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By Orange Tsai

English Version
中文版本

Accellion File Transfer Appliance (FTA) is a secure file transfer service which enables users to share and sync files online with AES 128/256 encryption. The Enterprise version further incorporates SSL VPN services with integration of Single Sign-on mechanisms like AD, LDAP and Kerberos.

In this research, the following vulnerabilities were discovered on the FTA version FTA_9_12_0 (13-Oct-2015 Release)

• Cross-Site Scripting x 3
• Pre-Auth SQL Injection leads to Remote Code Execution
• Known-Secret-Key leads to Remote Code Execution
• Local Privilege Escalation x 2

The above-mentioned vulnerabilities allow unauthenticated attackers to remotely attack FTA servers and gain highest privileges successfully. After the attackers fully controlled the servers, they will be able to retrieve the encrypted files and user data, etc.

After reporting to CERT/CC, these vulnerabilities were assigned 4 CVEs (CVE-2016-2350, CVE-2016-2351, CVE-2016-2352, CVE-2016-2353).

According to a public data reconnaissance, there are currently 1,217 FTA servers online around the world, most of which are located in the US, followed by Canada, Australia, UK, and Singapore.
Determine from the domain name and SSL Certificate of these servers, FTA is widely used by governmental bodies, educational institutions, enterprises, including several well-known brands.

https://<fta>/courier/move_partition_frame.html
?f2=’-prompt(document.domain);//

https://<fta>/courier/web/getimageajax.php
?documentname=”onerror=”prompt(document.domain)//

https://<fta>/courier/web/wmInfo.html
?msg=ssologout
&loginurl=”><svg/onload=”prompt(document.domain)

After code reviewing, a pre-authentication SQL Injection vulnerability was found in FTA. This vulnerability grants malicious users access to sensitive data and personal information on the server through SQL Injection, and launch remote code execution (RCE) by further exploiting privilege-escalating vulnerabilities.
The key to this problem lies in the client_properties( ... ) function called by security_key2.api!

Among these parameters, $g_app_id $g_username $client_id and $password are controllable by the attackers. And although the function _decrypt( ... ) handles the passwords, it does not involve in the triggering of the vulnerability.
One thing to pay special attention is that the value of $g_app_id will be treated as a global variable which represents the current Application ID in use, and will be applied in opendb( ) accordingly. The code in opendb( ) includes the following lines:

In mysql_select_db, the name of the database to be opened is controllable by the user. If wrong value was given, the program will be interrupted. Therefore, $g_app_id must be forged correctly.

The following lines are the most important function client_properties( $client_id ).

The parameters passed onto the function client_properties( ... ) will be assembled into SQL statements. Among all the functions joining the assembling, construct_where_clause( ... ) is the most crucial one.
In the function construct_where_clause( ... ), every parameter is protected by the string mysql_real_escape_string except for $client_id. Judging from the coding style of the source code, it might be a result of oversight. Therefore, SQL Injection can be triggered by sending out corresponding parameters according to the program flow.

In addition, FTA database user has root privileges with FILE_PRIV option enabled. By exploiting INTO OUTFILE and writing their own PHP code to write-enabled directory, user will be able to execute code remotely!

PoC

The created PHP file will be located at

http://<fta>/courier/themes/templates/.cc.php

In the previous vulnerability, one requirement to execute code remotely is the existence of a write-enabled directory for injecting webshell. But in reality, chances are there is no write-enabled directory available, thus fail to execute code through SQL Injection. But there is another way to help us accomplish RCE.

The precondition of this vulnerability is Known-Secret-Key stored in the database

This is not a problem, since the database can be accessed with the SQL Injection vulnerability mentioned earlier. Also, although there are some parameter filters in the code, they can be bypassed!

If Known-Secret-Key has been acquired, the output of decrypt( $_POST[fc] ) will be controllable. And despite that the succeeding regular expressions work as a function name whitelist filter, they do not filter parameters.
Therefore, the only restriction for injecting random codes in the parameters is to exclude ( ) in the strings. But thanks to the flexible characteristic of PHP, there are lots of ways to manipulate, just to name two examples here.

Execute system commands directly by using backticks (`)

user_profile_auth(`$_POST[cmd]`);

A more elegant way: use the syntax INCLUDE to include the tmp_name of the uploaded files, so that any protection will give way.

user_profile_auth(include $_FILES[file][tmp_name]);

After gaining PHP page privileges, we discovered that the privileges were assigned to user nobody. In order to engage in advanced recon, the web environment had been observed. After the observation, two possible privilege escalation vulnerabilities were identified.

The module name soggycat is readable and writable to anyone for the directory /home/soggycat/, therefore the SSH Key can be written into /home/soggycat/.ssh/ and then use the soggycat credential to login.

To enable system updates through web UI, the sudoers configuration in FTA exceptionally allows the user nobody to directly execute commands with root privileges and update software with the program yum-client.pl.

YUM_CLIENT is the command for proceeding updates. Part of the codes are as follows:

After taking a closer look on ymm-client.pl, a Command Injection vulnerability was found on the parameter --cdrom. This vulnerability enables attackers to inject any commands into the parameter and execute as root.

Thus, using the commands below

will grant execution freely as root!

After gaining the highest privilege and carrying out server recon, we identified that several backdoors had been already planted in FTA hosts. One of them is an IRC Botnet which had been mentioned in Niara’s Accellion File Transfer Appliance Vulnerability.
Apart from that, two additional PHP Webshells of different types which had NEVER been noted in public reports were also identified. Through reviewing Apache Log, these backdoors might be placed by exploiting the CVE-2015-2857 vulnerability discovered in mid-2015.

One of the backdoors is PHPSPY, it is found on 62 of the online hosts globally. It was placed in

https://<fta>/courier/themes/templates/Redirector_Cache.php

The other is WSO, found on 9 of the online hosts globally, placed in

https://<fta>/courier/themes/templates/imag.php

The vulnerability mentioned in this Advisory was identified in early 2016 while looking for vulnerabilities in Facebook, you can refer to the article “How I Hacked Facebook, and Found Someone’s Backdoor Script”.
Upon discovering the FTA vulnerability in early February, I notified Facebook and Accellion and both were very responsive. Accellion responded immediately, issuing patch FTA_9_12_40 on February 12th and notifying all affected customers about the vulnerability and instructions to install the patch. Accellion has been very communicative and cooperative throughout this process.

• Feb 6, 2016 05:21 Contact Accellion for vulnerability report
• Feb 7, 2016 12:35 Send the report to Accellion Support Team
• Mar 3, 2016 03:03 Accellion Support Team notifies patch will be made in FTA_9_12_40
• May 10, 2016 15:18 Request Advisory submission approval and report the new discovery of two backdoors to Accellion
• Jun 6, 2016 10:20 Advisory finalized by mutual consent

• R7-2015-08: Accellion File Transfer Appliance Vulnerabilities (CVE-2015-2856, CVE-2015-2857)
• Rapid7 Advisory R7-0039: Accellion File Transfer Appliance Multiple Vulnerabilities
• Threat Advisory: Accellion File Transfer Appliance Vulnerability

By Orange Tsai

English Version
中文版本

Accellion File Transfer Appliance (以下簡稱 FTA) 為一款安全檔案傳輸服務，可讓使用者線上分享、同步檔案，且所有檔案皆經 AES 128/256 加密，Enterprise 版本更支援 SSL VPN 服務並整合 AD, LDAP, Kerberos 等 Single Sign-on 機制。

在研究過程中，於 FTA 版本 FTA_9_12_0 (13-Oct-2015 Release) 上，發現了下列弱點：

• Cross-Site Scripting x 3
• Pre-Auth SQL Injection leads to Remote Code Execution
• Known-Secret-Key leads to Remote Code Execution
• Local Privilege Escalation x 2

以上弱點可使不需經過認證的攻擊者，成功遠端攻擊 FTA 伺服器並取得最高權限，當攻擊者完全控制伺服器後，可取得伺服器上的加密檔案與用戶資料等。

弱點經回報 CERT/CC 後取得共四個獨立 CVE 編號 (CVE-2016-2350, CVE-2016-2351, CVE-2016-2352, CVE-2016-2353)。

根據公開資料掃描，全球共發現 1217 台 FTA 存活主機，主要分布地點為美國，其次加拿大、澳洲、英國與新加坡。根據存活主機的域名、SSL Certificate 發現 FTA 使用客戶遍及政府、教育、企業等領域，其中不乏一些知名品牌。

https://<fta>/courier/move_partition_frame.html
?f2=’-prompt(document.domain);//

https://<fta>/courier/web/getimageajax.php
?documentname=”onerror=”prompt(document.domain)//

https://<fta>/courier/web/wmInfo.html
?msg=ssologout
&loginurl=”><svg/onload=”prompt(document.domain)

經過代碼審查後，在 FTA 中發現一個不須驗證的 SQL Injection，這使得惡意使用者可透過 SQL Injection 存取伺服器的敏感檔案及個人資料，並配合權限設定問題導致遠端代碼執行。問題出在 security_key2.api 中所呼叫到的 client_properties( ... ) 函數中！

其中 $g_app_id $g_username $client_id $password 皆為攻擊者可控參數，雖然有個 _decrypt( ... ) 函數對密碼進行處理，但是與弱點觸發並無相關。其中要注意是 $g_app_id 的值會被代入成全域變數，代表當前使用的 Application ID，並且在 opendb( ) 使用，其中在 opendb( ) 內有以下代碼：

mysql_select_db 中所開啟資料庫的名稱由使用者可控，如給錯誤的值將導致程式無法繼續執行下去，所以必須將 $g_app_id 偽造成正確的內容。

接著是最主要的函數 client_properties( $client_id )

client_properties( ... ) 中會將所傳進的參數進行 SQL 語句的拼裝，而 construct_where_clause( ... ) 為最關鍵的一個函數。 在 construct_where_clause( ... ) 中可以看到參數皆使用 mysql_real_escape_string 來防禦但唯獨缺少 $client_id，從原始碼的 Coding Style 觀察猜測應該是開發時的疏忽，因此根據程式流程送出對應的參數即可觸發 SQL Injection。

此外，在 FTA 中資料庫使用者為 root 具有 FILE_PRIV 權限，因此可使用 INTO OUTFILE 撰寫自己 PHP 代碼至可寫目錄達成遠端代碼執行！

PoC

生成的 PHP 檔案位置在

http://<fta>/courier/themes/templates/.cc.php

在前個弱點中，要達成遠端代碼執行還有一個條件是要存在可寫目錄，但現實中有機率找不到可寫的目錄放置 Webshell，因此無法從 SQL Injection 達成代碼執行，不過這時有另外一條路可以幫助我們達成遠端代碼執行。

這個弱點的前提條件是 已知資料庫中所存的加密 KEY

這點對我們來說不是問題，從前面的 SQL Injection 弱點可任意讀取資料庫內容，另外雖然在程式碼中有對參數進行一些過濾，但那些過濾是可以繞過的！

如果已知加密 KEY 的話，即可控制 decrypt( $_POST[fc] ) 的輸出，而後面的正規表示式雖然針對函數名稱進行白名單過濾，但是沒對參數進行過濾，如此一來我們可以在參數的部分插入任意代碼，唯一的條件就是不能有 ( ) 出現，但由於 PHP 的鬆散特性，玩法其實很多，這裡列舉兩個：

直接透過反引號執行系統指令：

user_profile_auth(`$_POST[cmd]`);

更優雅的方式可以透過 include 語法引入上傳檔案的 tmp_name，這樣各種保護都不用擔心：

user_profile_auth(include $_FILES[file][tmp_name]);

在取得 PHP 網頁權限後，發現所屬權限為 nobody，為了進行更深入的研究，在對環境進行審視後，發現兩個可用來提升權限之弱點。

其中模組名稱 soggycat 對 /home/soggycat/ 為任何人可讀可寫，所以可將 SSH Key 寫至 /home/soggycat/.ssh/ 後以 soggycat 身分登入

在 FTA 中，為了使系統可以直接透過網頁介面進行更新，因此在 sudoers 配置中特別針對 nobody 用戶允許直接使用 root 權限執行指令，並透過 yum-client.pl 這隻程式進行軟體更新

其中 YUM_CLIENT 就是進行更新的指令，部分代碼如下：

深入觀察 yum-client.pl 後可發現在 --cdrom 參數上存在 Command Injection，使得攻擊者可將任意指令插入參數內並以 root 身分執行

所以使用如下指令：

即可以 root 身分執行任意指令！

在取得最高權限後，開始對伺服器進行一些審視時，發現已有幾款後門藏在 FTA 主機中了，經過研究後首先確認一款 IRC BOT 為 Niara 所發布的 弱點報告 中有提及，此外，額外發現兩款不同類型的 PHP Webshell 並無在公開的報告中發現，透過 Apache Log 時間推測應該是透過 2015 年中的 CVE-2015-2857 所放置之後門。

PHPSPY 後門，全球 1217 台存活主機上共發現 62 台，放置路徑於：

https://<fta>/courier/themes/templates/Redirector_Cache.php

WSO 後門，全球 1217 台存活主機上共發現 9 台，放置路徑於：

https://<fta>/courier/themes/templates/imag.php

這份 Advisory 所提及的弱點為在 2016 二月時參加 Facebook Bug Bounty 時尋找到的，詳情可參考文章《滲透 Facebook 的思路與發現》，找到弱點的當下立即回報包括 Accellion 及 Facebook，Accellion 並在 2/12 號將此份弱點記錄在 FTA_9_12_40 並通知所有受影響的客戶安裝修補程式。

感謝 Facebook 以及 Accellion 的迅速反應跟配合 : )

• 2016/02/06 05:21 聯絡 Accellion 詢問何處可回報弱點
• 2016/02/07 12:35 將報告寄至 Accellion Support Team
• 2016/03/03 03:03 Accellion Support Team 通知會在 FTA_9_12_40 修復
• 2016/05/10 15:18 詢問將撰寫 Advisory 許可及通知發現兩款後門存在
• 2016/06/06 10:20 雙方討論定稿

• R7-2015-08: Accellion File Transfer Appliance Vulnerabilities (CVE-2015-2856, CVE-2015-2857)
• Rapid7 Advisory R7-0039: Accellion File Transfer Appliance Multiple Vulnerabilities
• Threat Advisory: Accellion File Transfer Appliance Vulnerability

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