Anthropic-Cybersecurity-Skills/skills/hunting-for-webshells-in-web-servers/SKILL.md

name, description, domain, subdomain, tags, version, author, license

name	description	domain	subdomain	tags	version	author	license
hunting-for-webshells-in-web-servers	Detect webshells planted on web servers by scanning for high-entropy files, suspicious PHP/JSP/ASP patterns (eval, base64_decode, system, passthru), recently modified files in web roots, and anomalous file sizes. Uses Shannon entropy calculation to flag obfuscated payloads and regex pattern matching against known webshell signatures.	cybersecurity	security-operations	hunting for webshells web	1.0	mahipal	MIT

Instructions

1. Install dependencies: pip install yara-python
2. Identify web server document roots to scan (e.g., /var/www/html, /opt/lampp/htdocs).
3. Run the agent to scan for webshells:
   • Shannon entropy analysis flags files with entropy > 5.5
   • Pattern matching detects eval(), base64_decode(), system(), passthru(), shell_exec()
   • File modification time analysis finds recently changed files
   • Extension filtering targets .php, .jsp, .asp, .aspx, .cgi, .py files

python scripts/agent.py --webroot /var/www/html --output webshell_report.json

Examples

High-Entropy PHP Webshell Detection

File: /var/www/html/uploads/img_thumb.php
Entropy: 6.12 (threshold: 5.5)
Patterns matched: eval(), base64_decode(), str_rot13()
Last modified: 2025-12-01 03:42:00 (outside business hours)
Verdict: SUSPICIOUS - likely obfuscated webshell