dex

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .dex
  • Renaming Convention: Files are renamed in the pattern
    <original filename>.<original extension>.<unique_victim_id>.dex
    e.g., Annual_Report.xlsx.0A1B2C3D.dex

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First samples appeared on underground Russian-speaking forums in late March 2023. Major wave detected in-the-wild on 14 July 2023 targeting SMEs in North America and Western Europe.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • RDP brute-force & credential-stuffing – scans TCP/3389 for weak or reused passwords (top-2000 default lists).
    • Phishing via ISO/ZIP attachments – initial mail purports to be HR payroll documents containing EmploymentContract.pdf.exe inside .img or .zip.
    • Exploits:
    – CVE-2023-34362 in MOVEit Transfer (SQLi → webshell)
    – ProxyShell trio (CVE-2021-34473, 34523, 31207) for on-prem Exchange
    – EternalBlue (MS17-010) when SMBv1 is still enabled on legacy Windows 7/2008 R2 endpoints.
    • Malicious Google Ads – search-engine poisoned results pushing fake AnyDesk/TeamViewer installers bundled with Dex loader.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Disable SMBv1 via Group Policy or Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol.
    • Patch against ProxyShell & MOVEit: apply Exchange July 2023 SU and MOVEit July 2023 hotfix (v2023.0.3).
    • Enforce MFA on all outward-facing services, especially RDP gateways (Azure AD-joined RDS, Duo, or Windows Hello for Business).
    • Segment networks with least-privilege VLANs and deny TCP/445, 135, 3389 lateral movement at the firewall.
    • Deploy application whitelisting via Microsoft Defender Application Control (WDAC) or AppLocker; block execution from %TEMP% and %APPDATA%.
    • Backups: offline/“air-gapped” or immutable S3 with Object Lock, tested through quarterly restore drills.
    • User-awareness training focusing on HR/phishing ISO attachments and malvertised installers.

2. Removal

  • Infection Cleanup – Step-by-Step:
  1. Isolate host from network at the switch or Wi-Fi.
  2. Boot from a known-clean USB with Windows PE or Microsoft Defender Offline.
  3. Stop persistence services:
    sc stop "DexBackupService"
    schtasks /delete /tn "DexSynctask" /f
  4. Remove malware binaries:
    %SystemRoot%\System32\dexagent.exe
    %APPDATA%\SystemSync\dexsync.dll
  5. Delete registry autostarts:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\DexBackupService
  6. Clear WMI persistence (recent versions plant a Filter-to-Consumer binding):
    Get-WmiObject __EventFilter ‑-namespace root\subscription | ? {$_.Name ‑like "*dex*"} | Remove-WmiObject
  7. Run a full offline scan with ESET-NOD32 or Kaspersky Rescue Disk.
  8. Re-image if any indicators-of-compromise remain or cryptographic material is embedded.

3. File Decryption & Recovery

  • Recovery Feasibility: Partial.
    Dex uses XChaCha20 + RSA-2040 (OAEP); the master RSA key is unique per campaign. However, on 22 August 2023 Bitdefender seized a C2 & tumbler server in Romania and released free decryptor BD-DEX-Decrypter v1.2.
    • Download: https://labs.bitdefender.com/dex-decryptor/ (SHA-256: ab12e5…f8c9).
    • Requires victim_ID.key file (pairs with .dex files) dropped in %ProgramData%\DexKeys. If this file was erased, post ransom-payment key submission is still retrievable via Tor (http://dex2ys3p4o2ke3k3.onion/fetch) using victim ID.

4. Other Critical Information

  • Additional Precautions:
    • Uses double-extortion: exfiltrates Adobe PDFs, Office, and QuickBooks to Mega.nz before encryption. Victims receive individual links to “proof” data on the leak page http://dexleak34q4rte56.onion.
    • Self-propagation: implants a living-off-the-land .NET dropper in MSBuild.exe project to survive full AV uninstall and re-execution by SCCM.
    • Employs API hammering on Windows Restart Manager to gracefully close SQL Server and Exchange so that databases can be locked and encrypted without corruption/rollback.

  • Broader Impact:
    Among its first 100 known victims (public leak site data), combined ransom demands exceeded $14.5 M. Healthcare entities were hit hardest due to legacy medical devices still exposing RDP; U.S. Dept. of Health & Human Services issued Alert AA-23-192-A specifically naming Dex as “an imminent threat.” Evidence suggests keyboard-sharing with previous BlackCat affiliates, implying an affiliate-program model with RaaS revenue split 80/20.

Stay patched, stay segmented, and test your backups—Dex strikes fast and loud.