AsyncRAT’s Cloud Shell Game: How Dropbox and TryCloudflare Are Becoming the Ultimate Smokescreen for Malware Delivery + Video

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Introduction:

In a concerning evolution of cyberattack methodologies, threat actors are increasingly weaponizing trusted, legitimate cloud infrastructure to distribute sophisticated Remote Access Trojans (RATs). The latest AsyncRAT campaign exemplifies this trend, exploiting Dropbox for initial payload delivery and TryCloudflare’s free tunneling service to host malicious servers and command-and-control (C2) infrastructure. By hiding malicious activities behind reputable services, attackers effectively bypass traditional security solutions, making detection and prevention significantly more challenging for organizations worldwide.

Learning Objectives:

  • Understand the multi-stage infection chain of the AsyncRAT campaign, from phishing email to final payload execution.
  • Learn how attackers abuse legitimate services like Dropbox and TryCloudflare to evade detection.
  • Acquire practical knowledge of detection, analysis, and mitigation techniques, including relevant Linux and Windows commands.

You Should Know:

  1. The Anatomy of the Attack Chain: From Phishing to Persistence

This campaign leverages a sophisticated, multi-stage infection chain that exploits trusted services at every step.

  • Stage 1: The Phishing Lure. The attack begins with a phishing email, often themed around an invoice (e.g., “Rechnung herunterladen” – Download invoice). The email contains a button or link that hides a legitimate Dropbox URL. Clicking this link downloads a ZIP archive, which appears innocuous to email filters that trust Dropbox domains.
  • Stage 2: The Internet Shortcut (.URL). Inside the ZIP file is an Internet Shortcut file (.url). This file is not a typical executable, making it less likely to be flagged. When opened, it connects to a TryCloudflare subdomain, which serves as the attackers’ hosting infrastructure. The `.url` file contains a link like URL=file[:]//inventory-card-thumbzilla-ip[.]trycloudflare[.]com@SSL/DavWWWRoot/DE.
  • Stage 3: Downloading the .LNK and .JS Files. The TryCloudflare subdomain hosts a Windows shortcut (.lnk) file. Executing this `.lnk` file triggers a PowerShell command to download a JavaScript (.js) file from the same tunnel.
  • Stage 4: The Obfuscated Batch File. The JavaScript file, when deobfuscated, reveals code that downloads a heavily obfuscated Batch (.bat) file. This batch file performs the critical payload delivery. It simultaneously opens a legitimate PDF file (e.g., from the German Chamber of Commerce) as a decoy to distract the victim while it downloads a second ZIP file containing the final malicious Python package.
  • Stage 5: Python Environment and Final Payload. The batch file checks if Python is installed on the system. If not, it downloads a legitimate, embedded version of Python 3.14.0 from the official Python website. It then executes a Python script (load.py) from the downloaded package.

Step‑by‑step guide for analyzing the attack chain:

To analyze this type of attack, security analysts can use a combination of static and dynamic analysis tools. The following steps outline a basic approach:

  1. Analyze the Phishing Email: Extract the full headers and the embedded URL. Use tools like `curl -v ` to follow the redirect chain and identify the final payload location without executing it.
  2. Inspect the .URL File: Open the `.url` file in a text editor (e.g., Notepad). Look for the `URL=` field to identify the TryCloudflare subdomain. 
    [bash]
URL=file://inventory-card-thumbzilla-ip.trycloudflare.com@SSL/DavWWWRoot/DE
  3. Analyze the .LNK File: Use a tool like `LnkParse3` (Python library) or `strings` on Linux to extract the command line from the `.lnk` file. 
    On Linux, to view strings in a .lnk file
strings malicious.lnk
  4. Deobfuscate the JavaScript: Use a JavaScript beautifier or a sandbox environment to deobfuscate the `.js` file. Look for the URL used to download the `.bat` file.
  5. Analyze the Batch File: The `.bat` file is likely heavily obfuscated. Use a sandbox or manual deobfuscation techniques to understand its actions. Look for commands that download the second ZIP file and set up the Python environment.
  6. Examine the Python Payload: Extract the final ZIP file and analyze the Python scripts. Pay close attention to `load.py` and the binary files (e.g., new.bin). Use a Python decompiler if the script is compiled.

  7. The Core Technique: Process Injection via Early Bird APC

The final stage of the attack relies on a sophisticated code injection technique known as “Early Bird Asynchronous Procedure Call (APC) Queue Injection”. This technique is used to execute the malicious shellcode within the memory of a legitimate Windows process, making it extremely difficult for traditional antivirus and endpoint detection and response (EDR) tools to detect.

  • How it Works:
  1. The Python script (load.py) uses the `ctypes` library, which allows it to call Windows system functions directly.
  2. It creates a new process in a “suspended” state (e.g., `explorer.exe` or notepad.exe).
  3. Before the process’s main thread begins execution, the malware allocates memory within this new process and writes the malicious shellcode (e.g., `new.bin` containing AsyncRAT shellcode) into it.
  4. It then queues an APC to the process’s main thread. An APC is a function that executes in the context of a specific thread.
  5. When the main thread is resumed, the APC is executed before any of the process’s legitimate code runs, effectively injecting and executing the malware within the trusted process.

– Evasion Techniques:
– Process Injection: Running within a legitimate process (like explorer.exe) helps the malware blend in and avoid suspicion.
– AMSI and ETW Patching: AsyncRAT is known to patch the Antimalware Scan Interface (AMSI) and disable Event Tracing for Windows (ETW) to hinder detection and logging by security tools.

Step‑by‑step guide for detecting process injection:

Detecting process injection requires monitoring for suspicious API calls and process behavior. Security tools and analysts can use the following methods:

  1. Monitor for Suspicious API Calls: Use Sysmon or EDR solutions to monitor for the following Windows API calls, which are indicative of process injection:
    – `CreateProcess` (with the `CREATE_SUSPENDED` flag)
    – `VirtualAllocEx` (to allocate memory in a remote process)
    – `WriteProcessMemory` (to write shellcode to the allocated memory)
    – `QueueUserAPC` (to queue an APC to a thread in the target process)
    – `ResumeThread` (to resume the suspended thread, triggering the APC)
  2. Use Process Monitor (ProcMon): Filter for process creation events and look for processes started in a suspended state.
  3. Memory Scanning: EDR solutions with memory scanning capabilities can detect shellcode within the memory of legitimate processes, even if it is not written to disk.
  4. PowerShell Logging: Enable PowerShell script block logging (via Group Policy) to capture the commands used in the attack chain. 
    Enable PowerShell Script Block Logging via Group Policy or Registry
Registry Path: HKLM\SOFTWARE\Policies\Microsoft\Windows\PowerShell\ScriptBlockLogging
Set "EnableScriptBlockLogging" = 1

3. Persistence and Living-off-the-Land (LotL) Techniques

To maintain access to the compromised system, the attackers employ multiple persistence mechanisms and utilize legitimate system tools to evade detection.

  • Startup Folder Scripts: The malicious batch file places files (e.g., ahke.bat, olsm.bat) in the Windows Startup folder. This ensures the malware executes every time the user logs in.
  • Living-off-the-Land (LotL): The attackers extensively use built-in Windows tools to execute their malicious activities. This includes:
  • Windows Script Host (WSH): Used to run `.js` and `.vbs` scripts.
  • PowerShell: Used to download files and execute commands.
  • WebDAV: The TryCloudflare tunnels are used to host a WebDAV server, from which files are downloaded.

Step‑by‑step guide for identifying persistence mechanisms:

  1. Check Startup Folders: Examine the common startup folders for suspicious files. 
    List files in the current user's Startup folder
dir "$env:APPDATA\Microsoft\Windows\Start Menu\Programs\Startup"
List files in the All Users Startup folder
dir "$env:PROGRAMDATA\Microsoft\Windows\Start Menu\Programs\Startup"
  2. Check Scheduled Tasks: List scheduled tasks that could be used for persistence. 
    List all scheduled tasks
schtasks /query /fo LIST /v
  3. Check Run Registry Keys: Examine common Run registry keys for malicious entries. 
    Check current user Run key
reg query HKCU\Software\Microsoft\Windows\CurrentVersion\Run
Check local machine Run key
reg query HKLM\Software\Microsoft\Windows\CurrentVersion\Run
  4. Analyze WebDAV Mounts: Check for any WebDAV shares that are mounted or have been accessed. 
    net use

4. Network Defense and Mitigation Strategies

Defending against these sophisticated attacks requires a multi-layered approach that combines email security, network controls, and endpoint protection.

  • Email Security: Implement advanced email filtering solutions that can inspect links and attachments, even those hosted on trusted services like Dropbox. Consider blocking or closely scrutinizing emails with `.url` attachments.
  • Network Controls: Update network controls and blocklists to include identified TryCloudflare domains and any associated malicious infrastructure. Use Next-Generation Firewalls (NGFWs) to terminate suspicious LNK file transfers and PowerShell connections.
  • Endpoint Protection: Deploy EDR solutions with behavioral analysis capabilities to detect RAT installation activity and abnormal remote access. Ensure that AMSI is enabled and properly configured.
  • Limit Tunnel Usage: Restrict inbound and outbound tunnels to authorized, authenticated infrastructure and enforce allowlists where possible.

Step‑by‑step guide for network-level detection and blocking:

  1. Monitor for TryCloudflare Domains: Use network traffic analysis tools to monitor for connections to `.trycloudflare.com` subdomains. While legitimate, an unusually high volume of traffic to these domains from internal hosts should be investigated.
  2. Block Suspicious URLs: Use a web filter or firewall to block known malicious URLs and patterns.
  3. Analyze Proxy Logs: Review proxy logs for connections to TryCloudflare and Dropbox that are associated with the download of archive files (.zip, .rar).
  4. Use Threat Intelligence: Integrate threat intelligence feeds that provide indicators of compromise (IOCs) related to this campaign, such as the specific TryCloudflare subdomains and file hashes. 
    Example: Using curl to check a URL for potential malicious content (DO NOT EXECUTE UNKNOWN FILES)
curl -I <suspicious_url>

5. IOCs and Threat Hunting Queries

Identifying indicators of compromise (IOCs) is crucial for threat hunting and incident response. Key IOCs for this campaign include:

  • File Names: Rechnung zu Auftrag W19248960825.pdf.zip, load.py, ahke.bat, olsm.bat, new.bin.
  • URL Patterns: Dropbox download URLs (dl[.]dropboxusercontent[.]com), TryCloudflare subdomains (.trycloudflare.com).
  • Process Names: explorer.exe, `notepad.exe` (as injection targets).
  • Command Lines: PowerShell commands used to download files from TryCloudflare.
  • Network Ports: The malware variants communicate with C2 servers over different ports.

Step‑by‑step guide for threat hunting:

  1. Search for Process Injection: Use EDR or SIEM to hunt for processes that were created in a suspended state (CreateProcess with `CREATE_SUSPENDED` flag) and subsequently had memory written to them (WriteProcessMemory).
  2. Search for Suspicious PowerShell Activity: Hunt for PowerShell commands that include -WindowStyle Hidden, -ExecutionPolicy Bypass, or that download files from the internet (e.g., using `Invoke-WebRequest` or Invoke-Expression).
  3. Search for New Python Installations: Monitor for the installation of Python, especially if it is not part of the standard corporate image. The campaign installs a specific version (3.14.0).
  4. Search for Suspicious .URL Files: Hunt for `.url` files in email attachments or on user endpoints. Use a tool like `Sysmon` to log the creation of `.url` files.

What Undercode Say:

  • Key Takeaway 1: The weaponization of legitimate cloud services (Dropbox, Cloudflare) represents a paradigm shift in malware delivery, rendering traditional blocklist-based defenses obsolete. Organizations must adopt a “zero-trust” approach to cloud traffic, assuming that all external connections are potentially malicious.
  • Key Takeaway 2: The use of fileless and in-memory execution techniques (like Early Bird APC injection) highlights the critical need for advanced endpoint protection that focuses on behavioral analysis rather than just signature-based detection. Security teams must prioritize EDR solutions and robust logging (e.g., PowerShell Script Block Logging) to detect these stealthy attacks.

Analysis:

This AsyncRAT campaign is a textbook example of how cybercriminals are evolving their tactics to bypass traditional security perimeters. By abusing the implicit trust placed in services like Dropbox and Cloudflare, attackers can easily deliver malicious payloads without raising immediate red flags. The multi-stage infection chain, heavily reliant on living-off-the-land techniques and process injection, makes it extremely difficult for even well-defended organizations to detect the intrusion. The attackers’ ability to dynamically create and discard TryCloudflare tunnels adds another layer of complexity, as it complicates efforts to block malicious infrastructure at the network edge. This campaign serves as a stark reminder that security strategies must shift from relying on static indicators to a more dynamic, behavior-focused approach that can identify and respond to sophisticated, cloud-abusing threats.

Prediction:

  • -1 The continued abuse of free-tier cloud services by threat actors will force providers like Cloudflare and Dropbox to implement more stringent security controls, potentially impacting legitimate users and increasing operational overhead for businesses.
  • -1 As detection and response mechanisms improve, attackers will likely adopt even more advanced evasion techniques, such as leveraging AI to generate highly convincing phishing lures and automating the creation of ephemeral infrastructure on a massive scale.
  • +1 The increasing sophistication of these campaigns will accelerate the adoption of AI-driven security solutions and Zero Trust architectures, pushing the cybersecurity industry towards more proactive and predictive defense models.

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