How to Detect and Mitigate Mass Encryption Attacks: Lessons from UMMC

The University of Mississippi Medical Center (UMMC) recently faced a severe encryption-based cyber incident that forced the organization to take systems offline. While the investigation is ongoing, the event highlights a harsh reality for healthcare defenders: ransomware and encryption attacks remain a primary threat to patient safety and operational continuity.

For defenders, understanding the mechanics of encryption-based attacks is not about studying the attacker's tools, but about recognizing the signs of compromise early enough to stop the exfiltration or encryption process. This post analyzes the technical implications of the UMMC incident and provides actionable queries and remediation steps to protect your environment.

Technical Analysis

The incident at UMMC is described as an "encryption-based cyber incident," which typically indicates a ransomware variant designed to lock files on servers and endpoints. In a healthcare setting, these attacks often target:

• Electronic Health Records (EHR): Databases containing sensitive patient data.
• Diagnostic Systems: Imaging and lab result repositories.
• Backend Services: Authentication and directory services.

Attackers usually gain access through exposed remote services (like VPNs or RDP), phishing, or unpatched vulnerabilities. Once inside, they move laterally to escalate privileges. The actual encryption phase is often the final step in a longer dwell time, preceded by data theft. The severity is high because encryption of medical records can directly delay patient care, making the recovery pressure intense and often leading to higher ransom demands.

Defensive Monitoring

Detecting a mass encryption event requires identifying rapid, anomalous file system changes or suspicious process executions. Defenders should look for processes that modify hundreds of files in seconds or the use of native administrative tools to delete backups (e.g., vssadmin).

Below are KQL queries for Microsoft Sentinel/Defender and a PowerShell script to help detect these indicators.

Detect Mass File Encryption (KQL)

This query identifies processes that have modified or created a high volume of files within a short time window, a common behavior of ransomware.

DeviceFileEvents
| where Timestamp > ago(1h)
| where ActionType in ("FileCreated", "FileModified")
| extend FileExtension = tostring(parse_path(FileName).Extension)
| where isnotempty(FileExtension)
| summarize FileCount = dcount(FileName) by DeviceId, DeviceName, InitiatingProcessFileName, InitiatingProcessAccountId, bin(Timestamp, 5m)
| where FileCount > 50 // Threshold for suspicious activity
| order by FileCount desc

Detect Backup Deletion Attempts (KQL)

Attackers often use vssadmin or wmic to delete shadow copies to prevent recovery. This query alerts on such commands.

DeviceProcessEvents
| where Timestamp > ago(1h)
| where FileName in~ ("vssadmin.exe", "wmic.exe", "wbadmin.exe")
| where ProcessCommandLine has_any ("delete", "shadow", "resize", "shadowstorage")
| project Timestamp, DeviceName, InitiatingProcessFileName, FileName, ProcessCommandLine, InitiatingProcessAccountId

Local Audit for Rapid File Changes (PowerShell)

This PowerShell script can be run locally on a suspected server to audit directories for bulk modifications.

$Path = "C:\\ImportantData" # Change to target path
$TimeSpan = (Get-Date).AddMinutes(-30)

Write-Host "Scanning for files modified in the last 30 minutes in $Path..."

$ModifiedFiles = Get-ChildItem -Path $Path -Recurse -ErrorAction SilentlyContinue | 
    Where-Object { $_.LastWriteTime -gt $TimeSpan }

if ($ModifiedFiles.Count -gt 100) {
    Write-Warning "Potential Ransomware Activity Detected: $($ModifiedFiles.Count) files modified recently."
    $ModifiedFiles | Select-Object FullName, LastWriteTime | Export-Csv -Path "C:\\Temp\\AuditResult.csv" -NoTypeInformation
} else {
    Write-Host "No bulk modification detected."
}

Remediation

To protect against encryption-based attacks like the one seen at UMMC, organizations must adopt a "Zero Trust" posture and assume breach. Implement the following steps immediately:

1. Strict Network Segmentation: Ensure that critical medical devices and EHR systems are on isolated VLANs. Ransomware should not be able to jump from a user workstation to a database server.

2. Implement Immutable Backups: Use WORM (Write Once, Read Many) storage or object lock technology for backups. If vssadmin is used to delete local shadow copies, immutable cloud backups ensure you can still recover.

3. Disable Unused Protocols: aggressively disable SMBv1 and restrict RDP access. Require VPNs with MFA for all remote access.

4. Patch aggressively: Prioritize patching of known vulnerabilities in internet-facing appliances (VPN, Firewalls) and operating systems.

5. Deploy Anti-Ransomware Policies: Configure Microsoft Defender or your EDR to enable "Ransomware protection" (Controlled folder access) which prevents unauthorized apps from writing to key directories.

Related Resources

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