0x01 前言
注：本文仅以安全研究为目的，分享对该漏洞的挖掘过程，文中涉及的所有漏洞均已报送给国家单位，请勿用做非法用途。
通达OA作为历史上出现漏洞较多的OA，在经过多轮的迭代之后已经很少前台的RCE漏洞了。一般来说通达OA是通过auth.inc.php文件来进行鉴权，如图1.1所示。整个通达全部的代码看下来很少有未鉴权的代码。
在通达中有一个模块
/general/appbuilder/web/index.php采用了yii框架实现，其鉴权逻辑与其他模块存在显著差异。
$url = $_SERVER["REQUEST_URI"];$strurl = substr($url, 0, strpos($url, "?"));if (strpos($strurl, "/portal/") !== false) {if (strpos($strurl, "/gateway/") === false) {header("Location:/index.php");sess_close();exit();}else if (strpos($strurl, "/gateway/saveportal") !== false) {header("Location:/index.php");sess_close();exit();}else if (strpos($url, "edit") !== false) {header("Location:/index.php");sess_close();exit();}}else if (strpos($url, "/appdata/doprint") !== false) {$_GET["csrf"] = urldecode($_GET["csrf"]);$b_check_csrf = false;if (!empty($_GET["csrf"]) && preg_match("/^\{([0-9A-Z]|-){36}\}$/", $_GET["csrf"])) {$s_tmp = __DIR__ . "/../../../../logs/appbuilder/logs";$s_tmp .= "/" . $_GET["csrf"];if (file_exists($s_tmp)) {$b_check_csrf = true;$b_dir_priv = true;}}if (!$b_check_csrf) {header("Location:/index.php");sess_close();exit();}}else {header("Location:/index.php");sess_close();exit();}
从上面的代码可以看出，如果访问的目标地址是/general/appbuilder/web/portal/gateway/?，则不需要授权就能访问对应的接口。
再来看一下通达中使用的yii版本，如图1.2所示。Yii2 < 2.0.38是存在反序列化利用链的，网上已经有很多分析文章，感兴趣的小伙伴可以关注。
https://www.anquanke.com/post/id/254429
虽然有了利用链，但如何利用一直是个难题。本文的目的是寻找反序列化利用点并构造反序列化利用链达到RCE效果。
0x02 反序列化点
在appbuilder模块中多数情况下会加载视图views/layouts/main.php。视图中会调用csrfMetaTags方法。
<?php$this->beginPage();echo "<!DOCTYPE html>\n<html lang=\"";echo Yii::$app->language;echo "\">\n<head>\n <meta charset=\"";echo Yii::$app->charset;echo "\">\n <meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\n ";echo yii\helpers\Html::csrfMetaTags();echo " <title>";echo yii\helpers\Html::encode($this->title);echo "</title>\n\t<link rel=\"stylesheet\" type=\"text/css\" href=\"";echo Yii::$app->params["MYOA_STATIC_SERVER"];echo "/static/theme/";echo Yii::$app->params["LOGIN_THEME"] == "" ? "1" : Yii::$app->params["LOGIN_THEME"];echo "/style.css\" />\n <link href=\"/static/js/bootstrap/css/bootstrap.css\" rel=\"stylesheet\">\n<!-- <link href=\"/module/appbuilder/css/bootstrap.css\" rel=\"stylesheet\">-->\n <link href=\"/module/appbuilder/css/site.css\" rel=\"stylesheet\"></head>\n\t<style>\n\ta.btn.btn-danger {\n\t\tcolor: #fff;\n\t}\n\t</style>\n ";$this->head();echo "</head>\n<body class=\"bodycolor\">\n";$this->beginBody();echo "\n<div><!--class=\"wrap\"-->\n ";
在yii框架中存在yii\helpers\Html::csrfMetaTags()方法，该方法的主要作用时用于生成csrf校验需要的meta标签。
public static function csrfMetaTags(){$request = Yii::$app->getRequest();if ($request instanceof Request && $request->enableCsrfValidation) {return static::tag('meta', '', ['name' => 'csrf-param', 'content' => $request->csrfParam]) . "\n". static::tag('meta', '', ['name' => 'csrf-token', 'content' => $request->getCsrfToken()]) . "\n";}return '';}
在方法中调用了$request->getCsrfToken()，跟踪该方法。
public function getCsrfToken($regenerate = false){if ($this->_csrfToken === null || $regenerate) {$token = $this->loadCsrfToken();if ($regenerate || empty($token)) {$token = $this->generateCsrfToken();}$this->_csrfToken = Yii::$app->security->maskToken($token);}return $this->_csrfToken;} public function getCsrfToken($regenerate = false){if ($this->_csrfToken === null || $regenerate) {$token = $this->loadCsrfToken();if ($regenerate || empty($token)) {$token = $this->generateCsrfToken();}$this->_csrfToken = Yii::$app->security->maskToken($token);}return $this->_csrfToken;}
在该方法中继续调用了loadCsrfToken方法，跟踪该方法。
protected function loadCsrfToken(){if ($this->enableCsrfCookie) {return $this->getCookies()->getValue($this->csrfParam);}return Yii::$app->getSession()->get($this->csrfParam);}
继续跟踪getCookies方法。
public function getCookies(){if ($this->_cookies === null) {$this->_cookies = new CookieCollection($this->loadCookies(), ['readOnly' => true,]);}return $this->_cookies;}
继续跟踪loadCookies方法，在这个方法中会调用unserialize方法对传入的Cookie的值进行反序列化。这也就会造成反序列化漏洞。
protected function loadCookies(){$cookies = [];if ($this->enableCookieValidation) {if ($this->cookieValidationKey == '') {throw new InvalidConfigException(get_class($this) . '::cookieValidationKey must be configured with a secret key.');}foreach ($_COOKIE as $name => $value) {if (!is_string($value)) {continue;}$data = Yii::$app->getSecurity()->validateData($value, $this->cookieValidationKey);if ($data === false) {continue;}if (defined('PHP_VERSION_ID') && PHP_VERSION_ID >= 70000) {$data = @unserialize($data, ['allowed_classes' => false]);} else {$data = @unserialize($data); //这里是反序列化点}if (is_array($data) && isset($data[0], $data[1]) && $data[0] === $name) {$cookies[$name] = Yii::createObject(['class' => 'yii\web\Cookie','name' => $name,'value' => $data[1],'expire' => null,]);}}} else {foreach ($_COOKIE as $name => $value) {$cookies[$name] = Yii::createObject(['class' => 'yii\web\Cookie','name' => $name,'value' => $value,'expire' => null,]);}}return $cookies;}
0x03 绕过限制
在上面的方法中会对传入的Cookie值进行签名校验，校验的方法是validateData，其中$this->cookieValidationKey是签名的key。
Yii::$app->getSecurity()->validateData($value, $this->cookieValidationKey)在validateData方法中会通过hash_hmac对传入的key和value进行签名校验。
public function validateData($data, $key, $rawHash = false){$test = @hash_hmac($this->macHash, '', '', $rawHash);if (!$test) {throw new InvalidConfigException('Failed to generate HMAC with hash algorithm: ' . $this->macHash);}$hashLength = StringHelper::byteLength($test);if (StringHelper::byteLength($data) >= $hashLength) {$hash = StringHelper::byteSubstr($data, 0, $hashLength);$pureData = StringHelper::byteSubstr($data, $hashLength, null);$calculatedHash = hash_hmac($this->macHash, $pureData, $key, $rawHash);if ($this->compareString($hash, $calculatedHash)) {return $pureData;}}return false;}
由于通达OA中的$this->cookieValidationKey来自于配置文件general/appbuilder/config/web.php。其中值是固定的。
<?php$params = require __DIR__ . "/params.php";$config = array("id" => "appbuilder","basePath" => dirname(__DIR__),"bootstrap" => array("log"),"charset" => "GB2312","language" => "zh-CN","runtimePath" => "@app/../../../logs/appbuilder","components" => array("request" => array("cookieValidationKey" => "tdide2"), //这是固定的密钥tdide2"cache" => array("class" => "app\\td\base\TDRedisCache"),"redis" => array("class" => "yii\\redis\Connection", "hostname" => $MYOA_REDIS_SERVERS[0]["host"], "port" => $MYOA_REDIS_SERVERS[0]["port"], "database" => $MYOA_REDIS_DB_ID + 1, "password" => $MYOA_REDIS_PASS),"errorHandler" => array("errorAction" => "site/error"),"log" => array("traceLevel" => 1,"targets" => array(array("class" => "yii\log\FileTarget","levels" => array("error")))
另外通达OA有全局的addslashes过滤，包括Cookie中的值。由于PHP反序列化中有大量的双引号，如果直接通过Cookie传递则会因为双引号被转义而失败。但是令人开心的是通达在进行全局addslashes的时候对部分值进行了例外排查。
if (0 < count($_COOKIE)) {foreach ($_COOKIE as $s_key => $s_value ) {if ((substr($s_key, 0, 7) == "_SERVER") || (substr($s_key, 0, 8) == "_SESSION") || (substr($s_key, 0, 7) == "_COOKIE") || (substr($s_key, 0, 4) == "_GET") || (substr($s_key, 0, 5) == "_POST") || (substr($s_key, 0, 6) == "_FILES")) {continue;}if (!is_array($s_value)) {$_COOKIE[$s_key] = addslashes(strip_tags($s_value));}$s_key = $_COOKIE[$s_key];}reset($_COOKIE);}
如果Cookie中字段名称的前面几位字符为_GET这种，则不进行addslashes操作。这也就给漏洞利用提供了便利。
0x04结论
反序列化利用链是直接采用yii中已经公开的反序列化链，本文作者调好的poc（实际是exp）如下所示，完整的poc可从ddpoc平台查看。
https://www.ddpoc.com/poc/DVB-2023-4705.html
使用该poc之后可以会在网站根目录生成哥斯拉的webshell。
在根目录生成文件/logon.php 111/111 哥斯拉本漏洞已向相关单位报送并已推出补丁，可适用于通达11.X全版本。对应12系列未做过多测试，本文提到的所有漏洞在最新版的通达12.4中已修复，使用此漏洞造成的任何攻击影响均与本文作者无关
