What is web shell? A clear guide for cybersecurity professionals

Web shells can be hard to spot, but it’s possible when you know what to look for:

• Unusual files: Keep an eye out for files with strange names or extensions in web directories.

• Unexpected server activity: Spikes in resource usage, odd processes, or unplanned data transfers can be red flags.

• Security tool alerts: Modern endpoint detection and response (EDR) and intrusion detection systems (IDS) can pick up on known shell patterns.

When a shell is discovered:

• Quarantine the affected system. Don’t just delete the web shell; the attacker may have added other backdoors or created new accounts.

• Analyze server and application logs. Check for signs of lateral movement or data exfiltration.

• Patch all exploited vulnerabilities. This could mean updating software, changing passwords, or fixing misconfigurations.

• Consider restoring from a clean backup. If in doubt, a fresh start ensures nothing malicious lingers behind.

• Notify the necessary parties. This could include your IT or security team, affected users, and, if required, regulatory authorities.

The National Security Agency (NSA) and CISA have published in-depth guides on detecting and mitigating web shells, which include step-by-step incident response checklists.