[FireEye]Targeted Attacks against Banks in the Middle East

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Targeted Attacks against Banks in the Middle East

  Introduction

 

In the first week of May 2016,     href="/content/fireeye-www/en_US/products/dynamic-threat-intelligence/dti-ati-atiplus-datasheet.html">FireEye’s
  DTI identified a wave of emails containing malicious attachments
  being sent to multiple banks in the Middle East region. The threat
  actors appear to be performing initial reconnaissance against would-be
  targets, and the attacks caught our attention since they were using
  unique scripts not commonly seen in crimeware campaigns.

 

In this blog we discuss in detail the tools, tactics, techniques and
  procedures (TTPs) used in these targeted attacks.

 

  Delivery Method

 

The attackers sent multiple emails containing macro-enabled XLS
  files to employees working in the banking sector in the Middle East.
  The themes of the messages used in the attacks are related to IT
  Infrastructure such as a log of Server Status Report or a list of
  Cisco Iron Port Appliance details. In one case, the content of the
  email appeared to be a legitimate email conversation between several
  employees, even containing contact details of employees from several
  banks. This email was then forwarded to several people, with the
  malicious Excel file attached.

 

  Macro Details

 

The macro first calls an Init() function
  (shown in Figure 1) that performs the following malicious activities:

 

1. Extracts base64-encoded content from the cells within a
       worksheet titled "Incompatible".
2. Checks for the
       presence of a file at the path       class="code">%PUBLIC%\Libraries\ update.vbs. If the file is
       not present, the macro creates three different directories under
         %PUBLIC%\Libraries, namely       class="code">up, dn, and tp.

   
3. The extracted content from step one is decoded using PowerShell
       and dropped into two different files:       class="code">%PUBLIC%\Libraries\update.vbs and
     %PUBLIC%\Libraries\dns.ps1
4. The macro then creates a
       scheduled task with name:     class="code">GoogleUpdateTaskMachineUI, which executes     class="code">update.vbs every three minutes.

 

Note: Due to the use of a hardcoded environment variable   class="code">%PUBLIC% in the macro code, the macro will only
  run successfully on Windows Vista and subsequent versions of the
  operating system.

 

 

 

  Figure 1: Macro Init() subroutine

 

  Run-time Unhiding of Content

 

One of the interesting techniques we observed in this attack was the
  display of additional content after the macro executed successfully.
  This was done for the purpose of social engineering – specifically, to
  convince the victim that enabling the macro did in fact result in the
  “unhiding” of additional spreadsheet data.

 

Office documents containing malicious macros are commonly used in
  crimeware campaigns. Because default Office settings typically require
  user action in order for macros to run, attackers may convince victims
  to enable risky macro code by telling them that the macro is required
  to view “protected content.”

 

In crimeware campaigns, we usually observe that no additional
  content is displayed after enabling the macros. However, in this case,
  attackers took the extra step to actually hide and unhide worksheets
  when the macro is enabled to allay any suspicion. A screenshot of the
  worksheet before and after running the macro is shown in Figure 2 and
  Figure 3, respectively.

 

 

 

  Figure 2: Before unhiding of content

 

 

 

  Figure 3: After unhiding of content

 

In the following code section, we can see that the subroutine   class="code">ShowHideSheets() is called after the   class="code">Init() subroutine executes completely:

 

  Private Sub Workbook_Open()
      Call Init
 
         Call ShowHideSheets
 
 End Sub

 

The code of subroutine ShowHideSheets(),
  which unhides the content after completion of malicious activities, is
  shown in Figure 4.

 

 

 

  Figure 4: Macro used to unhide content at runtime

 

  First Stage Download

 

After the macro successfully creates the scheduled task, the dropped
  VBScript, update.vbs (Figure 5), will be
  launched every three minutes. This VBScript performs the following operations:

 

1. Leverages PowerShell to download content from the URI       class="code">hxxp://go0gIe[.]com/sysupdate.aspx?req=xxx\dwn&m=d
         and saves it in the directory %PUBLIC%\Libraries\dn.

   
2. Uses PowerShell to download a BAT file from the URI       class="code">hxxp://go0gIe[.]com/sysupdate.aspx?req=xxx\bat&m=d
         and saves it in the directory %PUBLIC%\Libraries\dn.

   
3. Executes the BAT file and stores the results in a file in the
       path %PUBLIC%\Libraries\up.

   
4. Uploads this file to the server by sending an HTTP POST request
       to the URI   class="code">hxxp://go0gIe[.]com/sysupdate.aspx?req=xxx\upl&m=u.

   
5. Finally, it executes the PowerShell script     class="code">dns.ps1, which is used for the purpose of data
       exfiltration using DNS.

 

 

 

  Figure 5: Content of update.vbs

 

During our analysis, the VBScript downloaded a customized version of
  Mimikatz in the previously mentioned step one. The customized version
  uses its own default prompt string as well as its own console title,
  as shown in Figure 6.

 

 

 

  Figure 6: Custom version of Mimikatz used to
    extract user password hashes

 

Similarly, the contents of the BAT file downloaded in step two are
  shown in Figure 7:

 

  whoami & hostname & ipconfig /all & net
    user /domain 2>&1 & net group /domain 2>&1 &
    net group "domain admins" /domain 2>&1 & net
    group "Exchange Trusted Subsystem" /domain 2>&1
    & net accounts /domain 2>&1 & net user 2>&1
    & net localgroup administrators 2>&1 & netstat -an
    2>&1 & tasklist 2>&1 & sc query 2>&1
    & systeminfo 2>&1 & reg query
    "HKEY_CURRENT_USER\Software\Microsoft\Terminal Server
    Client\Default" 2>&1

 

  Figure 7: Content of downloaded BAT script

 

This BAT file is used to collect important information from the
  system, including the currently logged on user, the hostname, network
  configuration data, user and group accounts, local and domain
  administrator accounts, running processes, and other data.

 

  Data Exfiltration over DNS

 

Another interesting technique leveraged by this malware was the use
  of DNS queries as a data exfiltration channel. This was likely done
  because DNS is required for normal network operations. The DNS
  protocol is unlikely to be blocked (allowing free communications out
  of the network) and its use is unlikely to raise suspicion among
  network defenders.

 

The script dns.ps1, dropped by the macro,
  is used for this purpose. In the following section, we describe its
  functionality in detail.

 

1. The script requests an ID (through the DNS protocol) from
         go0gIe[.]com. This ID will then be saved
       into the PowerShell script.
2. Next, the script queries the C2
       server for additional instructions. If no further actions are
       requested, the script exits and will be activated again the next
       time update.vbs is called.
3. If an
       action is required, the DNS server replies with an IP with the
       pattern 33.33.xx.yy. The script then
       proceeds to create a file at     class="code">%PUBLIC%\Libraries\tp\chr(xx)chr(yy).bat. The
       script then proceeds to make DNS requests to fetch more data. Each
       DNS request results in the C2 server returning an IP address. Each
       octet of the IP address is interpreted as the decimal representation
       of an ASCII character; for example, the decimal number 99 is
       equivalent to the ASCII character ‘c’. The characters represented by
       the octets of the IP address are appended to the batch file to
       construct a script. The C2 server signals the end of the data stream
       by replying to a DNS query with the IP address 35.35.35.35.

   
4. Once the file has been successfully transferred, the BAT file
       will be run and its output saved as   class="code">%PUBLIC%\Libraries\tp\chr(xx)chr(yy).txt.

   
5. The text file containing the results of the BAT script will then
       be uploaded to the DNS server by embedding file data into part of
       the subdomain. The format of the DNS query used is shown in Table
     1.
6. The BAT file and the text file will then be deleted. The
       script then quits, to be invoked again upon running the next
       scheduled task.

 

The DNS communication portion of the script is shown in Figure 8,
  along with a table showing the various subdomain formats being
  generated by the script.

 

 

 

  Figure 8: Code Snippet of dns.ps1

 

Format of subdomains used in DNS C2 protocol:

 
   
     
   
     
   
     

Subdomain used to request for           BotID, used in step 2 above	valign="top"> [00][botid]00000[base36 random number]30
Subdomain used while performing           file transfers used in step 3 above	valign="top"> [00][botid]00000[base36 random number]232A[hex_filename][i-counter]
Subdomain used while performing           file upload, used in step 5 above	valign="top"> [00][botid][cmdid][partid][base36 random number][48-hex-char-of-file-content]

 

  Table 1: C2 Protocol Format

 

Conclusion

 

Although this attack did not leverage any zero-days or other
  advanced techniques, it was interesting to see how attackers used
  different components to perform reconnaissance activities on a
  specific target.

 

This attack also demonstrates that macro malware is effective even
  today. Users can protect themselves from such attacks by disabling
  Office macros in their settings and also by being more vigilant when
  enabling macros (especially when prompted) in documents, even if such
  documents are from seemingly trusted sources.

Source: Targeted Attacks against Banks in the Middle East