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:

Burkina Faso
South Africa
Saudi Arabia
United Kingdom

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

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


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

No argument or not matching the followingPersistence 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.
-iThe backdoor is restarted with the -k switch.
-kThe backdoor interpreter (described later) is called with a kill switch.
-dThe backdoor interpreter is called without a kill switch.


The kill switch gives the backdoor the privilege to uninstall or restart SparrowDoor.
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

domaincredits.offices-analytics[.]comC&C server domain
useruserProxy settings used to connect to C&C server
serviceNameWSearchIndexInformation 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
serviceDisplayNameWindows Search Index
serviceDescriptionProvides 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 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 IDAction
0x1C615632The current process is closed.
0x1DE15F35A 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.
0x1A6B561AA directory is created using the name provided by the C&C server.
0x18695638A file is renamed. Both the file to be renamed and the new name are provided by the C&C server.
0x196A5629A file is deleted, as specified in the incoming data.
0x17685647If 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.

0x15665665A new thread is created to exfiltrate the content of a specified file.
0x16675656If 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.
0x14655674A 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.
0x13645683If 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.

OtherRestarts 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

Indicators of Compromise

SHA-1FilenameESET detection nameDescription
76C430B55F180A85F4E1A1E40E4A2EA37DB97599dump.exeWin64/Kryptik.BSQLsass dumper
C36ECD2E0F38294E1290F4B9B36F602167E33614Indexer.exe-Legitimate K7 Computing binary
23E228D5603B4802398B2E7419187AEF71FF9DD5MpSvc.dllEncrypted shellcode
2560B7E28B322BB7A56D0B1DA1B2652E1EFE76EA–Decrypted shellcode
DomainIP addressComment
credits.offices-analytics[.]com45.192.178[.]206SparrowDoor C&C server
-27.102.113[.]240Delivery domain

MITRE ATT&CK techniques

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

Resource DevelopmentT1588.005Obtain Capabilities: ExploitsFamousSparrow used RCE vulnerabilities against Microsoft Exchange, SharePoint and Oracle Opera.
T1583.001Acquire Infrastructure: DomainsFamousSparrow purchased a domain at Hosting Concepts.
T1583.004Acquire Infrastructure: ServerFamousSparrow rented servers at Shanghai Ruisu Network Technology and DAOU TECHNOLOGY.
Initial AccessT1190Exploit Public-Facing ApplicationFamousSparrow used RCE vulnerabilities against Microsoft Exchange, SharePoint and Oracle Opera.
ExecutionT1059.003Command and Scripting Interpreter: Windows Command ShellFamousSparrow used cmd.exe to run commands to download and install SparrowDoor.
T1203Exploitation for Client ExecutionFamousSparrow used RCE vulnerabilities in Microsoft Exchange, SharePoint and Oracle Opera to install SparrowDoor.
PersistenceT1547.001Boot or Logon Autostart Execution: Registry Run Keys / Startup FolderSparrowDoor achieves persistence through the HKCU Run registry value WSearchIndex = Indexer.exe -i registry entry.
T1543.003Create or Modify System Process: Windows ServiceFamousSparrow installs SparrowDoor as a service named WSearchIndex.
T1574.001Hijack Execution Flow: DLL Search Order HijackingFamousSparrow loads the malicious K7UI.dll through DLL search order hijacking.
Defense EvasionT1055.001Process Injection: Dynamic-link Library InjectionMpSvc.dll (shellcode) is injected into processes by SparrowDoor.
T1134.002Access Token Manipulation: Create Process with TokenSparrowDoor creates processes with tokens of processes specified by the C&C server, using the CreateProcessAsUserA API.
T1134Access Token ManipulationSparrowDoor tries to adjust its token privileges to receive SeDebugPrivilege.
T1027Obfuscated Files or InformationThe shellcode, MpSvc.dll, is encrypted using XOR, along with the config embedded within SparrowDoor.
Credentials AccessT1003OS Credential DumpingFamousSparrow makes use of a custom Mimikatz version.
DiscoveryT1082System Information DiscoverySparrowDoor collects the username, computername, RDP session ID, and drive types in the system and sends this data to the C&C server.
T1083File and Directory DiscoverySparrowDoor 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.
CollectionT1005Data from Local SystemSparrowDoor has the ability to read file contents and exfiltrate them to the C&C server.
Command and ControlT1071.001Application Layer Protocol: Web ProtocolsSparrowDoor communicates with the C&C server using the HTTPS protocol.
T1573.001Encrypted Channel: Symmetric CryptographySparrowDoor encrypts/decrypts communications with its C&C server using different multi-byte XOR keys.
ExfiltrationT1041Exfiltration Over C2 ChannelSparrowDoor exfiltrates data over its C&C channel.

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