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Cybercriminals are evolving Fake CAPTCHA attacks using homoglyphs and Unicode tricks to bypass detection. Instead of a simple checkbox, victims see a CAPTCHA prompting them to press keys, triggering malicious code execution.
Evolution of Fake CAPTCHA Bypass Techniques
1️⃣ Early Detection (Basic Strings)
- Malicious CAPTCHAs used plain text like `”I am not a robot”` in command lines.
- Easy to detect via simple string matching.
- Example: https://lnkd.in/eqvKt4xU
2️⃣ Homoglyph Substitution
- Attackers replaced Latin letters with visually identical Unicode characters:
– `not` → `nοt` (Greek omicron,U+03BF)
– `robot` → `rоbоt` (Cyrillic o,U+043E) - Example: https://lnkd.in/einU2mGn
3️⃣ Zero-Width & Directional Unicode
- Added invisible characters to evade automated detection:
- Zero-Width Space (
U+200B) - Right-to-Left Override (
U+202E) → Reverses text (ABCbecomesCBA). - Example: https://lnkd.in/evxmY3vP
You Should Know: Detection & Mitigation
🔍 Regex to Detect Fake CAPTCHA
[regex]
r[ ][oоο][ ]b[ ][oоο][ ]t
[/regex]
🛡 Linux Command to Check for Unicode Tricks
echo -n "rоbоt" | xxd Inspect hex for homoglyphs
🖥 Windows PowerShell Unicode Detection
[System.Text.Encoding]::UTF8.GetBytes("nοt") | Format-Hex
📌 CyberChef Recipe
Use CyberChef to analyze Unicode manipulation:
– “To Hex” → Check for hidden characters.
– “Remove Diacritics” → Normalize text.
Threat Intelligence Lookup (IOCs)
- Query Fake CAPTCHA Hashes: https://lnkd.in/eHfG8VgA
What Undercode Say
Fake CAPTCHA attacks leverage Unicode deception to evade traditional security tools. Defenders must:
– Monitor command lines for unusual Unicode patterns.
– Use regex with hidden character support for detection.
– Leverage sandbox analysis (e.g., ANY.RUN) to study behavior.
Expected Output:
Fake CAPTCHA detected: rоbоt (Cyrillic 'o' substitution) Unicode manipulation found: Zero-Width Space (U+200B)
Related URLs:
References:
Reported By: Ouardi Mohamed – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
