Public Vulnerability Reports

PHP getimagesize() Multiple Denial of Service Vulnerabilities

03.31.05

BACKGROUND

PHP is a widely-used general-purpose scripting language that is
especially suited for Web development and can be embedded into HTML.
More information is available at:

   http://www.php.net

DESCRIPTION

Remote exploitation of multiple denial of service vulnerabilities in
the PHP Group's PHP scripting language allows attackers to consume CPU
resources. The vulnerable routines, php_handle_iff() and
php_handle_jpeg(), are reachable from the PHP function getimagesize(),
which is defined as follows:

    array getimagesize ( string filename [, array &imageinfo] )

The getimagesize() routine is used to determine the size and dimensions
of multiple image formats, including GIF, JPG, PNG, TIFF, etc.

ISSUE 1 - php_handle_iff() Denial of Service

Remote exploitation of a denial of service (DoS) condition in the PHP
Group's PHP scripting language allows attackers to consume CPU
resources.

The problem specifically exists within the function php_handle_iff()
defined in ext/standard/image.c. The vulnerability is demonstrated in
the following excerpt:

    static struct gfxinfo *php_handle_iff(php_stream * stream TSRMLS_DC)
    {
    ...
            /* loop chunks to find BMHD chunk */
            do {
    [1]             if (php_stream_read(stream, a, 8) != 8) {
                            efree(result);
                            return NULL;
                    }
                    chunkId = php_ifd_get32s(a+0, 1);
    [2]             size    = php_ifd_get32s(a+4, 1);
                    if ((size & 1) == 1) {
                            size++;
                    }
                if (chunkId == 0x424d4844) { /* BMHD chunk */
                        ...
                ...
            } else {
    [3]                 if (php_stream_seek(stream, size, SEEK_CUR)) {
                                efree(result);
                                return NULL;
                        }
                    }
            } while (1);
    }

In the excerpt above, at line [1], 8 bytes are read from the user-
supplied file stream. At line [2], the variables 'chunkId' and 'size'
are set to user-supplied values from the file stream. If the variable
'size' is set to -8, then on line [3] the current position within the
file stream is moved back 8 bytes, resulting in an infinite loop.

ISSUE 2 - php_handle_jpeg() Denial of Service

Local exploitation of an input validation vulnerability in The PHP
Group's PHP embedded scripting language allows attackers to consume CPU
resources. The vulnerability specifically exists due to insufficient
validation of JPEG image file headers in the php_handle_jpeg()
function. The JPEG file header contains a file length field which may
be manipulated to cause an infinate loop in the copying of file data to
memory as shown below from ext/standard/image.c:

    static struct gfxinfo *
    php_handle_jpeg (php_stream * stream, pval *info TSRMLS_DC)
    {
        struct gfxinfo *result = NULL;
        unsigned int marker = M_PSEUDO;
        unsigned short length, ff_read=1;
   
        for (;;) {
    [1]     marker = php_next_marker(stream, marker, 1, ff_read TSRMLS_CC);
            ff_read = 0;
            switch (marker) {
                ...
                default:
    [2]             php_skip_variable(stream TSRMLS_CC);
                                    break;
            }
        }
   
        return result; /* perhaps image broken -> no info but size */
    }
   
    static void php_skip_variable(php_stream * stream TSRMLS_DC)
    {
    [3] off_t length = ((unsigned int)php_read2(stream TSRMLS_CC));
   
        length = length-2;
        if (length)
        {
    [4]     php_stream_seek(stream, (long)length, SEEK_CUR);
        }
    }

The php_next_marker() call [1] reads the next byte in the stream to
determine handling of the associated data. If given an invalid marker
value, the case statement executes the default block which calls the
php_skip_variable() function [2]. The php_read2() call [3] will return
0 bytes if the file stream has reached its end, so the pointer math
causes a length value of -2 in the php_stream_seek() call[4]. This
results in an infinate loop as the stream now points to the last two
bytes of the file when the pointer is returned to the for loop in
php_handle_jpeg().

ANALYSIS

Exploitation of either vulnerability could allow unauthenticated remote
attackers to consume 100% CPU resources on vulnerable systems.
Exploitation requires that an attacker supply a malicious image to the
getimagesize() PHP routine. The getimagesize() routine is frequently
used when handling user-supplied image uploads, which increases the
feasibility of remote exploitation.

DETECTION

iDEFENSE has confirmed the existence of these vulnerabilities in PHP
versions 4.2.2, 4.3.9, 4.3.10 and 5.0.3.

WORKAROUND

iDEFENSE is currently unaware of any workaround for this issue.

VENDOR RESPONSE

These vulnerabilities are addressed in PHP 4.3.11 and 5.0.4 which are
available for download at:

   www.php.net/distributions/php-5.0.4.tar.gz
   www.php.net/distributions/php-5.0.4.tar.bz2
  
   www.php.net/distributions/php-4.3.11.tar.gz
   www.php.net/distributions/php-4.3.11.tar.bz2

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
following names to these issues:

CAN-2005-0524 - php_handle_iff()
CAN-2005-0525 - php_handle_jpeg()

These are candidates for inclusion in the CVE list
(http://cve.mitre.org), which standardizes names for security problems.

DISCLOSURE TIMELINE

02/23/2005       Initial vendor notification
02/23/2005       Initial vendor response
03/31/2005       Coordinated public disclosure

CREDIT

The discoverer of these issues wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2005 Verisign, Inc.

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at the time of publishing based on currently available information. Use
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