WellCMS 2.0 Beta3 后台任意文件上传

WellCMS¶

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。

复现过程¶

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：

源码分析¶

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'));