php反序列unserialize的一个小特点

php反序列unserialize的一个小特性

这几天wordpress的那个反序列漏洞比较火，具体漏洞我就不做分析了，看这篇：http://drops.wooyun.org/papers/596，?
你也可以去看英文的原文：http://vagosec.org/2013/09/wordpress-php-object-injection/。?

wp官网打了补丁，我试图去bypass补丁，但让我自以为成功的时候，发现我天真了，并没有成功绕过wp的补丁，但却发现了unserialize的一个小特性，在此和大家分享一下。?

1.unserialize()函数相关源码：?

if ((YYLIMIT - YYCURSOR) < 7) YYFILL(7);?
????????yych = *YYCURSOR;?
????????switch (yych) {?
????????case 'C':?
????????case 'O':????????goto yy13;?
????????case 'N':????????goto yy5;?
????????case 'R':????????goto yy2;?
????????case 'S':????????goto yy10;?
????????case 'a':????????goto yy11;?
????????case 'b':????????goto yy6;?
????????case 'd':????????goto yy8;?
????????case 'i':????????goto yy7;?
????????case 'o':????????goto yy12;?
????????case 'r':????????goto yy4;?
????????case 's':????????goto yy9;?
????????case '}':????????goto yy14;?
????????default:????????goto yy16;?
????????}

上边这段代码是判断序列串的处理方式，如序列串O:4:"test":1:{s:1:"a";s:3:"aaa";},处理这个序列串，先获取字符串第一个字符为O，然后case 'O':??goto yy13?

yy13:?
????????yych = *(YYMARKER = ++YYCURSOR);?
????????if (yych == ':') goto yy17;?
????????goto yy3;

从上边代码看出，指针移动一位指向第二个字符，判断字符是否为:，然后 goto yy17?

yy17:?
????????yych = *++YYCURSOR;?
????????if (yybm[0+yych] & 128) {?
????????????????goto yy20;?
????????}?
????????if (yych == '+') goto yy19;?

.......?

yy19:?
????????yych = *++YYCURSOR;?
????????if (yybm[0+yych] & 128) {?
????????????????goto yy20;?
????????}?
????????goto yy18;

从上边代码看出，指针移动，判断下一位字符，如果字符是数字直接goto yy20，如果是'+'就goto yy19，而yy19中是对下一位字符判断，如果下一位字符是数字goto yy20，不是就goto yy18，yy18是直接退出序列处理，yy20是对object性的序列的处理，所以从上边可以看出：?

O:+4:"test":1:{s:1:"a";s:3:"aaa";}?
O:4:"test":1:{s:1:"a";s:3:"aaa";}

都能够被unserialize反序列化，且结果相同。?

2.实际测试：?

<?php?
var_dump(unserialize('O:+4:"test":1:{s:1:"a";s:3:"aaa";}'));?
var_dump(unserialize('O:4:"test":1:{s:1:"a";s:3:"aaa";}'));?
?>

输出：?

object(__PHP_Incomplete_Class)#1 (2) { ["__PHP_Incomplete_Class_Name"]=> string(4) "test" ["a"]=> string(3) "aaa" }?
object(__PHP_Incomplete_Class)#1 (2) { ["__PHP_Incomplete_Class_Name"]=> string(4) "test" ["a"]=> string(3) "aaa" }

其实，不光object类型处理可以多一个'+',其他类型也可以，具体测试不做过多描述。?

3.我们看下wp的补丁：?

function is_serialized( $data, $strict = true ) {?
????????// if it isn't a string, it isn't serialized?
????????if ( ! is_string( $data ) )?
????????????????return false;?
????????$data = trim( $data );?
???????? if ( 'N;' == $data )?
????????????????return true;?
????????$length = strlen( $data );?
????????if ( $length < 4 )?
????????????????return false;?
????????if ( ':' !== $data[1] )?
????????????????return false;?
????????if ( $strict ) {//output?
????????????????$lastc = $data[ $length - 1 ];?
????????????????if ( ';' !== $lastc && '}' !== $lastc )?
????????????????????????return false;?
????????} else {//input?
????????????????$semicolon = strpos( $data, ';' );?
????????????????$brace???? = strpos( $data, '}' );?
????????????????// Either ; or } must exist.?
????????????????if ( false === $semicolon && false === $brace )?
????????????????????????return false;?
????????????????// But neither must be in the first X characters.?
????????????????if ( false !== $semicolon && $semicolon < 3 )?
????????????????????????return false;?
????????????????if ( false !== $brace && $brace < 4 )?
????????????????????????return false;?
????????}?
????????$token = $data[0];?
????????switch ( $token ) {?
????????????????case 's' :?
????????????????????????if ( $strict ) {?
????????????????????????????????if ( '"' !== $data[ $length - 2 ] )?
????????????????????????????????????????return false;?
????????????????????????} elseif ( false === strpos( $data, '"' ) ) {?
????????????????????????????????return false;?
????????????????????????}?
????????????????case 'a' :?
????????????????case 'O' :?
????????????????????????echo "a";?
????????????????????????return (bool) preg_match( "/^{$token}:[0-9]+:/s", $data );?
????????????????case 'b' :?
????????????????case 'i' :?
????????????????case 'd' :?
????????????????????????$end = $strict ? '$' : '';?
????????????????????????return (bool) preg_match( "/^{$token}:[0-9.E-]+;$end/", $data );?
????????}?
????????return false;?
}
补丁中的?

return (bool) preg_match( "/^{$token}:[0-9]+:/s", $data );

可以多一个'+'来绕过，虽然我们通过这个方法把序列值写入了数据库，但从数据库中提取数据，再次验证的时候却没法绕过了，我这个加号没能使数据进出数据库发生任何变化，我个人认为这个补丁绕过重点在于数据进出数据的前后变化。?

4.总结?
虽然没有绕过wp补丁，但这个unserialize()的小特性可能会被很多开发人员忽略，导致程序出现安全缺陷。?
以上的分析有什么错误请留言指出。?

5.参考?
《WordPress < 3.6.1 PHP Object Injection》?
http://vagosec.org/2013/09/wordpress-php-object-injection/?
《var_unserializer.c源码》?
https://github.com/php/php-src/blob/73cd2e0ab14d804c6bf0b689490bdd4fd6e969b1/ext/standard/var_unserializer.c?
《PHP string序列化与反序列化语法解析不一致带来的安全隐患》?
http://zone.wooyun.org/content/1664