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                                      Nitol Botnet makes a resurgence with evasive sandbox analysis technique

                                      Oct 14 2016
                                      Tags
                                      Cloud Best Practices
                                      Cloud Malware
                                      Cloud Security
                                      Evasive Malware
                                      Netskope Threat Research Labs
                                      Nitol
                                      Office Macro
                                      Tools and Tips

                                      Introduction

                                      Netskope Threat Research Labs recently observed a strain of macro-based malware that use fairly smart techniques to bypass malware sandbox analysis. The macro code is obfuscated and uses a multi-stage attack methodology to compromise the endpoint machines. Netskope Active Threat Protection detects and mitigates this macro-based malware as W97m.Downloader.

                                      Bypass the Malware Sandbox Analysis  

                                      Although bypassing sandbox analysis is not new for malware, this strain of malware uses a novel technique to bypass analysis. Specifically, the malicious macro-based documents we observed use two methods to bypass sandbox analysis.

                                      • Password Protection: The documents that we have observed are password protected, thus bypassing the sandbox entirely. The process to enter the password is a complex user interaction event, so it is difficult for automated analysis technologies (like a sandbox) to emulate this event. Figure 1 shows a password prompt while opening one such malicious macro based document.

                                       

                                      figure1

                                      Figure 1.  Password prompt while opening a protected malicious document

                                      • Delayed Execution: Typical malware use instructions such as sleep or other methods like stalling “for loops” or date and time checks to delay the execution in a sandbox, effectively bypassing the analysis. In case of these macro-based malware documents, we have observed that they use the “ping” utility to delay the execution. The malware invokes the command “ping 8.8.8.8 -n 250” and waits for the ping process to complete the execution. This typically takes a long time to complete (sometimes as long as approximately 5 minutes) and in most cases is enough to bypass the sandbox analysis since they sandboxes are typically configured with a smaller time threshold for executing samples. The ping command has long been used, mostly to ensure the connectivity to the Internet. In this case, however, the use of the ping command to delay the execution of a sample is novel. Figure 2 shows the snapshot from the process explorer indicating the ping command being invoked by the execution of a malicious document.
                                      figure2

                                      Figure 2: Sample using ping command for delayed execution

                                      Malicious Document Execution Analysis

                                      Analysis of vbscript

                                      The macro code in the malicious document drops and executes a vbscript (vbs) file as shown in Figure 2. The dropped vbscript file is responsible for downloading and executing the second stage payload. The vbscript file was obfuscated and for the purpose of demonstration in this blog  we will use debug feature of the vbscript editor to deobfuscate the file content.

                                      figure3

                                      Figure 3:  Vbscript deobfuscation using vbscript editor

                                      As shown in Figure 3, the vbscript file launches the “ping” utility to delay the execution and after that connects to “http://doktrine.fr/mg.txt” domain to download the second stage payload. The vbscript then saves the downloaded payload to the disk with a “.qsb” extension.

                                      The payload in “.qsb” file is xor encoded. The vbscript will decode the “.qsb” file and write the content to another file with “.fyn” extension and execute the file. Figure 4 and Figure 5 show the “.qsb” decoding routine and the execution of the “.fyn” file.

                                      figure4

                                      Figure 4: “.qsb” file decoding routine

                                      figure5

                                      Figure 5: Executing “.fyn” file

                                      Analysis of .fyn (PE) file

                                      The file with the “.fyn” is a Windows-executable file. As shown in the Figure 6 the execution started in the code section and then jumped to the marked region. The density of the API calls is higher in the region indicating the execution of the unpacked code.  

                                      figure6

                                      Figure 6: Distribution of API calls in the process address space

                                      As shown in Figure 7, there is a region of the code which is checking if the execution environment is VMware using the process enumeration.

                                      figure7

                                      Figure 7: Code checking for VMware execution environment

                                      The code also checks for active debugging using GetTickCount. After these checks the code will search for default browser in http//shell/open//command registry. After that, it will create a browser process in suspended mode and then it will unmap and write the browser process memory with a upx compressed file as shown in Figure 8 and Figure 9 respectively.

                                      figure8

                                      Figure 8: Create Process in suspended mode
                                      figure9Figure 9: Write the browser process memory with upx compressed file.

                                      Analysis of UPX compressed file (Nitol Botnet)

                                      The UPX compressed file is a nitol botnet binary. Nitol is a very old botnet and its C&C server domains are currently sinkholed. During our analysis, the binary tried to connect to d.googlex.me which is currently not active. It is interesting to note that the same domain was referenced as a C&C server in a blog published by McAfee in February 2016 on the Hydracrypt ransoware.

                                      It is currently not clear if the attackers are using Nitol binaries as a placeholder for the future threats or if they are testing a new attack methodology.

                                      Netskope Detection & Remediation

                                      Netskope Active Threat Protection detects these malicious macro based documents as W97m.Downloader. Customers who have deployed the Netskope Active Platform and Netskope Introspection can set the respective malware & threat detection policies to detect and remediate against this malware.

                                      figure10

                                      IOC

                                      1. 5866c53bd16a15d88f51415fde254b8edac9bc22495ad3ac2f12f5e5ef025923
                                      2. 4d977327390a13a2660da4f65872810245b57b34d990c22c547410fe3b7f3511
                                      3. e88f5c562bb894e452c88ac1c8f4fa2aea9e14275ca5a2e25655cb95491cc37f
                                      4. 2e42ca6c471ef2894ea407d482b0b6419afbd2e550a8688932064caabd48dfb6
                                      5. d76cf03299107defbb6270bbe0118aa2ceaa1197d7a0499bdb869ed02401b756
                                      6. e65b5b57f3dd913e24bb65bfb7f0a9f60fb53f2b12460b537d6b21a6d5a14eb8
                                      1. b14f8b2b8b82267be787b4b844a17554e5b6fa34ea0af197176c29dcbba60b52 (.qsb)
                                      2. 5041bf99f3010fd88ec0a37557cb2ee51aba5cb49fac5bb0aec120f2cc893128 (.fyn)

                                      ¡Mantente informado!

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