IBM X-Force has identified new capabilities in DBatLoader malware samples delivered in recent email campaigns, signaling a heightened risk of infection from commodity malware families associated with DBatLoader activity. X-Force has observed nearly two dozen email campaigns since late June leveraging the updated DBatLoader loader to deliver payloads such as Remcos, Warzone, Formbook, and AgentTesla. DBatLoader malware has been used since 2020 by cybercriminals to install commodity malware remote access Trojans (RATs) and infostealers, primarily via malicious spam (malspam).

DBatLoader

DBatLoader (aka ModiLoader) is a malware strain that has been observed since 2020 used to download and execute the final payload of commodity malware campaigns, namely a remote access tool/trojan (RAT) or infostealer such as Remcos, Warzone, Formbook, and AgentTesla. DBatLoader campaigns are frequently undertaken using malicious emails and are known to abuse cloud services to stage and retrieve additional payloads. Earlier this year, DBatLoader campaigns reportedly targeted entities in Eastern Europe to distribute Remcos and businesses in Europe to distribute Remcos and Formbook. Remcos was the most common payload that X-Force observed in these recent campaigns.

Remcos — short for Remote Control and Surveillance — is a remote access tool offered for sale by a company named Breaking Security but is widely used for malicious purposes. Like most such remote tools, Remcos can be used to provide backdoor access to Windows operating systems. Warzone (aka AveMaria), in use since 2018, is a remote access trojan that is also publicly available for purchase at the website warzone[.]ws. Formbook and AgentTesla are popular information stealers that are available on underground markets.

The recent campaigns observed by X-Force that deliver the updated DBatLoader follow and also improve on previously observed tactics. For example, in several observed campaigns the threat actors leveraged sufficient control over the email infrastructure to enable malicious emails to pass SPF, DKIM, and DMARC email authentication methods. A majority of campaigns leveraged OneDrive to stage and retrieve additional payloads, with a small fraction otherwise utilizing transfer[.]sh or new/compromised domains. Most email content appeared targeted toward English speakers, although X-Force also observed emails in Spanish and Turkish.

DBatLoader is still under active development and continues to improve its capabilities. The recently observed samples offer UAC-bypass, persistence, various process injection techniques, and support the injection of shellcode payloads. Furthermore, the signed Windows executable vulnerable to DLL-hijacking (easinvoker.exe), as well as a modified version of netutils.dll, may now be supplied as part of the downloaded payload and config, in order to decrease the size of the DBatLoader stager.

DBatLoader’s most recent iteration also attempts an unexpected technique of DLL hooking. DLL hooking is commonly used to bypass AMSI, however, most of DBatLoader’s current hooking implementations are flawed, rendering it ineffective. The experimental coding style and frequent implementation changes suggest that some of the loader’s functionality is still a work in progress.

Analysis

DBatLoader email campaigns

The email campaigns that X-Force observed used either ISO images or one of several different archive file formats — such as 7-Zip, tar, zip, or rar — to deliver the DBatLoader executable. Most of the campaigns relied on a variety of common email lures to persuade targets to open the file attachments, such as shipping orders or billing/invoice/purchase requests or inquiries. The graphics below provide a screenshot of emails delivering DBatLoader.

Figure 1: Malicious email delivering DBatLoader to install Formbook

Figure 2: Malicious email delivering DBatLoader to install Remcos

Figure 3: DBatLoader infection chain

DBatLoader: First stage

Broken AMSI-bypass

The first stage of DBatLoader is a Delphi-compiled executable. After initialization, execution transfers to the loader’s main function. DBatLoader makes heavy use of junk code and specifically displays an interesting behavior of faking DLL patching. It is not uncommon to see malware attempt to manipulate the behavior of specific DLLs in memory such as AMSI.dll in order to prevent antivirus detection. This is known as AMSI-bypass and is usually achieved by hooking or otherwise patching the AMSI.dll in memory. In the case of DBatLoader, the malware combines splitted strings to generate those commonly targeted API names, such as AmsiInitialize(), AmsiUacScan() or AmsiOpenSession().

Figure 4: AMSI function names splitted strings

The loader uses the strings in a function, which at first appears to locate those functions in memory and then call another function to patch them in order to break the malware detection capability. However, instead of passing the address of the targeted export, the code passes the address of the pointer to the export.

Figure 5: Faulty patching function

The function responsible for patching the memory does work as expected, so it overwrites the pointer it received with a jump instruction to an unrelated API call (GetBkMode). It also uses VirtualProtect, which would have been necessary, if the targeted address was in fact within AMSI.dll’s .text segment.

Figure 6: Patching memory

Multiple implementations of this were observed in different samples and both the first and second stages. The second stage for instance uses native API calls NtProtectVirtualMemory and NtWriteVirtualMemory to patch memory, with a jump instruction to the GetCPInfo export.

Figure 7: Faulty patching in Stage 2

All implementations display the same unexpected behavior of patching only the pointer, but not the actual DLL. Whether or not this behavior is intended, it renders the functionality completely ineffective as an AMSI bypass.

Payload decryption and execution

The encrypted second-stage PE is stored within the binary. Due to the simple ADD-based encryption, it is visible in the hexdump:

Figure 8: ADD-encrypted second stage

The payload is decrypted byte-by-byte using the ADD-based algorithm below:

Figure 9: Payload decryption

Once the payload is decrypted, the resulting PE is parsed and each section is manually mapped into memory. The loader also resolves all imports and applies the appropriate memory protection. Next, the faulty patching functions discussed above are used on several other APIs, associated with malware detection and antivirus behavior. Some of them are:

  • ReportEventW (used for event logging)
  • SaferiIsExecutableFileType (used to detect executable files that could potentially be malicious)
  • VerifySignature and SspiZeroAuthIdentity (used by Windows to verify security and identity)

Lastly, the loader transfers execution to the entry point of the second stage.

DBatLoader: Second stage

Downloading and decrypting the config

DBatLoader’s second stage is a Delphi-compiled DLL. It begins by initiating a timer event using timeSetEvent() and passes its main function as a callback, which is executed after 10 seconds. Just like the first stage, almost all functions contain large amounts of the faulty DLL patching functionality. First, the code attempts to locate and parse the encrypted download URL from its parent binary. The encrypted bytes and a key can be parsed using the delimiter “^^Nc”.

Figure 10: Encrypted URL in green, separator in red, key in blue

Next, the bytes are decrypted using a simple modulo-based algorithm and the hardcoded key highlighted above.

Figure 11: URL decryption

Decryption with the key “255” results in the following download URL:

https://onedrive[.]live[.]com/download?resid=E0CF7F9E6AAF27EF%211759&authkey=!APOw7kS6PG9JFg8

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In order to retrieve the payload, DBatLoader first resolves the CLSID for the object “WinHttp.WinHttpRequest.5.1” using the CLSIDFromProgID() API. The CLSID is then passed to CoCreateInstance() to initialize the HTTP object. The response to the GET request is a Base64 encoded blob of encrypted data containing various configuration parameters and payloads.

Figure 12: Base64 encoded response

After decoding, the response is decrypted using the same key and algorithm as the URL (see Figure 8). The next stage of decryption uses the custom algorithm shown below:

Figure 13: Custom decryption algorithm

The resulting binary blob contains a list of different config values, which are each parsed out by another separator:

Figure 14: Payload with separator (highlighted blue)

After splitting the blob into a list, the following config values are revealed:

  1. XOR key to decrypt payload
  2. Filename to be used for persistence
  3. Encrypted payload
  4. Option to enable UAC bypass
  5. Option to enable persistence
  6. Option to inject shellcode
  7. Option to inject into remote process
  8. Numeric decryption key (often same as used before)
  9. Unused
  10. easinvoker.exe payload
  11. netutils.dll payload
  12. Option to inject via process hollowing

Figure 15: DBatLoader parsing payload

Persistence

If the persistence option is enabled, DBatLoader writes its parent binary to “C:\Users\Public\Libraries\<config_filename>.PIF“. By using the .PIF extension, it will automatically be executed if opened.

It then writes a .URL file at the path “C:\Users\Public\<config_filename>.url“. The file is effectively a shortcut to the .PIF file:

Figure 16: Example shortcut file for persistence

Finally, DBatLoader writes the path of the shortcut file to the registry key:

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\<config_filename>

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This will ensure the execution of the DBatLoader binary every time the user logs on.

UAC bypass

When the UAC bypass option is enabled, DBatLoader will start to drop several files. The first file, dropped to C:\Users\Public\Libraries\Null, is used as a mutex and contains a random integer. Execution will only continue if the file doesn’t exist already.

Next, both downloaded files from the config, easinvoker.exe and netutils.dll are dropped to C:\Users\Public\Libraries\.

Figure 17: Building easinvoker.exe path to drop

DBatLoader also drops two .BAT files KDECO.bat and <config_filename>O.bat to the same directory and executes the latter:

Figure 18: UAC bypass .BAT file

The malicious .BAT file above creates a new directory “C:\Windows \System32” and copies both binaries and KDECO.bat into it. This technique is known as mocking trusted directories. The extra space in the “Windows “ directory name mocks the trusted directory “C:\Windows\System32” and ultimately leads to Windows automatically elevating the privilege of processes of specific system executables started from that location — without a UAC confirmation pop-up. The executable easinvoker.exe, which is run by the batch script, is a legitimate and signed Windows component that is vulnerable to DLL hijacking, meaning it will search for and load any DLL in its directory called “netutils.dll” and execute a specific export.

In this case, it will find the netutils.dll previously copied to the mock directoryThe DLL’s export NetpwNameValidate() was modified to execute a .BAT file in the same directory.

Figure 19: Modified netutils.dll export

Finally, KDECO.bat contains the following command, which is executed with elevated privileges:

start /min powershell -WindowStyle Hidden -inputformat none -outputformat none -NonInteractive -Command \”Add-MpPreference -ExclusionPath ‘C:\\Users’\” & exit

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This effectively disables antivirus protection for all files below the C:\\Users\ directory.

After this has been completed, all previously dropped files and directories are deleted by the first BAT file and DBatLoader’s second stage.

Process injection

The next task is to decrypt and execute the final payload that was downloaded. It can be decrypted using the XOR key from the config using another custom algorithm, which XORs the key as well as both lengths of the key and ciphertext.

Figure 20: XOR decryption algorithm

Afterward, it goes through another stage of modulo-based decryption with the integer key from the config (see Figure 12) and finally the already mentioned custom decryption algorithm (Figure 14).

The resulting payload is then injected into a legitimate process from the C:\Windows\System32\ directory. Each DBatLoader sample contains a list of targeted process names, from which it chooses the first executable present on the system. The following processes have been observed recently:

  • sndvol.exe
  • iexpress.exe
  • colorcpl.exe
  • wusa.exe

DBatLoader’s downloaded config also specifies how the payload is to be injected, either via regular process injection, shellcode injection (for shellcode payloads only), or process hollowing.

In the case of regular process injection, DBatLoader uses WinExec() to start the targeted process. It then uses CreateToolhelp32Snapshot(), Process32First() and Process32Next() to search for the process and retrieve the corresponding process handle to open the process. DBatLoader allocates memory in the remote process space, maps the payload, resolves imports, and writes the payload into the allocated memory buffer using the following API calls:

  • NtAllocateVirtualMemory()
  • LoadLibraryExA()
  • NtProtectVirtualMemory()
  • NtWriteVirtualMemory()

The payload is then executed in a new thread via RtlCreateUserThread().

Lastly, DBatLoader hooks two APIs NtSetSecurityObject() and NtOpenProcess() in the memory space of the newly created process, by writing a return instruction (0xC3) at the start of the functions. This is the only implementation of hooking that is not broken and works as expected.

Figure 21: Hooking ntdll 

Shellcode injection

DBatLoader also supports the injection of shellcode payloads. If the config has the respective option enabled, the loader starts the targeted process in a suspended state and opens it:

Figure 22: Create suspended process

The decrypted payload is written to the process memory in a buffer using NtAllocateVirtualMemory() and NtWriteVirtualMemory(). To execute the shellcode, an APC thread is created via the NtQueueApcThread() API and run via ResumeThread(). Lastly, DBatLoader hooks NtSetSecurityObject() in the new processes context.

Process hollowing

PE payloads may also be injected using a technique known as process hollowing. First, the target process is again created in a suspended state. Instead of injecting the payload into a new buffer within the process memory, this technique uses a series of API calls in order to overwrite the legitimate executable with the mapped malicious PE within the created process. The following API calls are made:

  • GetThreadContext()
  • NtReadVirtualMemory()
  • NtUnmapViewOfSection()
  • NtAllocateVirtualMemory()
  • NtWriteVirtualMemory()
  • NtFlushInstructionCache()
  • SetThreadContext()

After the process has been injected with the malicious PE, DBatLoader resumes the suspended thread using NtResumeThread(), which causes execution to continue at the malicious PE’s entry point. Once again, NtSetSecurityObject() is hooked in the new process.

Finally, before the DBatLoader’s process is terminated, it calls FlushInstructionCache() and hooks NtOpenProcess().

Improved DBatLoader heralds increased risk of associated infections

Due to the sophistication of DBatLoader phishing techniques and improvements to the malware itself, it is likely that infections with DBatLoader and follow-on payloads will rise. IBM X-Force reported on a surge in Remcos RAT activity in Q1 2023, and expects to see a future upward trend in infections from this malware, as well as other RATs and infostealers associated with DBatLoader. A rise in these infections signals a heightened risk of highly impactful post-compromise activity facilitated by malicious programs that collect credentials and enable remote control of systems.

To combat this, security teams are encouraged to renew vigilance around TTPs associated with DBatLoader campaigns, such as abuse of public cloud infrastructure, and characteristics of the new variants of the malware observed by X-Force. Policy and procedure changes in the form of multi-factor authentication implementation, monitoring for leaked enterprise credentials, and review of policies for ISO auto-mounting can also help mitigate the risk of this and other malicious activity.

To learn how IBM Security X-Force can help with anything regarding cybersecurity including incident response, threat intelligence or offensive security services, schedule a meeting here: IBM Security X-Force Scheduler.

If you are experiencing cybersecurity issues or an incident, contact IBM Security X-Force for help: US hotline 1-888-241-9812 | Global hotline (+001) 312-212-8034.

Indicators of Compromise

Indicator Indicator Type Context 
hxxp://doctorproff[.]ru/194_Hmoczcsvbok URL Payload Staging URL
hxxps://travelinspiration.sa[.]com/.xleet2/255_Oyvdqiogydx URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=168DC93239B65DF6%21216&authkey=!AFhcwjWlnon5LwE URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=B044AF3D48F7B886%21365&authkey=!AIpyTdc7_NVF6I8 URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=F253EE082321791B%21110&authkey=!AMAFiW2uLt6IzGM URL Payload Staging URL
hxxps://transfer[.]sh/get/6eSIqx4VYA/255_Xwgdedwtiyw URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=2F714EB1E9F0F34B%21131&authkey=!AB-Xgr3iPCVl3gc URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=D94EF82AD5BE7BDF%21120&authkey=!AI3c0hhcpsQ92lg URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=8AC261C876D2C5D0!230&authkey=!AJjFtmZbzh4E0lA URL Payload Staging URL
hxxps://biototec[.]co/youtubedrivedocumentsuploadgifterssocialiseapartmentsroomsdoors/211_Wbroctgfmht URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=DDFE20447411E22A!138&authkey=!ANsuuB_STyMMWaM URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=F21FE0453B44A092%21131&authkey=!AHYgqFp_4Em3JLI URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=445E8B425B247567%21164&authkey=!AMMd_FSLiwAEKhQ URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=445E8B425B247567%21152&authkey=!APbQBxaFQ4ZpNjQ URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=26943FEBC022618F%21339&authkey=!AMGXtmXOj3JDCls URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=4949CD367CC71D79!665&authkey=!AHrzsEuO8nQG9Ck URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=B044AF3D48F7B886%21307&authkey=!AND2XupI-UzvwZc URL Payload Staging URL
hxxps://ariso[.]eu/vorpruefung/255_Pbtrfmfsxud URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=E0CF7F9E6AAF27EF%211585&authkey=!APMIaCFn0CdoKkc URL Payload Staging URL
hxxp://balkancelikdovme[.]com/hjghgynyvbtvyugjhbugvdveksk/Xezdxpgykmk URL Payload Staging URL
hxxps://balkancelikdovme[.]com/work/Elpuxpkilck URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=B044AF3D48F7B886%21367&authkey=!AF8bdRvVB0L2ejQ URL Payload Staging URL
hxxps://onedrive.live[.]com/download?resid=B044AF3D48F7B886!369&authkey=!AA6HUemo3mWPD8E URL Payload Staging URL
40.74.95[.]186 IP Address Remcos C2
www.rainbow-industrie[.]com Domain Remcos C2
www.binccoco[.]com Domain Remcos C2
www.aconaus[.]org Domain Remcos C2
hxxp://chibb.ydns[.]eu/chibbori/inc/8fcde15698ce9a.php URL AgentTesla C2
20.231.24[.]237 IP Address Remcos C2
hxxp://jimbo.ydns[.]eu/jimboori/inc/def4f4924bdf6e.php URL AgentTesla C2
www.monarkpapes[.]com Domain Remcos C2
donelpacino.ddns[.]net Domain Warzone C2
nightmare4666.ddns[.]net Domain Warzone C2
www.zysnuy[.]com Domain Remcos C2
www.twyfordtille[.]com Domain Remcos C2
remcos1.ydns[.]eu Domain Remcos C2
greatzillart.ydns[.]eu Domain Remcos C2
www.playdoapp[.]online Domain Formbook C2
www.oldironmetalworksllc[.]com Domain Formbook C2
www.mattewigs[.]com Domain Formbook C2
www.dunia138[.]info Domain Formbook C2
www.transportlogistcs[.]com Domain Formbook C2
www.rva[.]info Domain Formbook C2
www.totomata[.]com Domain Formbook C2
www.janus[.]news Domain Formbook C2
www.bvgroupcos[.]com Domain Formbook C2
www.transportlogistcs[.]com Domain Formbook C2
www.purelyunorthodox[.]com Domain Formbook C2
www.660danm[.]top Domain Formbook C2
www.mytraderstore[.]com Domain Formbook C2
www.undoables[.]com Domain Formbook C2
www.azurefd-paitohk[.]xyz Domain Formbook C2
www.altralogos[.]com Domain Formbook C2
www.sinpercar[.]com Domain Formbook C2
55c34ff5126f2b46d623f802d1e0e1d886e671fb8fb7f75294bbf7726f13340d SHA256 Hash DBatLoader
352aac36d6ee5ce68679227aa27b082cbeae8990853a47b3d48ee7bc4cd7c613 SHA256 Hash DBatLoader
fef09480410315363b71b047f1a07100080cb970bae50ee0280586ab778089e8 SHA256 Hash DBatLoader
98a4d17d6dee54f9242c704af627da853d978d6d37738f875d08ea0e7eaca373 SHA256 Hash DBatLoader
43ff884128b4cee041776015abb9692e42db2cbf8b5a4364859d346c809ec5cd SHA256 Hash DBatLoader
cf39a14a2dc1fe5aa487b6faf19c63bc97103db670fa24c62832895e3002eca2 SHA256 Hash DBatLoader
d168a3b56994a97374be1c208e6e3aa01e1c512829ee4cceafceeeee1b5ddcc1 SHA256 Hash DBatLoader
1ba931f3d786284d056bd83659afabe498c61c999fd5d64837da8c2b737e3746 SHA256 Hash DBatLoader
147ccc27801c86734963bf547721517bddbc76c4b80225d557c373cd5e16da3d SHA256 Hash DBatLoader
0d2f7e49186d74f6e8a320d41283d88fcd785f4b1e06abd18553ebc14b8c9f17 SHA256 Hash DBatLoader
b9e4e58572b93ecd81ebcb6ef411b6fa447c7c9177a1ea2fdf26558d76e0ca3a SHA256 Hash DBatLoader
ad5e18d32f403ca4871f3d4b222c84821a6b6ba74ec858cc99eb00c66bb6bddb SHA256 Hash DBatLoader
0cc5de13ddde8a5dbbe9ce4f14a595e8f8bed743a0f4a7bbdba4d8de44d88b30 SHA256 Hash DBatLoader
a08cd110a928227dd4b3b42b1801bc1c907dd042bea8494ac701142c5eb345da SHA256 Hash DBatLoader
d9b2b28698fd4b81fc602305bd73e060dc35acb6b72264e75ba9bee47a3501e2 SHA256 Hash DBatLoader
203146e788d7a0afa679721e1581f5cdcf8e2c4d4367a7ce53c433184d988fcc SHA256 Hash DBatLoader
9474ca0fa771bd4dd2202e312ada0090f6890635b9039b5be855cc7cb8eab6ee SHA256 Hash DBatLoader
921a295f8a722340f6cf979c9e3fb0f9a762fe45c94407d1e1a32a4dc35e2854 SHA256 Hash DBatLoader
31eed753e4fc1e7fb831c38bddd30577a41a727fabb73360fa90a6d93fc61d02 SHA256 Hash DBatLoader
7db150c239b11e729433ce9ea99939f08bf35aac1dda071917c4a7e694a7258d SHA256 Hash DBatLoader
e9352253e3211314faee670cf457e3f6732d7d93eb52f46aebf4f79cb22cbf7e SHA256 Hash DBatLoader
1ba55bb7d2d33d7892669c2e96c351fe59ce60144429508d6251d5dcbfc5ff86 SHA256 Hash DBatLoader
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