__________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Korn Shell (ksh) suid_exec Vulnerability December 12, 1996 23:00 GMT Number H-15 ______________________________________________________________________________ PROBLEM: A problem has been identified in the "suid_exec" program often supplied with the Korn Shell (ksh) distribution. PLATFORM: All operating systems with "suid_exec" program supplied with the Korn Shell (ksh) distribution. DAMAGE: Local users may gain root privileges. SOLUTION: Apply the workaround described below. ______________________________________________________________________________ VULNERABILITY Exploit details involving this vulnerability have been widely ASSESSMENT: distributed. ______________________________________________________________________________ [ Start AUSCERT Advisory ] =========================================================================== AA-96.17 AUSCERT Advisory Korn Shell (ksh) suid_exec Vulnerability 5 December 1996 Last Revised: -- ---------------------------------------------------------------------------- AUSCERT has received information that there is a vulnerability in the "suid_exec" program often supplied with the Korn Shell (ksh) distribution. ksh may be part of the standard vendor distribution or may have been installed as additional software by system administrators. This vulnerability may allow local users to gain root privileges. Exploit details involving this vulnerability have been widely distributed. At this stage, AUSCERT is unaware of any official vendor patches. AUSCERT recommends that sites apply the workaround given in Section 3 until vendor patches are made available. This advisory will be updated as new information becomes available. ---------------------------------------------------------------------------- 1. Description suid_exec is a program that is often installed when the Korn shell (ksh) interpreter is installed. suid_exec is installed to allow the execution of setuid/setgid shell scripts and shell scripts which do not have read permissions set. All versions of suid_exec are currently vulnerable. The vulnerability in suid_exec may allow arbitrary commands to be executed with root privileges. Exploit information involving this vulnerability has been made publicly available. suid_exec is known to be present in the default installation on the following Unix operating systems: IRIX 5.x IRIX 6.x This list is not necessarily complete, and other operating systems may have suid_exec installed by default. Sites may also have installed suid_exec when installing the publicly available version of the Korn Shell (ksh). This version of suid_exec is also vulnerable. All sites are encouraged to check their systems for the presence of the suid_exec program. By default, suid_exec will be found in /etc. To determine the location of any other copies of suid_exec, the following command can be executed as root: # find / -name suid_exec -perm -4000 -exec ls -l {} \; If suid_exec is installed, sites should take the steps outlined in Section 3. 2. Impact Local users may gain root privileges. 3. Workarounds/Solution AUSCERT recommends that sites prevent the exploitation of the vulnerability in suid_exec by immediately applying the workaround given in Section 3.1. Currently there are no vendor patches available that address this vulnerability. AUSCERT recommends that official vendor patches be installed when they are made available. 3.1 Remove setuid and execute permissions Until official vendor patches are made available, sites should remove the setuid root and execute permissions from suid_exec. For example, if suid_exec is located in /etc, the following command should be run as root: # chmod 400 /etc/suid_exec # ls -l /etc/suid_exec -r-------- 1 root sys 14384 May 30 1996 /etc/suid_exec Note that the removing these permissions from suid_exec may stop non-root users from executing setuid/setgid shell scripts and scripts upon which read permissions have not been set. 4. Additional measures Most Unix systems ship with numerous programs which have setuid or setgid privileges. Often the functionality supplied by these privileged programs is not required by many sites. The large number of privileged programs that are shipped by default are to cater for all possible uses of the system. AUSCERT encourages sites to examine all the setuid/setgid programs and determine the necessity of each program. If a program does not absolutely require the setuid/setgid privileges to operate (for example, it is only run by the root user), the setuid/setgid privileges should be removed. Furthermore, if a program is not required at your site, then all execute permissions should be removed. A sample command to find all setuid/setgid programs is (run as root): # find / \( -perm -4000 -o -perm -2000 \) -exec ls -ld {} \; It is AUSCERT's experience that many vulnerabilities are being discovered in setuid/setgid programs which are not necessary for the correct operation of most systems. Sites can increase their security by removing unnecessary setuid/setgid programs. For example, the functionality provided by the suid_exec program is not needed by most sites. If sites had previously disabled the suid_exec program, they would not have been vulnerable to this latest exploit. ---------------------------------------------------------------------------- AUSCERT thanks Silicon Graphics Inc. for their assistance and technical expertise essential for the production of this advisory. ---------------------------------------------------------------------------- [ End AUSCERT Advisory ] _______________________________________________________________________________ CIAC wishes to acknowledge the contributions of AUSCERT for the information contained in this bulletin. _______________________________________________________________________________ CIAC, the Computer Incident Advisory Capability, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. 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A list of FIRST member organizations and their constituencies can be obtained via WWW at http://www.first.org/. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. 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