Posté par: igor51 le février 09, 2018, 16:00:18
UPDATE (Dec. 8, 2017): We now attribute this campaign to APT34, a
suspected Iranian cyber espionage threat group that we believe has
been active since at least 2014. Learn more about
their late 2017 targeting of a government organization in the Middle East.
Introduction
In the first week of May 2016, 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:
- Extracts base64-encoded content from the cells within a
worksheet titled "Incompatible". - Checks for the
presence of a file at the path
not present, the macro creates three different directories under
%PUBLIC%\Libraries, namely - The extracted content from step one is decoded using PowerShell
and dropped into two different files:
%PUBLIC%\Libraries\dns.ps1 - The macro then creates a
scheduled task with name:
Note: Due to the use of a hardcoded environment variable
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
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:
- Leverages PowerShell to download content from the URI
and saves it in the directory %PUBLIC%\Libraries\dn. - Uses PowerShell to download a BAT file from the URI
and saves it in the directory %PUBLIC%\Libraries\dn. - Executes the BAT file and stores the results in a file in the
path %PUBLIC%\Libraries\up. - Uploads this file to the server by sending an HTTP POST request
to the URI - Finally, it executes the PowerShell script
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.
- The script requests an ID (through the DNS protocol) from
go0gIe[.]com. This ID will then be saved
into the PowerShell script. - 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. - 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
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. - Once the file has been successfully transferred, the BAT file
will be run and its output saved as - 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. - 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 | valign="top"> |
Subdomain used while performing | valign="top"> |
Subdomain used while performing | valign="top"> |
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 (http://)