50172dfba8e6f9ad0c83f0e087e8

Note: When the above file is unpacked, the md5 checksum referenced in the README.pcnfsd.93.02.16-cert matches the following:

MD5 (pcnfsd.93.02.16-cert.tar) = 3a33f392d66b166cbc630275d8aba6f7

ftp://ftp.cert.org/pub/tools/pcnfsd/pcnfsd_misc.c-diffs
MD5 (pcnfsd_misc.c-diffs) = e9a83e6d540ab4683767ecf6d66dda9d

ftp://ftp.cert.org/pub/tools/pcnfsd/pcnfsd_print.c-diffs
MD5 (pcnfsd_print.c-diffs) = 7d9dac3c14b258e855517894e2934b14


Appendix A: Vendor Information

Below is information we have received from vendors concerning the vulnerability described in this advisory. If you do not see your vendor's name, please contact the vendor directly for information.

Berkeley Software Design, Inc. (BSDI)

The problem described in these vulnerabilities is present in all versions of BSD/OS. There is a patch (our patch number U210-007) for our 2.1 version of BSD/OS and associated products available from our patch and ftp servers <patches@BSDI.> or ftp://ftp.BSDI.COM/bsdi/patches/patches-2.1/U210-007

Data Design Systems, Inc.

The Tandem NonStop Kernel (NSK) system, does NOT contain either of the vulnerabilities cited in the advisory.

Digital Equipment Corporation

For updated information, please refer to the Digital Equipment Corporation Vendor Bulletin #96.0383, available in

ftp://ftp.cert.org/pub/vendors/dec/dec_96.0383

Note: Non-contract/non-warranty customers should contact local Digital support channels for information
regarding these kits.

As always, Digital urges you to periodically review your system management and security procedures. Digital will continue to review and enhance the security features of its products and work with customers to maintain and improve the security and integrity of their systems.

FreeBSD Inc.

There are two seperate ways of upgrading. The patch listed below is a source code patch, and is available from:

ftp://ftp.FreeBSD.ORG/pub/FreeBSD/FreeBSD-current/ports/net/pcnfsd/patches/patch-ad
MD5 (patch-ad) = 6dfdf6229632e53cb060961ac09bbd1a

This is part of the ports collection and anyone using current revisions of the ports system will automatically have this patch applied.

You can also get a FreeBSD "package" (pre-compiled binary) from:

ftp://ftp.FreeBSD.ORG/pub/FreeBSD/packages-current/net/pcnfsd-93.02.16.tgz
MD5 (pcnfsd-93.02.16.tgz) = 59c54dae46d1b4fd41887877b0a7097a

Hewlett-Packard Company

1. The rpc.pcnfsd binary that ships with HP systems contains a vulnerability that could allow a user to change permissions on a restricted file or directory.

Hewlett Packard is delivering a set of operating system dependent patches which contain a new version of rpc.pcnfsd. Accompanying each patch is a README file which discusses the general purpose of the patch and describes how to apply it to your system.

Recommended solution:

Apply one of the following patches based on your system hardware and operating system revision:

s300/s400 9.X - PHNE_7371 (rpc.pcnfsd)
s700/s800 9.X - PHNE_7072 (NFS Megapatch)
s700/s800 10.X - PHNE_7073 (NFS Megapatch)

The patches described above provide a new version of the rpc.pcnfsd executable which fixes the vulnerability.

2. The rpc.pcnfsd binary that ships with most Unix systems contains a vulnerability that could allow users to execute arbitrary commands on the machine where pcnfsd runs.

The rpc.pcnfsd daemon that ships with Hewlett Packard systems does not make the system call that allows this vulnerability. Since HP systems are not vulnerable - there is no fix!

To subscribe to automatically receive future NEW HP Security Bulletins please refer to information in

ftp://ftp.cert.org/pub/vendors/hp/HP.contact_info

IBM Corporation

See the appropriate release below to determine your action.

Until these fixes are applied, pcnfsd should be turned off and commented out in /etc/inetd.conf.

WARNING: If the line in /etc/inetd.conf has only one comment character, it will be uncommented (and exploitable) when mknfs is run! The inetd.conf entry must look like the following to remain turned off:

## pcnfsd sunrpc_udp udp wait root /usr/sbin/rpc.pcnfsd pcnfsd 150001 1-2

AIX 3.2

Apply the following fix to your system:
APAR - IX68084 (PTF - U447684 U450406)

To determine if you have this PTF on your system, run the following command:
lslpp -lB U447684 U450406

AIX 4.1

Apply the following fix to your system:
APAR - IX68086

To determine if you have this APAR on your system, run the following command:
instfix -ik IX68086

Or run the following command:
lslpp -h bos.net.nfs.client bos.net.nis.server

Your version of bos.net.nfs.client should be 4.1.5.5 or later. Your version of bos.net.nis.server should be 4.1.5.1 or later.

AIX 4.2

Apply the following fix to your system:
APAR - IX68087

To determine if you have this APAR on your system, run the following command:
instfix -ik IX68087

Or run the following command: lslpp -h bos.net.nfs.client bos.net.nis.server

Your version of bos.net.nfs.client should be 4.2.1.1 or later. Your version of bos.net.nis.server should be 4.2.1.3 or later.

To Order

APARs may be ordered using Electronic Fix Distribution (via FixDist) or from the IBM Support Center. For more information on FixDist, reference http://service.software.ibm.com/aixsupport/ or send e-mail to aixserv@austin.ibm.com with a subject of "FixDist".

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

NCR Corporation

The pcnfsd binary that shipped with some older NCR MP-RAS SVR4 releases contains a vulnerability that could allow a user to change permissions on a restricted file or directory.

NCR is delivering a set of operating system dependent patches which contain a new version of pcnfsd. Accompanying each patch is a README file which discusses the general purpose of the patch and describes how to apply it to your system.

Recommended solution:

Apply one of the following patches based on your operating system revision:

MP-RAS 2.03.x - PNFS203 (Version after 5/24-96)
MP-RAS 3.00.x - PNFS300 (Version after 5/28-96)
MP-RAS 3.01.x and later - Not vulnerable

The patches described above provide a new version of the pcnfsd executable which fixes the vulnerability.

NEC Corporation

Some systems are vulnerable and patches are available through anonymous FTP from ftp://ftp.meshnet.or.jp in the /pub/48pub/security directory.
UP-UX/V (Rel4.2MP) R5.x NECu5s003.COM.pkg
/pub/48pub/security/up/r5/pkg
Results of sum = 3060 266
md5 = 79E626B99A55FB0DBCE6EE642874570A
R6.x NECu6s003.COM.pkg
/pub/48pub/security/up/r6/pkg
Results of sum = 47304 272
md5 = 9FC9E993A5AB51291BF4817D3D70FBFD
R7.x NECu7s003.COM.pkg
/pub/48pub/security/up/r7/pkg
Results of sum = 46470 291
md5 = 59CA6887078AF88EA165AFD3BF5A1374
EWS-UX/V (Rel4.2) R7.X NECe7s004.COM.pkg
/pub/48pub/security/ews/r7/pkg
Results of sum = 3827 194
md5 = 4D40D9258DAB7EA41C30789609818330
R8.x NECe8s004.COM.pkg
/pub/48pub/security/ews/r8/pkg
Results of sum = 24399 199
md5 = 40B4CB1140791C14D1B604B6E8CB5FCB
R9.x
(except EWS4800/110N)
NECe9s008.COM.pkg
/pub/48pub/security/ews/r9/pkg
Results of sum = 23250 203
md5 = 5AD8BED137AAE7D0067EF3120574786C
R9.x
(EWS4800/110N)
NECe9s007.COM.pkg
/pub/48pub/security/ews/r9n/pkg
Results of sum = 3972 201
md5 = 28B2FA99F5200F81C5465571EF27E08B
R10.x NECeas004.COM.pkg
/pub/48pub/security/ews/ran/pkg
Results of sum = 51969 205
md5 = B6E12017E66DC8DC38FBE78CA1F0B0F0
EWS-UX/V (Rel4.2MP) R10.x NECmas007.COM.pkg
/pub/48pub/security/ews/ra/pkg
Results of sum = 48060 291
md5 = 42F8AE832071F033E21D8718A3670D76
UX/480O R11.x NECmbs010.COM.pkg
/pub/48pub/security/ews/rb/pkg
Results of sum = 24885 335
md5 = 7A14CBE4EA9B2470E340B5EEFD523F95

For further information contact: UX48-security-support@nec.co.jp . We encourage you contact the vendor directly if you have any questions.

NeXT Software, Inc.

NEXTSTEP is vulnerable. This will be fixed in the 4.0 release of OpenStep for Mach (aka NEXTSTEP 4.0, due out 2Q96).

Novell

CERT staff do not know whether Novell's enhanced version of PCNFSD (LWPNFSD) is vulnerable to this problem. We encourage you contact the vendor directly if you have any questions.

The Santa Cruz Operation, Inc.

Patches for pcnfsd are currently being developed for the following releases:

SCO OpenServer 5
SCO UnixWare 2.1.

These releases, as well as all prior releases, are vulnerable to both issues mentioned in the advisory. Should you not need to use pcnfs, SCO recommends that you not run pcnfsd. This can be done by commenting out pcnfsd in the appropriate script that starts pcnfsd, located in /etc/rc2.d.

This CERT advisory will be updated when further patch information is available.

Silicon Graphics Corporation

pcnfsd was only released for IRIX 5.3 and IRIX 6.2.

SGI is producing patch1179 for IRIX 5.3.

IRIX 6.2 is not vulnerable.

Sun Microsystems, Inc.

Sun has made patches available:

Solaris 2.4, 2.5 (Sparc) 103095-02
Solaris 2.4, 2.5 (X86) 103457-01
SunOS 4.1.X 103096-02

TGV Software, Inc./Cisco Systems, Inc.

These vulnerabilities are UNIX-specific and are not present in any version of MultiNet for OpenVMS.

The CERT Coordination Center thanks Josh Daymont, Ben G., and Alfred H. of Avalon Security Research for providing information for this advisory. We thank Wolfgang Ley of DFN-CERT for his help in understanding these problems.



Copyright 1996 Carnegie Mellon University.


Revision History
Dec. 5, 1997  Appendix A - Added information for NCR Corporation.
Oct. 31, 1997 Updated vendor information for IBM.
Sep. 24, 1997 Updated copyright statement
Apr. 03, 1997 Minor changes: corrected a name in the acknowledgments;
              indicated that CERT is now a registered service mark
Aug. 30, 1996 Information previously in the README was inserted into the
              advisory.
              Appendix B was moved to Sec. III.C. Appendix A - updated IBM URL
              in "To Order" section.
Aug. 01, 1996 Appendix A - updated Hewlett-Packard patch information.
July 26, 1996 Appendix A - modified NEC patch information.
July 5, 1996  Appendix A - added pointer to updated vendor information
              for Digital Equipment Corporation.
June 26, 1996 Appendix A - updated vendor information for NEC.
              Appendix A - added vendor information for Data Design Systems, Inc.
May 8, 1996   Appendix A - added patch information for FreeBSD.
May 6, 1996   Section II -added additional clarification about the impact of
              the vulnerability described.
              Appendix B - replaced the patch information originally contained
              in Appendix B with updated information.
              Appendix A - added updates for Digital Equipment Corporation,
              Novell, Sun Microsystems, Inc, and
              TGV Software, Inc./Cisco Systems, Inc.
Apr. 23, 1996 Appendix A - added information from NEC Corporation.
Apr. 19, 1996 Appendix B - new information on the fix referred to in Appendix
              B of the advisory.