CERT Advisory - SunOS Environment Variables

Wed May 27 14:45:36 UTC 1992

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CA-92:11                        CERT Advisory
                                May 27, 1992
            SunOS Environment Variables and setuid/setgid Vulnerability
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The Computer Emergency Response Team/Coordination Center (CERT/CC) has
received information concerning a vulnerability involving environment
variables and setuid/setgid programs under Sun Microsystems Computer
Corporation SunOS.  This vulnerability exists on all Sun architectures
running SunOS 4.0 and higher.

In-house and third-party software can also be impacted by this
vulnerability.  For example, the current versions of rnews, sudo,
smount, and npasswd are known to be vulnerable under SunOS.  See the
Description section of this advisory for details of how to identify
software which may be vulnerable.

The workaround detailed in this advisory can be used to protect
vulnerable software on SunOS operating system versions for which
patches are unavailable, or for local or third party software which
may be vulnerable.

Sun has provided patches for SunOS 4.1, 4.1.1, and 4.1.2 programs
which are known to be impacted by this vulnerability.  They are
available through your local Sun Answer Center as well as through
anonymous ftp from the ftp.uu.net (137.39.1.9) system in the
/systems/sun/sun-dist directory.

Fix                     PatchID        Filename            Checksum
login and su            100630-01      100630-01.tar.Z     36269    39
sendmail                100377-04      100377-04.tar.Z     14692   311

Note: PatchID 100630-01 contains the international version of
/usr/bin/login.  PatchID 100631-01 contains the domestic version
of /usr/bin/login and is only available from Sun Answer Centers for
sites that use the US Encryption Kit.

Please note that Sun will occasionally update patch files.  If you
find that the checksum is different please contact Sun or the CERT/CC
for verification.

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I.   Description

     A security vulnerability exists if a set-user-id program changes
     its real and effective user ids to be the same (but not to the
     invoker's id), and subsequently causes a dynamically-linked program 
     to be exec'd.  A similar vulnerability exists for set-group-id programs.

     In particular, SunOS /usr/lib/sendmail, /usr/bin/login,
     /usr/bin/su, and /usr/5bin/su are vulnerable to this problem.

II.  Impact

     Local users can gain unauthorized privileged access to the system.

III. Solution

     A.  Obtain and install the patches from Sun or from ftp.uu.net following 
         the provided instructions.

     B.  The following workaround can be used to protect vulnerable binaries
         for which patches are unavailable for your SunOS version,
         or for local or third party software which may be vulnerable. 
         The example given is a workaround for /usr/lib/sendmail.  

         1.  As root, rename the existing version of /usr/lib/sendmail
             and modify the permissions to prevent misuse.

             # mv /usr/lib/sendmail /usr/lib/sendmail.dist
             # chmod 755 /usr/lib/sendmail.dist

         2.  In an empty temporary directory, create a file wrapper.c
             containing the following C program source (remember to
             strip any leading white-space characters from the #define lines).

             /* Start of C program source */

             /* Change the next line to reflect the full pathname
                of the file to be protected by the wrapper code   */

             #define COMMAND "/usr/lib/sendmail.dist"
             #define VAR_NAME "LD_"

             main(argc,argv,envp)
             int argc;
             char **argv;
             char **envp;
             {
                     register char  **cpp;
                     register char  **xpp;
                     register char   *cp;

                     for (cpp = envp; cp = *cpp;) {
                             if (strncmp(cp, VAR_NAME, strlen(VAR_NAME))==0) {
                                     for (xpp = cpp; xpp[0] = xpp[1]; xpp++);
                                     /* void */ ;
                             }
                             else {
                                     cpp++;
                             }
                     }

                     execv(COMMAND, argv);
                     perror(COMMAND);
                     exit(1);
             }
             /* End of C program source */

         3.  As root, compile the C program source for the wrapper and
             install the resulting binary.

             # make wrapper
             # mv ./wrapper /usr/lib/sendmail
             # chown root /usr/lib/sendmail
             # chmod 4711 /usr/lib/sendmail

         4.  Steps 1 through 3 should be repeated for other vulnerable
             programs with the appropriate substitution of pathnames and file
             names. The "COMMAND" C preprocessor variable within the C program
             source should also be changed to reflect the appropriate renamed
             system binary.

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The CERT/CC wishes to thank the following for their assistance: CIAC,
PCERT, and in particular Wietse Venema of Eindhoven University, The
Netherlands, for his support in the analysis of and a workaround for
this problem.  We also wish to thank Sun Microsystems Computer
Corporation for their prompt response to this vulnerability.
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If you believe that your system has been compromised, contact CERT/CC 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/CC personnel answer 7:30 a.m.-6:00 p.m. EST(GMT-5)/EDT(GMT-4),
           on call for emergencies during other hours.

Computer Emergency Response Team/Coordination Center (CERT/CC)
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

Past advisories, information about FIRST representatives, and other
information related to computer security are available for anonymous ftp
from cert.org (192.88.209.5).