xalgord Hunting For Webshell Activity
When to Use
- When proactively hunting for indicators of hunting for webshell activity in the environment
- After threat intelligence indicates active campaigns using these techniques
- During incident response to scope compromise related to these techniques
- When EDR or SIEM alerts trigger on related indicators
- During periodic security assessments and purple team exercises
Detection Gaps & Validation
- File-create hunting misses a lot: shells appended to existing legit
.aspx/.php, stored in a database, loaded as an in-memory IIS module (T1505.004), or served via handler mapping under a benign extension. Hash/signature scans miss obfuscated one-liners like<?php @eval($_POST['x']);?>. - Process lineage is the durable signal: w3wp.exe / httpd / tomcat / php-cgi spawning cmd.exe/powershell/whoami (EID 1, ParentImage = web server) catches even unknown shells; EID 11 FileCreate in webroot by the web-server process (not a deploy pipeline) is high-signal.
- Low-and-slow operators: China Chopper uses tiny intermittent POSTs that hide in netflow — needs web/proxy log content inspection, not volume thresholds.
- Validate: drop a benign test
.aspxthat runswhoami; confirm the w3wp→cmd EID 1 chain and the EID 11 write both fire. - FP tuning: baseline legitimate CI/CD deploys and admin maintenance scripts that legitimately write to webroot.
Prerequisites
- EDR platform with process and network telemetry (CrowdStrike, MDE, SentinelOne)
- SIEM with relevant log data ingested (Splunk, Elastic, Sentinel)
- Sysmon deployed with comprehensive configuration
- Windows Security Event Log forwarding enabled
- Threat intelligence feeds for IOC correlation
Workflow
- Formulate Hypothesis: Define a testable hypothesis based on threat intelligence or ATT&CK gap analysis.
- Identify Data Sources: Determine which logs and telemetry are needed to validate or refute the hypothesis.
- Execute Queries: Run detection queries against SIEM and EDR platforms to collect relevant events.
- Analyze Results: Examine query results for anomalies, correlating across multiple data sources.
- Validate Findings: Distinguish true positives from false positives through contextual analysis.
- Correlate Activity: Link findings to broader attack chains and threat actor TTPs.
- Document and Report: Record findings, update detection rules, and recommend response actions.
Key Concepts
| Concept | Description |
|---|---|
| T1505.003 | Web Shell |
| T1190 | Exploit Public-Facing Application |
| T1059.001 | PowerShell |
Tools & Systems
| Tool | Purpose |
|---|---|
| CrowdStrike Falcon | EDR telemetry and threat detection |
| Microsoft Defender for Endpoint | Advanced hunting with KQL |
| Splunk Enterprise | SIEM log analysis with SPL queries |
| Elastic Security | Detection rules and investigation timeline |
| Sysmon | Detailed Windows event monitoring |
| Velociraptor | Endpoint artifact collection and hunting |
| Sigma Rules | Cross-platform detection rule format |
Common Scenarios
- Scenario 1: China Chopper web shell via IIS vulnerability
- Scenario 2: ASPXSpy through vulnerable upload
- Scenario 3: PHP shell hidden in image file
- Scenario 4: JSP shell via Tomcat manager console
Output Format
Hunt ID: TH-HUNTIN-[DATE]-[SEQ]
Technique: T1505.003
Host: [Hostname]
User: [Account context]
Evidence: [Log entries, process trees, network data]
Risk Level: [Critical/High/Medium/Low]
Confidence: [High/Medium/Low]
Recommended Action: [Containment, investigation, monitoring]