CERT Advisory - NeXT NetInfo "_writers" Vulnerabilities

CERT Advisory cert-advisory-request at cert.org
Wed Jan 20 20:11:24 UTC 1993

CA-93:02                        CERT Advisory
                              January 20, 1993
                   NeXT NetInfo "_writers" Vulnerabilities


The CERT Coordination Center has received information from NeXT Computer,
Inc. concerning a vulnerability in the distributed printing facility of NeXT
computers running all releases of NeXTSTEP software through NeXTSTEP Release
3.0.  For more information, please contact your authorized support center.
If you are an authorized support provider, please contact NeXT through your
normal channels.


I.   Description

     The default NetInfo "_writers" properties are configured to allow
     users to install printers and FAX modems and to export them to the
     network without requiring assistance from the system administrator.
     They also allow a user to configure other parts of the system, such as
     monitor screens, without requiring help from the system administrator.
     Vulnerabilities exist in this facility that could allow users to gain
     unauthorized privileges on the system.

II.  Impact

     In the case of the "/printers" and the "/fax_modems" directories, the
     "_writers" property can permit users to obtain unauthorized root
     access to a system.

     In the "/localconfig/screens" directory, the "_writers" property can
     potentially permit a user to deny normal login access to other users.

III. Solution

     To close the vulnerabilities, remove the "_writers" properties from
     the "/printers", "/fax_modems", and "/localconfig/screens" directories
     in all NetInfo domains on the network, and from all immediate
     subdirectories of all "/printers", "/fax_modems", and
     "/localconfig/screens" directories.  The "_writers" properties may be
     removed using any one of the following three methods:

     A. As root, use the "niutil" command-line utility.  For example, to
        remove the "_writers" property from the "/printers" directory:

          # /usr/bin/niutil -destroyprop . /printers _writers

     B. Alternatively, use the NetInfoManager application: open the
        desired domain, open the appropriate directory, select the
        "_writers" property, choose the "Delete" command [Cmd-r] from
        the "Edit" menu, and save the directory.

     C. To assist system administrators in editing their NetInfo
        domains, a shell script, "writersfix", is available via
        anonymous FTP from next.com (

          Filename                                   Size   Checksum
          --------                                   ----   --------
          pub/Misc/Utilities/WritersFix.compressed   5515   53660 6

        After transferring this file using BINARY transfer type,
        double-click on the file.  A "WritersFix" directory will be
        created in your file system, containing the script
        ("writersfix") and some documentation ("WritersFix.rtf").

     Consider removing "_writers" from other NetInfo directories as well
     (for example, "/locations"), noting the following trade-off between
     ease-of-use and security.  By removing the "_writers" properties, the
     network and the computers on the network become more secure, but a
     system administrator's assistance is required where it previously was
     not required.

     Please refer to the NeXTSTEP Network and System Administration manual
     for additional information on "_writers".  Note that the
     subdirectories of the "/users" directory have "_writers_passwd" set to
     the user whose account is described by the directory.  This is
     essential if users are to be able to change their own passwords, and
     this does not compromise system security.

The CERT Coordination Center wishes to thank Alan Marcum and Eric Larson of
NeXT Computer, Inc. for notifying us about the existence of these
vulnerabilities and for providing appropriate technical information.

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in FIRST (Forum of Incident
Response and Security Teams).

Internet E-mail: cert at cert.org
Telephone: 412-268-7090 (24-hour hotline)
           CERT personnel answer 7:30 a.m.-6:00 p.m. EST(GMT-5)/EDT(GMT-4),
           on call for emergencies during other hours.

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

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