Incident Response | Root Cause Analysis | Malware | Windows Security
Identified and resolved a persistent W32/Virut infection that was reinfecting workstations daily across two corporate branches — after weeks of failed remediation attempts.
A corporate file server at DHmont (Lima, Perú) was compromised by W32/Virut, a polymorphic file infector. Despite repeated Bitdefender scans and PC reformats, workstations continued experiencing BSODs, random reboots, and slowness within days of being cleaned.
The IT team was trapped in an endless loop — because no one had identified the root cause.
The insight: The file server was never remediated. Every formatted PC reconnected to the infected server and reinfected immediately.
Environment: Windows XP corporate network — two branches (Lima, Perú)
Malware: W32/Virut — polymorphic file infector
Symptom: BSOD (svchost.exe) + random reboots + slowness — recurring
Root cause: Infected file server reinfecting all workstations on reconnect
Why AV failed: Virut corrupts files when cleaned → BSOD
Fix: Server remediation FIRST → OS migration on all workstations
MITRE: T1080 · T1027 · T1554
File server (infected)
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PC formatted and reinstalled
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PC reconnects to network → accesses shared folder
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Virut reinfects via .exe files on server
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BSOD returns within days
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└──────────────────────── endless loop
Breaking the loop required remediating the server first — something the standard procedure had never addressed.
W32/Virut injects itself into .exe and .dll system files. When antivirus removes the malicious code, the host file is left corrupted and unusable. Loading a corrupted svchost.exe at startup causes an immediate BSOD.
Virut is one of the few malware families where the industry consensus is: do not clean, reinstall.
- Isolate file server from network
- Remediate server — full OS reinstall
- Migrate workstations to newer Windows version (XP was EOL since 2014)
- Reconnect workstations only after server confirmed clean
- Deploy antivirus with valid licenses across all endpoints
👉 Read the complete technical writeup
Includes: root cause analysis methodology, malware classification, reinfection chain diagram, MITRE ATT&CK mapping, remediation plan, and lessons learned.
📄 README.md ← This file
📄 virut-writeup.md ← Full technical writeup
Alvaro Martinez | IT Infrastructure Specialist
Google Cybersecurity · Cisco CyberOps · ~10 years experience
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