Subject Korn Shell (ksh) suid_exec vulnerability Date 05-dec-96
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Security Advisory CERT-NL
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Author/Source : Gert Meijerink Index : S-96-74
Distribution : World Page :
Classification: External Version:
Subject : Korn Shell (ksh) suid_exec vulnerability Date : 05-dec-96
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By courtesy of AUSCERT, the Australian CERT, we received information on a
vulnerability Korn Shell (ksh) suid_exec.
This information is made publicly available by
AUSCERT advisory AA-96.17, dated 5-dec-96.
CERT-NL recommends that sites apply the workaround given in Section 3 until
vendor patches are made available.
Keywords: ksh, suid_exec
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AA-96.17 AUSCERT Advisory
Korn Shell (ksh) suid_exec Vulnerability
5 December 1996
Last Revised: --
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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.
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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.
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AUSCERT thanks Silicon Graphics Inc. for their assistance and technical
expertise essential for the production of this advisory.
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