CERT Advisory CA-1996-10 NIS+ Configuration Vulnerability

Original issue date: May 28, 1996
Last revised: October 20, 1997
Vendor information for Sun has been added to the UPDATES section.

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

The text of this advisory was originally released by the Australian Emergency Response Team on May 20, 1996, and updated on May 27, 1996, as AUSCERT advisory AA-96.02a.

Because of the seriousness of the problem, we are reprinting the AUSCERT advisory here with their permission. Only the contact information at the end has changed: AUSCERT contact information has been replaced with CERT/CC contact information.

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

AUSCERT has received information that a vulnerability exists under some configurations of NIS+. In vulnerable installations of NIS+, the access rights on the NIS+ passwd table are left in an unsecure state.

This vulnerability is known to exist in NIS+ installations initially created on Solaris 2.5 servers. Similar vulnerabilities in NIS+ configurations may also exist in previous versions of Solaris 2.

This vulnerability may allow any user with valid NIS+ credentials to gain root privileges.

AUSCERT recommends that any site which has NIS+ installed take this opportunity to check their installations and apply the appropriate workarounds as described in Section 3.

This updated advisory contains clarifications for sites requiring password aging facilities and sites running their NIS+ servers in NIS compatibility mode.

1. Description

NIS+ provides distributed network access to information sources such as password, group and host information. It maintains this information in the form of NIS+ tables. NIS+ tables contain the administrative information normally supplied by local files (such as /etc/passwd). As with the standard Unix administration files, setting secure permissions on the NIS+ tables is of utmost importance in maintaining system security.

NIS+ provides a comprehensive set of access rights for NIS+ tables. This includes permissions not only on NIS+ tables but also individual columns and entries in those tables. Due to the added complexity, sites need to be particularly diligent in ensuring that permissions on NIS+ tables (and associated entries and columns) are secure.

AUSCERT encourages sites running NIS+ to gain a good understanding of the permission model used by NIS+. A complete description may be found in the NIS+ documentation set. The rest of this advisory assumes a good understanding of NIS+ permission controls.

AUSCERT has received information that under some installations of NIS+ the permissions on the NIS+ passwd table are left in an unsecure state.

This vulnerability is known to exist in NIS+ installations initially created on Solaris 2.5 servers. Similar vulnerabilities in NIS+ configurations may also exist in previous versions of Solaris 2.

2. Impact

Any user with login access to a client or server that uses NIS+ for authentication may gain root privileges.

3. Workarounds

NIS+ uses an access control mechanism for granting access to NIS+ tables which is similar (but not identical) to that used by the standard Unix file system. NIS+ tables are assigned permissions for the NIS+ user categories nobody, owner, group and world. NIS+ also has permissions associated with columns and individual entries in NIS+ tables.

Under some installations of NIS+ the permissions of the NIS+ passwd table and its columns are left in an unsecure state. These permissions can be viewed using niscat(1).

To check the permissions on the NIS+ passwd table, sites can use:

# niscat -o passwd.org_dir This should produce output similar to:


Object Name   : passwd
Owner         : myhost.mydomain.org.
Group         : admin.mydomain.org.
Domain        : org_dir.mydomain.org.
Access Rights : ----rmcdrmcd----
Time to Live  : 12:0:0
Object Type   : TABLE
Table Type          : passwd_tbl
Number of Columns   : 8
Character Separator : :
Search Path         :
Columns             :
        [0]     Name          : name
                Attributes    : (SEARCHABLE, TEXTUAL DATA, CASE SENSITIVE)
                Access Rights : r---------------
        [1]     Name          : passwd
                Attributes    : (TEXTUAL DATA)
                Access Rights : -----m----------
        [2]     Name          : uid
                Attributes    : (SEARCHABLE, TEXTUAL DATA, CASE SENSITIVE)
                Access Rights : r---------------
        [3]     Name          : gid
                Attributes    : (TEXTUAL DATA)
                Access Rights : r---------------
        [4]     Name          : gcos
                Attributes    : (TEXTUAL DATA)
                Access Rights : r---------------
        [5]     Name          : home
                Attributes    : (TEXTUAL DATA)
                Access Rights : r---------------
        [6]     Name          : shell
                Attributes    : (TEXTUAL DATA)
                Access Rights : r---------------
        [7]     Name          : shadow
                Attributes    : (TEXTUAL DATA)
                Access Rights : ----------------

This output shows two types of access rights associated with the NIS+ passwd table. First, the default access rights for the table, which are given at the start of the output (----rmcdrmcd----). Second, the access rights associated with each column.

In particular, sites should check the access rights on the columns of the NIS+ passwd table. It should be noted that it appears that individual entries of the passwd table are owned by individual users. The above access rights do not allow a user to modify any part of their passwd table entry besides their own passwd field. For many environments this is acceptable.

However, depending on the local site configuration and requirements, additional access rights may also be needed.

Sites that wish users to be able to change their shell or gcos information may have the (m)odify bit set for owner on the shell or gcos column as needed.
Sites that have their NIS+ servers running in NIS compatibility mode to serve NIS clients may require (r)ead permission for nobody on the NIS+ passwd table.
Sites that are using password aging may require additional access rights on the shadow column. The exact access rights will depend on the particular NIS+ version (including patches). Sites are encouraged to check their local documentation for more information.

Other than this, the access rights on columns should appear as shown in the niscat(1) output above.

Any additional access rights on the table or its columns besides those shown above may allow a user to gain additional privileges, including possibly root. Sites should completely understand the ramifications if they allow additional access rights.

Sites may set the access rights on the NIS+ passwd table, as shown in the above output, by issuing the following commands as root on the master NIS+ server.

To set the default access rights for the NIS+ passwd table:

# nischmod na-rmcd,og+rmcd passwd.org_dir

To set the column access rights on the NIS+ passwd table:

After making any changes in access rights, AUSCERT recommends that sites verify the changes they have made using niscat(1), as shown previously.

Sites that have replica NIS+ servers may use nisping(1m) to propagate the changes to the replica servers in a timely manner.

4. Additional measures

AUSCERT recommends that sites take this opportunity to ensure that all NIS+ tables have access rights in accordance with the local site security policy. This also includes checking access rights on all the columns and entries of the NIS+ tables in addition to the default access rights of the tables themselves.

AUSCERT wishes to thank Ivan Angus and David Clarke of ANU for reporting this vulnerability and for their advice in the preparation of this advisory. AUSCERT also acknowledges Marek Krawus of UQ, Reinhard Uebel and Mark McPherson of QTAC for their assistance.

The AUSCERT team have made every effort to ensure that the information contained in this document is accurate. However, the decision to use the information described is the responsibility of each user or organisation. The appropriateness of this document for an organisation or individual system should be considered before application in conjunction with local policies and procedures. AUSCERT takes no responsibility for the consequences of applying the contents of this document.

UPDATES

CERT/CC received information concerning an additional problem with the ROW access rights in the NIS+ password table. Accounts created on Solaris 2.4 and 2.5 systems have excessive rights on the system. These new super accounts have read, modify, create, and delete access rights on their own rows in the nisplus password table. This means they can alter all attributes on their own entries.

To determine if your system is so affected, execute the following:


        % niscat -o '[name=juke],passwd.org_dir' | egrep "Access"

If the output displays information similar to the following:


          Access Rights : ----rmcdr---r---
                              ^^^^

then the owner can read, modify, change, and delete information.

The rights at this level should be more restrictive, and the individual rights on entries should be less restrictive. The less restrictive rights on entries allow a user to change their password entry, the GECOS field, and even the shell depending upon how the entry rights are set.

The output from the niscat above should look like the following:


          Access Rights : ----r-----------

This allows only the user to read information from the password table.

One way to determine which entries in the password table need to be changed is the following:


        % niscat -o '[ ],passwd.org_dir' | egrep "Owner|rmc"

To fix the entries, use the following:


          % nischmod o=r,ngw-rmdc '[ ],passwd.org_dir'

This sets the owner permissions to r (read) and removes all permissions from nobody, group, and world.

Vendor Information

Below is information we have received from vendors. If you do not see your vendor's name below, contact the vendor directly for information.

Sun Microsystems, Inc.

Sun Microsystems has provided the following list of patches in response to this advisory:

103266-01 5.5
103267-01 5.5_x86
103270-01 5.4
103271-01 5.4_x86
103269-01 5.3

Revision History

Oct. 27, 1997  Vendor information for Sun has been added to the UPDATES
	       section.
Sep. 24, 1997  Updated copyright statement 
Aug. 30, 1996  Information previously in the README was inserted into the
               advisory.
               Beginning of the advisory - removed AUSCERT advisory header
               to avoid confusion.
June 12, 1996  Updates section - added clarification concerning ROW access
               rights.

Space shortcuts

Child pages