Discuz x3.4前台SSRF

补丁: https://gitee.com/ComsenzDiscuz/DiscuzX/commit/41eb5bb0a3a716f84b0ce4e4feb41e6f25a980a3

可跨协议打
缺点: 由于payload构造中有第二次跳转，所以要求对方服务器的127.0.0.1:80也是dz
PHP版本: php ver > 5.3

触发漏洞点

漏洞点: source/module/misc/misc_imgcropper.php:55

$prefix = $_GET['picflag'] == 2 ? $_G['setting']['ftp']['attachurl'] : $_G['setting']['attachurl'];
	if(!$image->Thumb($prefix.$_GET['cutimg'], $cropfile, $picwidth, $picheight)) {

因为$_G['setting']['ftp']['attachurl']默认是/，传入Thumb中的$prefix.$_GET['cutimg']的后缀也可控

一路跟进，source/class/class_image.php:52 -> source/class/class_image.php:118

function init($method, $source, $target, $nosuffix = 0) {
		global $_G;

		$this->errorcode = 0;
		if(empty($source)) {
			return -2;
		}
		$parse = parse_url($source);
		if(isset($parse['host'])) {
			if(empty($target)) {
				return -2;
			}
			$data = dfsockopen($source);
			$this->tmpfile = $source = tempnam($_G['setting']['attachdir'].'./temp/', 'tmpimg_');
			if(!$data || $source === FALSE) {
				return -2;
			}
			file_put_contents($source, $data);
}

可以看到如果能够被parse_url函数解析出host即可进入dfsockopen里面进行curl请求
所以这个就是一个前缀限定为/，跟入parse_url函数底层会发现，它还支持这种作为url: //www.baidu.com

/php-5.4.45/ext/standard/url.c

继续跟入dz的dfsockopen函数
source/function/function_filesock.php:14

$matches = parse_url($url);
$scheme = $matches['scheme'];
$host = $matches['host'];
$path = $matches['path'] ? $matches['path'].($matches['query'] ? '?'.$matches['query'] : '') : '/';
...省略
curl_setopt($ch, CURLOPT_URL, $scheme.'://'.($ip ? $ip : $host).($port ? ':'.$port : '').$path);
...省略
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
...省略

由于协议是为null的，所以最后请求的url地址就是http://://google.com/aaa，也就是http://127.0.0.1:80/google.com/aaa

所以此处需要一个url跳转才能进行下一步的ssrf攻击，当然这也能够攻击本地，不过很鸡肋.

PHP版本问题

仔细研究parse_url处理无协议的url时候，//www.baidu.com在不同的php版本还有一些小差别
相对url是在php5.4才有进行处理

大致总结为
php5.4，能解析ok: //www.baidu.com/../aaa
php5.4后，需要加上端口号: //www.baidu.com:80/../aaa

本地url跳转构造

对于跳转的要求比较高，因为需要的是get型，、不登录、任意地址跳转
找了很久发现了一个很符合要求，logout的时候会获取referer，然后进入301跳转

其中跳转的地址referer会有验证，验证其是否和本身host匹配，也就是限制了你不能进行任意地址跳转

/source/function/function_core.php:1498

function dreferer($default = '') {
	global $_G;

	$default = empty($default) && $_ENV['curapp'] ? $_ENV['curapp'].'.php' : '';
	$_G['referer'] = !empty($_GET['referer']) ? $_GET['referer'] : $_SERVER['HTTP_REFERER'];
	$_G['referer'] = substr($_G['referer'], -1) == '?' ? substr($_G['referer'], 0, -1) : $_G['referer'];

	if(strpos($_G['referer'], 'member.php?mod=logging')) {
		$_G['referer'] = $default;
	}

	$reurl = parse_url($_G['referer']);

	if(!$reurl || (isset($reurl['scheme']) && !in_array(strtolower($reurl['scheme']), array('http', 'https')))) {
		$_G['referer'] = '';
	}

	if(!empty($reurl['host']) && !in_array($reurl['host'], array($_SERVER['HTTP_HOST'], 'www.'.$_SERVER['HTTP_HOST'])) && !in_array($_SERVER['HTTP_HOST'], array($reurl['host'], 'www.'.$reurl['host']))) {
		if(!in_array($reurl['host'], $_G['setting']['domain']['app']) && !isset($_G['setting']['domain']['list'][$reurl['host']])) {
			$domainroot = substr($reurl['host'], strpos($reurl['host'], '.')+1);
			if(empty($_G['setting']['domain']['root']) || (is_array($_G['setting']['domain']['root']) && !in_array($domainroot, $_G['setting']['domain']['root']))) {
				$_G['referer'] = $_G['setting']['domain']['defaultindex'] ? $_G['setting']['domain']['defaultindex'] : 'index.php';
			}
		}
	} elseif(empty($reurl['host'])) {
		$_G['referer'] = $_G['siteurl'].'./'.$_G['referer'];
	}

	$_G['referer'] = durlencode($_G['referer']);
	return $_G['referer'];
}

因为跳转地址是否合法性的验证是通过parse_url解析出host，与$_SERVER['HTTP_HOST']进行判断。后面跳转后的地址是进入了curl中进行请求。所以这里牵涉到一个东西就是parse_url与curl的差异性。

当地址为下面链接时，parse_url解析出来为localhost，但是进入curl后便是www.baidu.com

http://localhost#@www.baidu.com/

所以最终跳转的链接如下

htp://localhost/code-src/dz/Discuz_TC_BIG5/upload/member.php?mod=logging&action=logout&XDEBUG_SESSION_START=13904&referer=http://localhost%23%40www.baidu.com&quickforward=1