WellCMS是一款开源、倾向移动端的轻量级CMS，高负载CMS，亿万级CMS，是大数据量、高并发访问网站最佳选择的轻CMS。登陆该CMS后台，某图片上传处，由于上传文件类型可控，可修改上传文件类型获取webshell。

这个漏洞来自一次偶然的测试，一次幸运的测试，那就直接写出我的测试过程。

第一步，登陆该CMS后台：

第二步，进入“后台管理“，定位利用点，点击下图红框中图片进行上传：

上传并抓取数据包：

第三步，修改post包中“filetype”参数类型为“php”；经分析 “data”参数为base64加密，这里我们将测试数据“<?php phpinfo();?>”经过base64加密等构造，形成“data”参数的数据：data%3Aimage%2Fjpeg%3Bbase64%2CPD9waHAgcGhwaW5mbygpOz8%2B，最后数据包放行，返回成功上传为php文件的路径：

最后，尝试访问，成功：

根据漏洞定位代码文件：route/attach.php，代码如下：

if ($action == 'create') {
    // hook attach_create_start.php
    user_login_check();
    // hook attach_create_check_after.php
    $backstage = param(2, 0);
    $width = param('width', 0);
    $height = param('height', 0);
    $is_image = param('is_image', 0); // 图片
    $name = param('name');
    $data = param_base64('data');
    $mode = param('mode', 0); // 上传类型 1主图
    $filetype = param('filetype'); // 压缩图片后缀jpeg jpg png等
    $convert = param('convert', 0); // 图片转换压缩 = 1
    $n = param('n', 0); // 对应主图赋值
    $type = param('type', 0); // type = 0则按照SESSION数组附件数量统计，type = 1则按照传入的n数值
    // hook attach_create_before.php
    // 允许的文件后缀名
    //$types = include _include(APP_PATH.'conf/attach.conf.php');
    //$allowtypes = $types['all'];
    empty($group['allowattach']) AND $gid != 1 AND message(2, '您无权上传');
    // hook attach_create_center.php
    empty($data) AND message(1, lang('data_is_empty'));
    //$data = base64_decode_file_data($data);
    $size = strlen($data);
    $size > 20480000 AND message(1, lang('filesize_too_large', array('maxsize' => '20M', 'size' => $size)));
    // hook attach_create_file_ext_start.php
    // 获取文件后缀名 111.php.shtmll
    $ext = file_ext($name, 7);
    $filetypes = include APP_PATH . 'conf/attach.conf.php';
    // hook attach_create_file_ext_before.php
    //主图必须为图片
    if ($is_image == 1 && $mode == 1 && !in_array($ext, $filetypes['image'])) message(1, lang('well_up_picture_error'));

    // hook attach_create_file_ext_center.php

    // 如果文件后缀不在规定范围内 改变后缀名
    //!in_array($ext, $filetypes['all']) AND $ext = '_' . $ext;
    if (!in_array($ext, $filetypes['all'])) {
        $ext = '_' . $ext;
    } else {
        // CMS上传图片
        $t == 1 AND $convert == 1 AND $is_image == 1 AND $ext = $filetype;
    }
    // hook attach_create_file_ext_after.php
    $tmpanme = $uid . '_' . xn_rand(15) . '.' . $ext;
    // hook attach_create_tmpanme_after.php
    $tmpfile = $conf['upload_path'] . 'tmp/' . $tmpanme;
    // hook attach_create_tmpfile_after.php
    $tmpurl = $conf['upload_url'] . 'tmp/' . $tmpanme;
    // hook attach_create_tmpurl_after.php
    $filetype = attach_type($name, $filetypes);
    // hook attach_create_save_before.php
    file_put_contents($tmpfile, $data) OR message(1, lang('write_to_file_failed'));
    // hook attach_create_save_after.php
    // 保存到 session，发帖成功以后，关联到帖子。
    // save attach information to session, associate to post after create thread.
    // 抛弃之前的 $_SESSION 数据，重新启动 session，降低 session 并发写入的问题
    // Discard the previous $_SESSION data, restart the session, reduce the problem of concurrent session write
    sess_restart();
    empty($t) AND empty($_SESSION['tmp_files']) AND $_SESSION['tmp_files'] = array();
    $t == 1 AND empty($_SESSION['tmp_website_files']) AND $_SESSION['tmp_website_files'] = array();

    // hook attach_create_after.php
    // type = 0则按照SESSION数组附件数量统计，type = 1则按照传入的n数值
    empty($type) AND $n = ($t == 1) ? count($_SESSION['tmp_website_files']) : count($_SESSION['tmp_files']);
    $filesize = filesize($tmpfile);
    $attach = array(
        'backstage' => $backstage, // 0前台 1后台
        'url' => $backstage ? '../' . $tmpurl : '' . $tmpurl,
        'path' => $tmpfile,
        'orgfilename' => $name,
        'filetype' => $filetype,
        'filesize' => $filesize,
        'width' => $width,
        'height' => $height,
        'isimage' => $is_image,
        'downloads' => 0,
        'aid' => '_' . $n
    );
    // hook attach_create_array_after.php
    if ($mode == 1) {
        // hook attach_create_thumbnail_beofre.php
        $_SESSION['tmp_thumbnail'] = $attach;
        // hook attach_create_thumbnail_after.php
    } else {
        // hook attach_create_website_files_beofre.php
        // 0 BBS 1 CMS
        $t == 1 ? $_SESSION['tmp_website_files'][$n] = $attach : $_SESSION['tmp_files'][$n] = $attach;
        // hook attach_create_website_files_after.php
    }
    // hook attach_create_session_after.php
    unset($attach['path']);
    // hook attach_create_end.php
    message(0, $attach);
}

大致流程：
1、 首先，接受相关参数，将filetype自行设置成“php”：

$data = param_base64('data');
$filetype = param('filetype'); /

2、 进行逻辑判断：

if (!in_array($ext, $filetypes['all'])) {
        $ext = '_' . $ext;
    } else {
        // CMS上传图片
        $t == 1 AND $convert == 1 AND $is_image == 1 AND $ext = $filetype;
    }

3、 最后成功写入：

$tmpanme = $uid . '_' . xn_rand(15) . '.' . $ext;
    // hook attach_create_tmpanme_after.php
    $tmpfile = $conf['upload_path'] . 'tmp/' . $tmpanme;
    // hook attach_create_tmpfile_after.php
    $tmpurl = $conf['upload_url'] . 'tmp/' . $tmpanme;
    // hook attach_create_tmpurl_after.php
    $filetype = attach_type($name, $filetypes);
    // hook attach_create_save_before.php
    file_put_contents($tmpfile, $data) OR message(1, lang('write_to_file_failed'));