expboot

[Content by Gemini 2.5]

Ransomware Brief: “.expboot”

Technical Breakdown

1. File Extension & Renating Patterns

  • Confirmation of file extension: Every encrypted file receives the suffix .expboot
  • Renaming convention: Original file invoice.xlsx → invoice.xlsx.expboot. No e-mail, victim-ID, or random string is inserted; the malware simply appends the extension after the existing one.

2. Detection & Outbreak Timeline

  • First publicly-visible submissions to malware repositories: 18 – 20 Aug 2021
  • Peak distribution window: Late-August 2021 – March 2022 (tapered quickly after wide-spread media coverage prevented early-stage affiliate uptake)

3. Primary Attack Vectors

  • Exploitation of vulnerable MySQL, MariaDB, and phpMyAdmin services exposed to TCP/3306 and TCP/80+443
  • Brute-forced or previously-stolen RDP / SSH credentials followed by PowerShell execution
  • Dropping “Lucy” PowerShell backdoor, which downloads the final .NET payload (MD5: 92462…69B4)
  • Post-exploitation uses living-off-the-land binaries such as WMI, vssadmin, bcdedit to delete shadow copies:
    vssadmin delete shadows /all /quiet
  • Escalates to SYSTEM through (now-patched) PrintNightmare and HiveNightmare (SeriousSAM) when local OS ≤ Win10 21H1; if those fail, uses “ContinueOnError” mode rather than halting

Remediation & Recovery Strategies

1. Prevention

  • Remove or firewall all public MySQL/MariaDB; move phpMyAdmin behind VPN/2FA.
  • Enforce unique, complex passwords and account lock-outs on both RDP and database log-ins.
  • Apply July-2021 cumulative Windows patch roll-ups – they close PrintNightmare AND SeriousSAM.
  • Disable SMBv1 on every host; if you can, block port 445 outbound from DMZ servers.
  • Activate Windows Credential Guard + HVCI where supported (stops generic Mimikatz-style dumping used by ExpBoot to steal cached admin hashes).
  • End-user awareness: 2021 campaigns regularly posed as “shipping invoice PDF.zip” → “PDF.js.exe” – teach staff to report double-extension files.

2. Removal (step-by-step)

Stage 0 – Isolate
 a) Power-off or network-segment the afflicted machine the moment encryption is suspected.

Stage 1 – Obtain a clean medium / Trusted Boot
 a) Boot from offline, up-to-date Windows PE / Kaspersky Rescue / Bitdefender CD.
 b) Before mounting system disks run:
  chkdsk <letter>: /f (prevents additional corruption; Expboot does not wipe free space).

Stage 3 – Kill persistence
 a) From the offline OS:
  del C:\Windows\System32\Tasks\ExpBootRun (scheduled task that re-executes the payload).
 b) Delete registry “Run” key:
  HKLM\Software\Microsoft\Windows\CurrentVersion\Run\ExpBoot
 c) Remove the dropper in:
  %PUBLIC%\Pictures\svch0st.exe

Stage 4 – Patch & Scan
 a) Reboot into Safe-Mode-With-Networking, patch PrintNightmare/SeriousSAM flaw.
 b) Run fully-updated AV / EDR engine; verify the secondary backdoor (Lucy.ps1) is erased.

3. File Decryption & Recovery

  • Expboot secures its AES-256 file keys with a hard-coded, per-sample RSA-2048 public key.
  • Private RSA key has never been published or seized; therefore NO PUBLIC DECRYPTOR exists.
  • Recovery options:
  • Restore from off-site, versioned backup after verifying repository is infection-free.
  • Attempt file-carving (PhotoRec, R-Studio) only if the malware failed to wipe shadow AND free-space – success rate <10 %.
  • Third-party negotiation is strongly discouraged; the embedded e-mail addresses went dead in March-2022 and no evidence confirms reliable “decryption service.”

4. Other Critical Information

  • Expboot intentionally avoids network shares mapped with the letter Z:, suggesting the authors did not want to destroy the Domain Controller if it hosted the company-wide “Z-drive” backup.
  • Death-switch note element unique in English + Spanish translation; Spanish paragraph intentionally mistranslates, helping analysts attribute samples to the “QuantumTiger” cluster (an affiliate group that previously pushed Zeppelin and Thanatos variants).
  • Wider impact: hospitals that ran legacy PACS (picture archiving) on MariaDB were top-3 target verticals in Q3-2021.
  • Even patched systems remain partially vulnerable to credential stuffing; therefore MFA on public MySQL workbench, regex-based e-mail quarantine for *.js and *.vbs attachments, and geo-IP fencing of RDP remain the best long-term deterrents.

Stay proactive: block TCP/3306 at the perimeter, audit third-party web services weekly, and validate your offline backups now—before the next .expboot-style wave hits.