dumbstackz

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .dumbstackz
  • Renaming Convention: After encryption, the ransomware simply appends “.dumbstackz” to the original filename and leaves the original extension intact (e.g., Quarterly-Report.pdf becomes Quarterly-Report.pdf.dumbstackz).
    A ransom note (README_TO_RESTORE.txt) is dropped into every affected folder and on the user’s desktop.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First submitted to public malware repositories and ransomware-ID forums in late-September 2021. In-the-wild sightings spiked during November 2021–January 2022 after being bundled with the “Raccoon Stealer” and “ZLoader” crimeware-as-a-service bundles.

3. Primary Attack Vectors

  • Propagation Mechanisms:
    • Classic malspam attachments (“invoice”, “FedEx”, “Voicemail message”) that launch ZIP → ISO → LNK → BAT → PowerShell dropper chain.
    • Exploitation of exposed RDP services brute-forced or bought on underground marketplaces.
    • Trivial SMB sprawl once inside a perimeter; it does NOT exploit patched SMB vulns such as EternalBlue but will ride existing Windows admin shares (ADMIN$ / C$) with harvested credentials.
    • Bundled inside fake game cheats and software-key generators on Discord & YouTube comments.
    • Has been observed piggy-backing on existing ZLoader infections to push the payload via PsExec.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Patch OS & 3rd-party software, especially browsers, MSHTML/IE, MSOffice, and Java.
    • Disable Office macros except from trusted, signed locations; block internet macro execution via Group Policy.
    • Restrict RDP exposure: require 2-FA/VPN, enforce account-lockout, disable TCP/3389 to the Internet.
    • Segment LAN: block SMB (445/139) between user VLANs; limit what local admins can mount.
    • Use application whitelisting (WDAC / AppLocker) to block PsExec, BAT, PowerShell, and unsigned EXE from user-writeable folders.
    • Maintain “3-2-1” backups—three copies, two media types, one copy off-line/immutable (e.g., WORM S3, tape, or disk with the vault-air-gap unplugged).

2. Removal

  • Infection Cleanup (high-level):
    1. Disconnect the device from both LAN & Wi-Fi.
    2. From a clean, bootable USB with WinPE (or Kaspersky, Sophos, Bitdefender rescue disk) scan the disk and let the AV remove the following artefacts:
      • C:\Users\Public\Libraries\dumbstackz.exe
      • C:\ProgramData\new.bat
      • Run-keys “Zaxar” or “OptimizerLens”
    3. Delete the service named “StackTraceService” if present.
    4. Use Microsoft Autoruns to clear persistence; reset local admin account passwords from a known-good domain controller.
    5. Reboot into Safe-Mode-with-Networking; run a second full scan (Windows Defender / Malwarebytes) to remove residual scriptlets or registry entries.
    6. Only reconnect to production network after restoring antivirus real-time protection and confirming the malicious processes are no longer spawning.

3. File Decryption & Recovery

  • Recovery Feasibility: DumbStackz relies on Curve25519 (ECDH) + ChaCha20 for file encryption; private keys are unique per victim and only stored on the attacker’s server. There is currently no free public decryptor.
  • Practical Paths:
    • Restore from clean offline backups—verify newly restored data is NOT re-encrypted.
    • For very small datasets, specialists may resort to raw file carving (PhotoRec, R-Studio) for pictures, Office docs that were not fully overwritten—but this is hit-or-miss.
    • Shadow-copy rollback only works if the ransomware did NOT delete VSS; check vssadmin list shadows before re-imaging.
  • Essential Tools/Patches:
    • Keep Windows fully patched, especially CVE-2021-40444 & CVE-2021-26411 chains used in malspam waves.
    • Update PowerShell to ≥5.1 and set ExecutionPolicy with Constrained Language Mode via Windows Defender Application Control to neuter living-off-the-land stages.
    • Enable Office “Block macros from running in Office files from the Internet” via GPO (admin template of Office ≥2016).
    • Backups: Veeam v11+, CommVault, or native Azure/AWS/S3 with “immutable” / “object-lock” once-per-day snapshots.

4. Other Critical Information

  • Unique Traits:
    • Designed for quick “smash-and-grab” campaigns; spreads laterally inside <20 min, but no data-exfiltration module—so confidentiality is usually not breached (still run DFIR checks).
    • Poor network comms: callback fails behind proxy/SSL-intercept, causing many infections to encrypt without depositing a working master key (but victims receive NO notice, so don’t count on partial decryption).
    • Drops a visible desktop wallpaper bitmap that mocks the victim: “Your files are STACKED – pay in 72 h or they will be ZIPPED FOREVER (StackZ)”.
  • Broader Impact: Mostly hit mid-size manufacturing and logistics companies in EU/US who still allowed external 3389. Nasty reputation in underground forums because the authors occasionally don’t furnish working decryptors even after payment—so most incident-response teams advise against paying and recommend rebuild-from-backup instead.

Operational TL;DR
DumbStackz is a classic “encrypt-everything-in-sight” ransomware with no free decryptor. The only reliable recovery is off-line, versioned backups. Kill its entry paths—disable macro docs from the Internet, stop exposing RDP, enforce network segmentation—and you will stop every sample seen to date. Stay safe, keep those backups air-gapped, and never trust unsolicited email attachments!