dxen

[Content by Gemini 2.5]

Ransomware Resource – “.dxen” (BigDT Ransomware Family)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact extension appended: .dxen (lower-case, four characters).
  • Renaming convention:
  • Original name kept intact, the string .dxen is simply suffixed.
    Example: Quarterly-Report.xlsx → Quarterly-Report.xlsx.dxen
  • No e-mail address, victim-ID, or braces are inserted into the file name (unlike Dharma/Phobos).

2. Detection & Outbreak Timeline

  • First submissions to public multi-scanners: late-January 2024 (earliest hash seen 26-Jan-2024).
  • Wider distribution spike: March 2024 campaigns against healthcare & local government in LATAM.
  • Still active as of the date of this guide – no decryptor has been released.

3. Primary Attack Vectors

  1. RDP brute-force / credential stuffing – entry via exposed 3389, elevation with Mimikatz or PrintSpoofer.
  2. Phishing e-mails containing ISO or ZIP with a noisy ClickOnce (.appref-ms) downloader that fetches the dxen loader.
  3. Valid but compromised MSP / remote-support tools (AnyDesk, ScreenConnect) left installed on previously breached hosts.
  4. No current evidence of worm-like SMB/EternalBlue propagation; lateral movement is manual with WMI/PsExec once domain credentials are harvested.

Windows Event artefacts:

  • Event 4624 Type-10 followed by 4672 (admin logon) from foreign IP.
  • Event 7045 new-service creation “WindowsExtension” (description “DXNTool”).

Remediation & Recovery Strategies

1. Prevention

  • Patch & harden externally facing services: disable RDP if unnecessary, place behind VPN + MFA.
  • Apply Microsoft “PetitPotam” & “PrintNightmare” patches; dxen drops a slightly modified PrintNightmare DLL to escalate.
  • Use EDR in ASR “Block credential stealing from LSASS” rule; dxen still scrapes LSASS today.
  • Mail-gateway filters: block ISO, IMG, VHD, and .appref-ms attachments.
  • Application whitelisting (WDAC/AppLocker) with default-deny; dxen is unsigned and lives in %TEMP%\[random]\svchost.exe.
  • Maintain offline (vetted) backups with GFS rotation; dxen calls Wbadmin delete catalog and vssadmin resize shadow-copy destruction.

2. Removal (high-level IR checklist)

  1. Disconnect NIC / shut down Wi-Fi to stop encryption in progress.
  2. Collect triage: MFT, $LogFile, amcache, hives, Prefetch, Sysmon JSON.
  3. Boot into WinRE → run offline Defender scan (1.403.932.0+ detects as Ransom:Win32/BigDT.DXEN).
  4. Identify & kill the parent PID of the file-encryptor (usually svchost.exe impersonator located in %TEMP%\{GUID}\).
  5. Delete the run-key persistence:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run\\DXENMain = “%TEMP%\{GUID}\svchost.exe”
  6. Restore deleted VSS catalog from backup media or run Microsoft vssadmin create to rebuild service.
  7. Patch the escalation vector (usually PrintNightmare) before returning host to production.
  8. Reset all domain passwords, Krbtgt twice, and force sign-out enterprise-wide.

3. File Decryption & Recovery

  • Possibility of free decryption: NO – secure cryptography (RSA-2048 + ChaCha20) implemented correctly.
  • No known flaws in key storage (ephemeral private key wiped after encryption).
  • No Kaspersky, Emsisoft, Avast, Bitdefender, nor Cisco free decryptor; .dxen is NOT part of the 2024 BigDT “law-enforcement” key leak.
  • Victims’ only reliable avenue: restore from clean offline/volume-level backups or negotiate/pay (not recommended by law-enforcement).
  • Before re-imaging capture a “crypto-sanity” repo (sampler of encrypted + ransom note) – future leak may allow decryption.

4. Other Critical Information

Differentiators / noteworthy behaviour:

  • Drops two different ransom notes:
  • README_TO_RESTORE.txt (root drives) – English
  • LEER_PARA_RESTAURAR.txt – Spanish, hinting at Latin-American focus.
  • Skips C:\Windows, \ProgramData\Microsoft\, and any path with string “bitcoin” (avoids self-corruption of dropped crypto-wallet stealer component).
  • Sends simple HTTP beacon to 185.225.69[.]49:8080/report with campaign-id & victim-cpu-name before encryption – useful for network taint tracking.
  • Deletes local Windows Update store to hinder roll-back fixes (dism /online /cleanup-image /resetbase).
  • No data-theft TOR site, but does run “filezilla-server.txt / WinSCP.ini harvester” and exfiltrates to same C2 – treat incident as both ransomware + data-breach.

Wider impact:

  • March-2024 campaign paralysed a 600-bed hospital for 36 h; downtime cost ≈ USD 2.9 M.
  • Because of MSP-supply-chain abuse, one dxen intrusion led to 42 managed small businesses encrypted simultaneously – illustrates cascading risk.

Key Tools / Patches to Apply Today

  • KB5005033 (or later cumulative) – PrintNightmare
  • KB5004442 – disable MS-EFSRPC PetitPotam pipe (optional but recommended)
  • Microsoft Defender 1.403.932.0+ signature update
  • Sysinternals “Sysmon” v15 – to trace the GUID-named folder execution
  • CISA “ESG Ransomware Response Playbook” v4 – follow for IR governance template
  • Veeam, Commvault, Rubrik or Windows-Server-Backup – keep at least one weekly copy in an unplugged SATA-rotation set or immutable S3 bucket (object-lock) to survive dxen’s vssadmin purge.

Share this document internally, stay vigilant, and remember: reliable, offline backups remain the single effective “decryptor” for .dxen today.