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TUCoPS :: Unix :: General :: ciacg021.txt

Vulnerabilities pcnfsd


                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

                       Vulnerabilities in PCNFSD Program

April 22, 1996 17:00 GMT                                           Number G-21
PROBLEM:       Two vulnerabilities exist in the pcnfsd program. 
PLATFORM:      The pcnfsd program (also called rpc.pcnfsd) runs on a UNIX 
DAMAGE:        The first vulnerability allows local users to change the 
               permissions on any file accessible to the local system that the 
               root user can change. The second vulnerability allows remote 
               users to execute arbitrary commands as root on the machine 
               where pcnfsd runs. 
SOLUTION:      Check for vulnerabilities and install the proper patch. If 
               patch is unavailable from vendor, consider using the public 
               available version describe below. 
VULNERABILITY  Knowledge of how to exploit these vulnerabilities are widely 
ASSESSMENT:    known. 

CIAC has obtained information from CERT pertaining to the vulnerabilities 
in the pcnfsd program. CIAC recommends that you install the proper patch 
and/or follow the solutions described below. 

[ Start of CERT Advisory ]

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. 

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, including how to obtain
         patches, is provided in the  appendix of this advisory and reproduced
         in the CA-96.xx.README file. We will update the README file as we 
         receive more information.

         If your vendor's name is not on this list, please contact the vendor

         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.
         NEXTSTEP                  Vulnerable. Will be fixed in version 4.0.
         SCO OpenServer 5          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 and included in Appendix B.

     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

                chmod 755 /usr/spool/pcnfs

     C.  Appendix B contains a patch 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

The CERT Coordination Center thanks Josh D., 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

Appendix A: Vendor Information

Current as of April 18, 1996
See CA-96.08.README for updated 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

Hewlett-Packard Company
Patches in process, watch for HP an security bulletin for this

IBM Corporation
  See the appropriate release below to determine your action.

  AIX 3.2
    Apply the following fixes to your system:

       APAR - IX57623 (PTF - U442633)
       APAR - IX56965 (PTF - U442638)

    To determine if you have these PTFs on your system, run the following

       lslpp -lB U442633
       lslpp -lB U442638

  AIX 4.1
    Apply the following fixes to your system:

        APAR - IX57616
        APAR - IX56730

    To determine if you have these APARs on your system, run the following

       instfix -ik IX57616
       instfix -ik IX56730

  To Order
    APARs may be ordered using FixDist or from the IBM Support Center.
    For more information on FixDist, reference URL:

    or send e-mail to with a subject of "FixDist".

  IBM and AIX are registered trademarks of International Business Machines

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).

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.

The README file for this 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.

Appendix B: Patch Information

Here is the patch for pcnfsd_print.c. It is also available as:

        MD5 (pcnfsd_print.c-diffs) = ec44046ff5c769aa5bf2d8d155b61f1f

---------------------------------CUT HERE---------------------------------
*** /tmp/T0a002c1       Fri Apr  5 13:14:50 1996
--- pcnfsd_print.c      Fri Apr  5 13:14:46 1996
*** 221,226 ****
--- 221,227 ----
  int    dir_mode = 0777;
  int rc;
+ mode_t oldmask;
        *sp = &pathname[0];
        pathname[0] = '\0';
*** 231,241 ****
        /* get pathname of current directory and return to client */
        (void)sprintf(pathname,"%s/%s",sp_name, sys);
        (void)mkdir(sp_name, dir_mode); /* ignore the return code */
-       (void)chmod(sp_name, dir_mode);
        rc = mkdir(pathname, dir_mode); /* DON'T ignore this return code */
        if((rc < 0 && errno != EEXIST) ||
-          (chmod(pathname, dir_mode) != 0) ||
           (stat(pathname, &statbuf) != 0) ||
           !(statbuf.st_mode & S_IFDIR)) {
--- 232,242 ----
        /* get pathname of current directory and return to client */
        (void)sprintf(pathname,"%s/%s",sp_name, sys);
+       oldmask = umask(0);
        (void)mkdir(sp_name, dir_mode); /* ignore the return code */
        rc = mkdir(pathname, dir_mode); /* DON'T ignore this return code */
+       umask(oldmask);
        if((rc < 0 && errno != EEXIST) ||
           (stat(pathname, &statbuf) != 0) ||
           !(statbuf.st_mode & S_IFDIR)) {
*** 381,387 ****
                   ** filter with the appropriate arguments.
!                  (void)run_ps630(new_pathname, opts);
                ** Try to match to an aliased printer
--- 382,391 ----
                   ** filter with the appropriate arguments.
!                  (void)sprintf(tempstr,
!                       "rpc.pcnfsd: ps630 filter disabled for %s\n", 
!                       msg_out(tempstr);
!                       return(PS_RES_FAIL);
                ** Try to match to an aliased printer
---------------------------------CUT HERE---------------------------------

[ End of CERT Advisory ]


CIAC wishes to acknowledge the contributions of CERT for the 
information contained in this bulletin.

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