This blog was written byVaradharajan Krishnasamy, Karthickkumar, Sakshi Jaiswal

Introduction

Ransomware attacks are one of the most common cyber-attacks among organizations; due to an increase in Ransomware-as-a-service (RaaS) on the black market. RaaS provides readily available ransomware to cyber criminals and is an effective way for attackers to deploy a variety of ransomware in a short period of time.

Usually, RaaS model developers sell or rent their sophisticated ransomware framework on the black market. After purchasing the license from the ransomware developer, attackers spread the ransomware to other users, infect them, encrypt files, and demand a huge ransom payment in Bitcoin.  Also, there are discounts available on the black market for ransomware frameworks in which the ransom money paid is shared between developers and the buyer for every successful extortion of ransom from the victims. These frameworks reduce the time and effort of creating a new ransomware from scratch using latest and advanced programming languages.

REvil is one of the most famous ransomware-as-a-service (RaaS) providers. The group released the Sodinokibi ransomware in 2019, and McAfee has since observed REvil using a DLL side loading technique to execute ransomware code. The actual ransomware is a dropper that contains two embedded PE files in the resource section.  After successful execution, it drops two additional files named MsMpEng.exe and MpSvc.dll in the temp folder. The file MsMpEng.exe is a Microsoft digitally signed file having a timestamp of March 2014 (Figure 1).

Figure-1: Image of Microsoft Digitally signed File

DLL SIDE LOADING

The malware uses DLL side loading to execute the ransomware code. This technique allows the attacker to execute malicious DLLs that spoof legitimate ones. This technique has been used in many APTs to avoid detection. In this attack, MsMpEng.exe loads the functions of MpSvc.dll during the time of execution. However, the attacker has replaced the clean MpSvc.dll with the ransomware binary of the same name. The malicious DLL file has an export function named ServiceCrtMain, which is further called and executed by the Microsoft Defender file. This is a clever technique used by the attacker to execute malicious file using the Microsoft digitally signed binary.

Figure-2: Calling Export function

PAYLOAD ANALYSIS

The ransomware uses the RC4 algorithm to decrypt the config file which has all the information that supports the encryption process.

Figure-3: REvil Config File

Then it performs a UI language check using GetSystemDefaultUILanguage/GetUserDefaultUILanguage functions and compares it with a hardcoded list which contains the language ID of several countries as shown in below image.

Figure-4: Language Check

Countries excluded from this ransomware attack are mentioned below:

GetUserDefaultUILanguage Country name
0x419 Russian
0x422 Ukranian
0x423 Belarusian
0x428 Tajik (Cyrilic from Tajikistan)
0x42B Armenian
0x42C Azerbaijani (Latin from Azerbaijan)
0x437 Georgian
0x43F Kazakh from Kazakhastan
0x440 Kyrgyzstan
0x442 Turkmenistan
0x443 Latin from Uzbekistan
0x444 Tatar from Russia Federation
0x818 Romanian from Moldova
0x819 Russian from Moldova
0x82C Cyrilic from Azerbaijan
0x843 Cyrilic from Uzbekistan
0x45A Syriac
0x281A Cyrilic from Serbia

 

Additionally, the ransomware checks the users keyboardlayout and it skips the ransomware infection in the machine’s which are present in the country list above.

Figure-5: Keyboardlayout check

Ransomware creates a Global mutex in the infected machine to mark its presence.

Figure-6: Global Mutex

After creating the mutex, the ransomware deletes the files in the recycle bin using the SHEmptyRecycleBinW function to make sure that no files are restored post encryption.

Figure-7: Empty Recycle Bin

Then it enumerates all the active services with the help of the EnumServicesStatusExW function and deletes services if the service name matches the list present in the config file. The image below shows the list of services checked by the ransomware.

Figure-8: Service List check

It calls the CreateToolhelp32Snapshot, Process32FirstW and Process32NextW functions to enumerate running processes and terminates those matching the list present in the config file.  The following processes will be terminated.

  • allegro
  • steam
  • xtop
  • ocssd
  • xfssvccon
  • onenote
  • isqlplussvc
  • msaccess
  • powerpnt
  • cad
  • sqbcoreservic
  • thunderbird
  • oracle
  • infopath
  • dbeng50
  • pro_comm_msg
  • agntsvc
  • thebat
  • firefox
  • ocautoupds
  • winword
  • synctime
  • tbirdconfig
  • mspub
  • visio
  • sql
  • ocomm
  • orcad
  • mydesktopserv
  • dbsnmp
  • outlook
  • cadence
  • excel
  • wordpad
  • creoagent
  • encsvc
  • mydesktopqos

 

Then, it encrypts files using the Salsa20 algorithm and uses multithreading for fast encryption of the files. Later, background wallpaper will be set with a ransom message.

Figure-9: Desktop Wallpaper

Finally, the ransomware displays ransom notes in the victim’s machine. Below is an image of readme.txt which is dropped in the infected machine.

Figure-10: Ransom Note

IOCs and Coverage

Type Value Detection Name Detection Package Version (V3)
Loader 5a97a50e45e64db41049fd88a75f2dd2 REvil.f 4493
Dropped DLL 78066a1c4e075941272a86d4a8e49471 REvil.e 4493

 

Expert rules allow McAfee customers to extend their coverage. This rule covers this REvil ransomware behaviour.

MITRE

Technique ID Tactic Technique Details
T1059.003 Execution Command and Scripting Interpreter
T1574.002 DLL Side-Loading Hijack Execution Flow
T1486 Impact Data Encrypted for Impact
T1036.005 Defense Evasion Masquerading
T1057 Discovery Process Discovery
T1082 Discovery System Information Discovery

Conclusion

McAfee observed that the REvil group has utilized oracle web logic vulnerability (CVE-2019-2725) to spread the ransomware last year and used kaseya’s VSA application recently for their ransomware execution, with the help of DLL sideloading. REvil uses many vulnerability applications for ransomware infections, however the encryption technique remains the same. McAfee recommends making periodic backups of files and keeping them isolated off the network and having an always updated antivirus in place.





Source link