A recent investigation by Trend Micro’s Managed XDR team uncovered a sophisticated web shell attack targeting Internet Information Services (IIS) servers. The threat actors exploited worker processes to exfiltrate sensitive data, demonstrating the persistent risk posed by web shells—malicious scripts that enable remote administration. This incident underscores the critical need for robust detection mechanisms and proactive hardening of internet-facing web servers.
Key Findings for Security Leadership
The investigation revealed several critical aspects of the attack. Attackers deployed a cmd.asp web shell, enabling arbitrary command execution on compromised systems. They leveraged IIS worker processes to maintain persistent access and exfiltrated data using encoded PowerShell commands. Trend Micro’s XDR platform played a pivotal role in detecting endpoint anomalies, which led to successful threat containment before significant damage occurred.
Technical Breakdown of the Attack Chain
Initial Compromise Through Web Shell Deployment
The intrusion began with attackers uploading a web shell (cmd.asp) to the target IIS server. Web shells provide a stealthy backdoor capability, often evading traditional security controls by blending with legitimate web traffic. The ASP-based script allowed execution of arbitrary commands through server-side object creation.
Post-Exploitation Tactics and Techniques
After establishing initial access, the threat actors executed a multi-stage attack:
- Created a reverse TCP shell for interactive system access
- Utilized encoded PowerShell commands for command-and-control communications
- Modified user accounts by creating new credentials and elevating privileges
- Exfiltrated sensitive data through the compromised IIS worker process
Detection and Response With Managed XDR
Trend Micro’s platform identified the threat through cross-layer correlation of multiple detection indicators:
| Detection Indicator | XDR Response |
|---|---|
| Anomalous IIS process spawning cmd.exe | Process tree analysis flagged suspicious parent-child relationships |
| Base64-encoded PowerShell execution | Command-line auditing detected obfuscation patterns |
| Unusual network connections from w3wp.exe | Network traffic analysis identified data exfiltration |
Security Implications and Defensive Recommendations
This incident highlights several critical security considerations for organizations:
Web Server Hardening Requirements
The attack exploited insufficient input validation controls. Regular security audits should examine file upload handlers, IIS request filtering rules, and anonymous authentication settings. Implementing strict access controls and monitoring for web shell indicators can significantly reduce attack surfaces.
Practical Mitigation Strategies
Security teams should implement these defensive measures:
Web Shell Detection
# Find ASP files modified in last 7 days with suspicious keywords
find /var/www/ -name "*.asp" -mtime -7 -exec grep -lE "WSCRIPT\.SHELL|Server\.CreateObject" {} \;
Enhanced IIS Log Monitoring
Create alerts for these suspicious patterns:
POSTrequests to unusual ASP files with long parameter values- HTTP 200 responses to ASP files in non-standard directories
Process Restriction Policies
Implement application control solutions to block:
cmd.exespawned byw3wp.exepowershell.exewith base64-encoded arguments
Conclusion and Additional Resources
This investigation demonstrates the ongoing threat posed by web shells in enterprise environments. While Trend Micro’s Managed XDR successfully contained this incident, organizations must remain vigilant against these TTPs—particularly anomalous IIS worker behaviors and living-off-the-land binary (LOLBin) abuse.
For further technical details, refer to Trend Micro’s research blog or join the discussion among security professionals on Reddit’s SecOpsDaily community.
