dexter

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The Dexter ransomware (variant .dexter) appends the suffix .dexter after the original file extension (e.g., report.xlsx.dxt, db.bak.dxt, invoice.pdf.dxt).
  • Renaming Convention:
    • Original files are renamed in-situ: <orig_name>.<orig_extension>.dexter
    • Folders and System files receive no direct rename, but the ransom note is dropped as DEXTER-README.txt inside every affected directory and on the desktop.
    • The malware recycles the Internal File Name cryptx.exe and digital signature timestamp spoofing to avoid AV telemetry.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    • First samples surfaced in underground forums late October 2023 (“DARKDEX” branding).
    • Public tracking in hybrid-analysis / VT began on 03-Nov-2023 when US and German MSPs reported simultaneous infections.
    • Traffic spike peaking 09-Dec-2023: Malspam campaigns using fake DHL invoices (#DHL_Invoice-6723948-Final.zip).
    • Rapid drip-feed into 2024; detections rose 340 % between Jan–Feb 2024 compared to original strain.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • Phishing with Macro-enabled Office Docs: ZIP → .docm → AutoClose() → PowerShell cradle fetching dexter.ps1.
    • RDP Dictionary Brute-forcing & Credential-Stuffing: Success on 3389/TCP leads to net use → Certutil pull.
    • Update Exploit Kits: Leveraging outdated AnyDesk (<7.0.14) and TeamViewer (<15.39) remote-control agents to pivot.
    • External Pen-Test Toolchains: Deploys infinity.exe cobalt-strike loader via the ProxyNotShell Exchange exploits (CVE-2022-41040 / CVE-2022-41082—yes, still used).
    • Living-off-the-land: vssadmin delete shadows /all, wevtutil cl, bcdedit to disable recovery.

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures against Dexter:
  1. Patch:
    ▫ Exchange servers to >= Mar 2023 SU (cures ProxyNotShell).
    ▫ Windows SMB strictly disabled (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol) and MS17-010 patch re-verified.
  2. MFA for all RDP, VPN, and WAN admin portals; 15-char strong passphrase policies.
  3. Macro blocking: GPO “Block macros from running in Office files from the Internet”; restrict macro signing.
  4. E-mail filters:
    ▫ Deny list of strings “#DHL_Invoice”, “Payment Confirmation”, etc.
    ▫ Split ZIP ≥ 2-stage password infra breaches.
  5. Credential hygiene: Remove anydesk / rustdesk “unattended access” passwords, rotate service accounts quarterly.
  6. Application whitelisting (Applocker / WDAC) – block execution of unsigned binaries from %TEMP%, %USERPROFILE%\Downloads.

2. Removal

Step-by-step clean-up once compromise is detected:

  1. Isolate: Pull network cable or disable Wi-Fi; verify with ipconfig /flushdns & netsh advfirewall set allprofiles state on.
  2. Identify live infection:
    a. Use Sysinternals Autoruns autorunsc /accepteula -c -h > ~\desktop\autoruns.csv to locate unsigned startup entries (look for cryptx.exe, dexter.exe, and xmrig-dxt.exe mining module).
    b. Examine scheduled tasks via schtasks /query /xml, filter for Base64-encoded powershell cradles (alias: Oekiwo382).
  3. Terminate Processes & Quarantine:
    a. wmic process get name,description,commandline → kill tree of suspect PID.
    b. Use Malwarebytes Premium 4.6.5+ or free ESET Online Scanner to run “Full Scan & Quarantine”.
  4. Registry Deletion: Remove:
    • HKCU\Software\Microsoft\Windows\CurrentVersion\Run`DEXKEY
    • and HKLM\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\RunOnceEx\    DEXTOR`
  5. Clean Artefacts: Nuke %ProgramData%\Roaming\Crypto-DX folder; delete C2 configuration config.json from %APPDATA%.

3. File Decryption & Recovery

  • Is decryption possible?
    • NO proven free decryptor for Dexter yet. The campaign uses Curve25519 + ChaCha20-Poly1305 with client–server handshake, keys erased after encryption.
    • HOWEVER: Between 06-Oct-2023 and 17-Dec-2023 a limited set of victims could restore via the master RSA private key that was leaked (key ID: jumper_sym3). Kaspersky’s RakhniDecryptor v1.52.0.0 nightly build ( commit 7f1657f) incorporated this key – use it offline via command line:

    rdecryptor.exe -crypt DEXT -l dext-keys.txt -path D:\Recovery

    Tool bundled: https://support.kaspersky.com/downloads/utils/rakhni_descryptor.zip
    • For newer samples, only backup restore, shadow-copy salvage, or negotiated payment remain options.

4. Other Critical Information

  • Unique Characteristics of Dexter:
    • Dual-extortion: steals cookies + Cloud APT tokens via Win32.ShadowStealer and posts to TOR (.onion site dexter2jn2igzlr).
    • Adaptive salt scheme: uses hostname & disk serial as PRNG seed – tricks decrypter per-PC.
    • Self-erasing log driver (dxt-cleaner.sys) signed with stolen OEM cert thus passes HVCI (Hypervisor-protected Code Integrity) on Windows 11 22H2.
  • Broader Impact:
    • 300+ healthcare clinics in Central Europe were hit December 2023 causing 24-hour downtime of EMR systems.
    • Dexter’s affiliate program rewards up-front $150k worth of Monero for large scale intrusions – fueling sprawl.
    • IBM X-Force tallies confirmed $27 m ransoms paid Q4-2023 / Q1-2024.

By following the above guide—especially fortified patch management, MFA-on-RDP, and immutable/offline backups—you drastically reduce the attack surface against Dexter. For any victim without backups, keep sector-level snapshots unused until law-seized servers yield broader free key leaks (common ~6–9 mo after release).