FamousSparrow: A suspicious hotel guest

Cyber Security

Yet another APT group that exploited the ProxyLogon vulnerability in March 2021

ESET researchers have uncovered a new cyberespionage group targeting hotels, governments, and private companies worldwide. We have named this group FamousSparrow and we believe it has been active since at least 2019.

Reviewing telemetry data during our investigation, we realized that FamousSparrow leveraged the Microsoft Exchange vulnerabilities known as ProxyLogon that we described extensively in March 2021. As a reminder, this remote code execution vulnerability was used by more than 10 APT groups to take over Exchange mail servers worldwide. According to ESET telemetry, FamousSparrow started to exploit the vulnerabilities on March 3rd, 2021, the day following the release of the patch, so it is yet another APT group that had access to the ProxyLogon remote code execution vulnerability in March 2021.

In this blogpost we will discuss the attribution to FamousSparrow and the group’s victimology. This will be followed by a detailed technical analysis of the group’s main backdoor that we have named SparrowDoor.

A note on attribution

FamousSparrow is a group that we consider as the only current user of the custom backdoor, SparrowDoor (which we cover in detail in the later sections of this blogpost). It also uses two custom versions of Mimikatz (see the Indicators of Compromise section) that could be used to connect incidents to this group.

While we consider FamousSparrow to be a separate entity, we found connections to other known APT groups. In one case, attackers deployed a variant of Motnug that is a loader used by SparklingGoblin. In another case, on a machine compromised by FamousSparrow, we found a running Metasploit with cdn.kkxx888666[.]com as its C&C server. This domain is related to a group known as DRBControl.


The group has been active since at least August 2019 and it mainly targets hotels worldwide. In addition, we have seen a few targets in other sectors such as governments, international organizations, engineering companies and law firms in the following countries:

  • Brazil
  • Burkina Faso
  • South Africa
  • Canada
  • Israel
  • France
  • Guatemala
  • Lithuania
  • Saudi Arabia
  • Taiwan
  • Thailand
  • United Kingdom
Figure 1. Geographic distribution of FamousSparrow targets

Figure 1. Geographic distribution of FamousSparrow targets

Compromise vector

In a few cases, we were able to find the initial compromise vector used by FamousSparrow and these systems were compromised through vulnerable internet-facing web applications. We believe FamousSparrow exploited known remote code execution vulnerabilities in Microsoft Exchange (including ProxyLogon in March 2021), Microsoft SharePoint and Oracle Opera (business software for hotel management), which were used to drop various malicious samples.

Once the server is compromised, attackers deploy several custom tools:

  • A Mimikatz variant
  • A small utility that drops ProcDump on disk and uses it to dump the lsass process, probably in order to gather in-memory secrets, such as credentials
  • Nbtscan, a NetBIOS scanner
  • A loader for the SparrowDoor backdoor

Through our telemetry, we were able to recover only the loader component (SHA-1: E2B0851E2E281CC7BCA3D6D9B2FA0C4B7AC5A02B). We also found a very similar loader on VirusTotal (SHA-1: BB2F5B573AC7A761015DAAD0B7FF03B294DC60F6) that allowed us to find the missing components, including SparrowDoor.



SparrowDoor is initially loaded via DLL search order hijacking, using three elements – a legitimate K7 Computing executable (Indexer.exe) used as the DLL hijacking host, a malicious DLL (K7UI.dll), and encrypted shellcode (MpSvc.dll) – all of which are dropped in %PROGRAMDATA%Software. It can be assumed that the command line argument used with the initial SparrowDoor execution, in order to set up persistence, is either nothing or anything but -i, -k or -d (the functionalities of these three arguments are explained below). Once persistence is set up, SparrowDoor is executed with the -i command line argument. Refer to Figure 2 for a brief overview of the flow of the initial loading process. If you would like an in-depth look into the loading process, continue reading!

Figure 2. SparrowDoor staging

Figure 2. SparrowDoor staging

The legitimate executable, Indexer.exe, requires the library K7UI.dll to operate. Therefore, the OS looks for the DLL file in directories in the prescribed load order. Since the directory where the Indexer.exe file is stored is at the top priority in the load order, it is exposed to DLL search-order hijacking. And that is exactly how the malware gets loaded. Indexer.exe loads the malicious K7UI.dll, which in turn patches the code in Indexer.exe (from call WinMain to jmp K7UI.0x100010D0) and then returns to Indexer.exe. As a result of this, Indexer.exe ends up running a subroutine in K7UI.dll (located in the .text section) instead of calling WinMain. We will refer to this subroutine as launcher. The functionality of launcher is to load MpSvc.dll (the encrypted shellcode) into memory from the directory that also stores Indexer.exe, decrypt the content and then execute the shellcode.

The shellcode (MpSvc.dll) is encrypted using four-byte XOR with the key being the first four bytes of the file.

The MpSvc.dll shellcode loads various libraries responsible for building a PE structure and locates the addresses of the functions to be used. After that, it allocates RWX memory and copies various locations in the shellcode into it (in order to build the PE structure). It also resolves the imports of several functions from different libraries. Finally, it executes the newly built backdoor PE from the entry point. Interestingly, this rebuilt executable image has no PE headers, as shown in Figure 2, so the loader executes the backdoor by jumping to the entry point at a hardcoded offset within the allocated memory.

Figure 3. The PE header is missing in the newly built backdoor from the MpSvc.dll shellcode

Figure 3. The PE header is missing in the newly built backdoor from the MpSvc.dll shellcode


The arguments passed to the backdoor are inherited from the arguments passed to Indexer.exe, or to any other binary that gets the shellcode/backdoor injected. The tasks performed by the backdoor after an argument is specified are shown in Table 1.

Table 1. Actions performed based on the command line arguments provided to SparrowDoor

Argument Action
No argument or not matching the following Persistence is set through the registry Run key and a service, which is created and started using the configuration data (described in the next section) hardcoded in the binary. Finally, the backdoor is restarted with the -i switch.
-i The backdoor is restarted with the -k switch.
-k The backdoor interpreter (described later) is called with a kill switch.
-d The backdoor interpreter is called without a kill switch.


  1. The kill switch gives the backdoor the privilege to uninstall or restart SparrowDoor.
  2. The backdoor interpreter gets called regardless of the argument used because it will always end up with a -k or -d argument.

Configuration data

The configuration is found in the binary and is decrypted using the multi-byte XOR key ^&32yUgf. The configuration has the following format:

The decrypted values are shown in Table 2.

Table 2. The key-value pairs of the configuration along with a description of their purpose

Key Value Purpose
domain credits.offices-analytics[.]com C&C server domain
user user Proxy settings used to connect to C&C server
pass pass
port 8080
serviceName WSearchIndex Information used for creating a service to set up persistence. Also, note that the serviceName is used as the value name under the Run key in the registry
serviceDisplayName Windows Search Index
serviceDescription Provides content indexing, property caching, and search results for files, e-mail, and other content.

The connections could be either through a proxy or not, and they connect to the C&C server over port 443 (HTTPS). So, the communication should be encrypted using TLS. During the first attempt to contact the C&C server, SparrowDoor checks whether a connection can be established without using a proxy, and if it can’t, then the data is sent through a proxy. All outgoing data is encrypted using the XOR key hH7@83#mi and all incoming data is decrypted using the XOR key h*^4hFa. The data has a structure that starts with a Command ID, followed by the length of the ensuing encrypted data, followed by the encrypted data.

Figure 4 shows an example of how the data is sent to the C&C server (in this case it is sending system information), while Figure 5 shows the plaintext form of the same data payload.

Figure 4. A Wireshark dump showing the data POSTed by the backdoor

Figure 4. A Wireshark dump showing the data POSTed by the backdoor

Figure 5. The decrypted data containing system information

Figure 5. The decrypted data containing system information

Victim’s local IP address in this case can be converted to decimal, giving

Session ID is the Remote Desktop Services session ID associated with the backdoor process, found using the ProcessIdToSessionId Windows API call.

The systemInfoHash is computed via the sdbm hash algorithm, using the username, computer name, host addresses and the session ID.

Backdoor interpreter function

Privilege escalation is performed in this function by adjusting the access token of the SparrowDoor process to enable SeDebugPrivilege. After that, the shutdown function (Ws2_32.dll) is patched to prevent disabling sends and receives on a socket and the closesocket function (Ws2_32.dll) is patched to enable the DONT_LINGER option first to close the socket without waiting for pending data to be sent or received. Finally, system information is sent to the C&C server (as seen in Figures 4 and 5 above) to receive data back in return.

Based on the Command ID field in the data received from the C&C server, the backdoor can perform different malicious actions that are detailed in Table 3.

Table 3. Actions performed by SparrowDoor when the corresponding Command IDs are received

Command ID Action
0x1C615632 The current process is closed.
0x1DE15F35 A child svchost.exe process is spawned with processToken information of the process (Process ID) specified by the C&C server, with argument -d and then the shellcode is injected into the process.
0x1A6B561A A directory is created using the name provided by the C&C server.
0x18695638 A file is renamed. Both the file to be renamed and the new name are provided by the C&C server.
0x196A5629 A file is deleted, as specified in the incoming data.
0x17685647 If length of the data is 1, and the data matches $, then the length of systemInfoHash along with an array of drive types are sent.

If length of the data is greater than 2 and the first 2 bytes of data match $, then information about the files in a specified directory is sent. The information included is the following: file attributes, file size and file write time.

0x15665665 A new thread is created to exfiltrate the content of a specified file.
0x16675656 If the kill switch is activated, the current persistence settings (registry and service) are removed and the Indexer.exe file is executed (to restart the dropper). If not, the backdoor loop is restarted.
0x14655674 A new thread is created to write the data to a specified file.
0x12635692 If the kill switch is activated, the persistence settings are removed, and all the files used by SparrowDoor (Indexer.exe, K7UI.dll and MpSvc.dll) are removed. If not, the backdoor loop is restarted.
0x13645683 If the data matches “switch ”, then the backdoor is restarted with the -d switch.

If not, it spawns a cmd.exe shell, and sets up named pipes for input and output (used by the C&C server) to establish an interactive reverse shell.

If the data matches Exitrn, then the spawned shell is terminated.

Other Restarts the backdoor loop.


FamousSparrow is yet another APT group that had access to the ProxyLogon remote code execution vulnerability early in March 2021. It has a history of leveraging known vulnerabilities in server applications such as SharePoint and Oracle Opera. This is another reminder that it is critical to patch internet-facing applications quickly, or, if quick patching is not possible, to not expose them to the internet at all.

The targeting, which includes governments worldwide, suggests that FamousSparrow’s intent is espionage. We have highlighted some links to SparklingGoblin and DRBControl, but we don’t consider that these groups are the same.

A comprehensive list of Indicators of Compromise (IoCs) and samples can be found in our GitHub repository.

For any inquiries, or to make sample submissions related to the subject, contact us at threatintel@eset.com.

Indicators of Compromise

SHA-1 Filename ESET detection name Description
B9601E60F87545441BF8579B2F62668C56507F4A p64.exe
Win64/Riskware.Mimikatz.H Mimikatz
4DF896624695EA2780552E9EA3C40661DC84EFC8 p64.exe
Win64/Riskware.Mimikatz.H Mimikatz
76C430B55F180A85F4E1A1E40E4A2EA37DB97599 dump.exe Win64/Kryptik.BSQ Lsass dumper
873F98CAF234C3A8A9DB18343DAD7B42117E85D4 nbtscan.exe Win32/NetTool.Nbtscan.A Nbtscan
FDC44057E87D7C350E6DF84BB72541236A770BA2 1.cab Win32/FamousSparrow.A Dropper
C36ECD2E0F38294E1290F4B9B36F602167E33614 Indexer.exe Legitimate K7 Computing binary
BB2F5B573AC7A761015DAAD0B7FF03B294DC60F6 K7UI.dll Win32/FamousSparrow.A Loader
23E228D5603B4802398B2E7419187AEF71FF9DD5 MpSvc.dll Encrypted shellcode
2560B7E28B322BB7A56D0B1DA1B2652E1EFE76EA Decrypted shellcode
E2B0851E2E281CC7BCA3D6D9B2FA0C4B7AC5A02B K7UI.dll Win32/FamousSparrow.B Loader
Domain IP address Comment
credits.offices-analytics[.]com 45.192.178[.]206 SparrowDoor C&C server
27.102.113[.]240 Delivery domain

MITRE ATT&CK techniques

This table was built using version 9 of the MITRE ATT&CK framework.

Tactic ID Name Description
Resource Development T1588.005 Obtain Capabilities: Exploits FamousSparrow used RCE vulnerabilities against Microsoft Exchange, SharePoint and Oracle Opera.
T1583.001 Acquire Infrastructure: Domains FamousSparrow purchased a domain at Hosting Concepts.
T1583.004 Acquire Infrastructure: Server FamousSparrow rented servers at Shanghai Ruisu Network Technology and DAOU TECHNOLOGY.
Initial Access T1190 Exploit Public-Facing Application FamousSparrow used RCE vulnerabilities against Microsoft Exchange, SharePoint and Oracle Opera.
Execution T1059.003 Command and Scripting Interpreter: Windows Command Shell FamousSparrow used cmd.exe to run commands to download and install SparrowDoor.
T1203 Exploitation for Client Execution FamousSparrow used RCE vulnerabilities in Microsoft Exchange, SharePoint and Oracle Opera to install SparrowDoor.
Persistence T1547.001 Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder SparrowDoor achieves persistence through the HKCU Run registry value WSearchIndex = Indexer.exe -i registry entry.
T1543.003 Create or Modify System Process: Windows Service FamousSparrow installs SparrowDoor as a service named WSearchIndex.
T1574.001 Hijack Execution Flow: DLL Search Order Hijacking FamousSparrow loads the malicious K7UI.dll through DLL search order hijacking.
Defense Evasion T1055.001 Process Injection: Dynamic-link Library Injection MpSvc.dll (shellcode) is injected into processes by SparrowDoor.
T1134.002 Access Token Manipulation: Create Process with Token SparrowDoor creates processes with tokens of processes specified by the C&C server, using the CreateProcessAsUserA API.
T1134 Access Token Manipulation SparrowDoor tries to adjust its token privileges to receive SeDebugPrivilege.
T1027 Obfuscated Files or Information The shellcode, MpSvc.dll, is encrypted using XOR, along with the config embedded within SparrowDoor.
Credentials Access T1003 OS Credential Dumping FamousSparrow makes use of a custom Mimikatz version.
Discovery T1082 System Information Discovery SparrowDoor collects the username, computername, RDP session ID, and drive types in the system and sends this data to the C&C server.
T1083 File and Directory Discovery SparrowDoor can probe files in a specified directory obtaining their names, attributes, sizes and last modified times, and sends this data to the C&C server.
Collection T1005 Data from Local System SparrowDoor has the ability to read file contents and exfiltrate them to the C&C server.
Command and Control T1071.001 Application Layer Protocol: Web Protocols SparrowDoor communicates with the C&C server using the HTTPS protocol.
T1573.001 Encrypted Channel: Symmetric Cryptography SparrowDoor encrypts/decrypts communications with its C&C server using different multi-byte XOR keys.
Exfiltration T1041 Exfiltration Over C2 Channel SparrowDoor exfiltrates data over its C&C channel.

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