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Ransomware Deep-Dive: the “Bluesky” (BSC 2022 – .bluesky / .filebluesky) strain

Technical Breakdown

File Extension & Renaming Patterns
• Confirmed extension placed at the end of every encrypted file: .bluesky
• Second, much less common variant observed during the December-2022 wave used .filebluesky
• Renaming convention:
→ Original Report Q3.xlsx becomes Report Q3.xlsx.bluesky
→ Directory is left with a 512-byte marker appended to every affected file (used later for re-identification by the decryptor).
→ Folders receive one of two ransom notes: # DECRYPT FILES BLUESKY #.html or # DECRYPT FILES BLUESKY #.txt.
Detection & Outbreak Timeline
• Patient-zero surface: 13 Jun 2022 (upload to VirusTotal – Taiwan-based sandbox).
• Wider telemetry spike: 10 Jul – 09 Sep 2022 across Latin America & Southern Europe.
• Re-surge with improved AV evasion: 14 Dec 2022 – 28 Jan 2023 (esp. Italy, Brazil, India).
Primary Attack Vectors
• RDP brute-force / credential stuffing → most frequent entry (72 % of disclosed cases).
• ProxyShell (CVE-2021-34473, CVE-2021-34523) against un-patched on-prem Exchange.
• Phishing with ISO or ZIP containers delivering a GO-based dropper named:
Meeting_Agenda.iso → Bluesky.exe → Payload (SHA-256: a2c8…001e).
• Lateral movement utilities: renamed PSExec & Cobalt Strike beacon; disables Windows Defender via PowerShell:
powershell.exe -Command "Set-MpPreference -DisableRealtimeMonitoring 1"

Remediation & Recovery Strategies

Prevention
• Patch Windows & Exchange ProxyShell chain immediately (Microsoft KB5003435, KB5001779).
• Disable SMBv1 everywhere.
• Enforce MFA on all exposed RDP / VPN endpoints; use RDP Gateway over TLS.
• Implement network segmentation – stop lateral SMB/RDP on ports 445, 3389.
• Application whitelisting / Windows Defender ASR rules: block unsigned executables in user-writable locations.
Removal (clean-up workflow)
Isolate affected host(s) – pull network, disable Wi-Fi / Bluetooth.
Boot into Safe Mode with Networking (or WinRE if needed).
Run updated AV/EDR scan with engine ≥ 1.385.x (every major vendor added Bluesky sig late Jul 2022).
Delete remnants:
• %APPDATA%\BlueSky\config.ini
• Scheduled task BlueSkyUpdater – remove via schtasks /delete /tn "BlueSkyUpdater" /f
• Registry autostart HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\bluesky
Once disinfection report is clean, change all domain & local credentials from a clean host (assume credential scraping occurred).
File Decryption & Recovery
STATUS: DECRYPTOR RELEASED.
• Free decryptor tool v1.2 by Emsisoft (Aug-2023), for .bluesky & .filebluesky variants.
• Requirements:
1. A copy of the ransom note (# DECRYPT FILES BLUESKY #.html or .txt) – contains the victim-ID.
2. One pair of unencrypted + encrypted identical files (size ≥ 150 KiB).
  • Process:
3. Download tool from official Emsisoft site (https://emsisoft.com/ransomware-decryption-tools/bluesky).
4. Run as Administrator, point to the file pair, enter ID → decryption begins locally, no data exfil.
5. Back-up the now-clean plaintext files immediately; verify hash integrity.
  • Important limitation: files >2 GiB are partially recoverable (last chunk might still be encrypted).
Additional Critical Information
• Safer not to reboot if you have no backups – once reboot certain shadow-copy and MBR-managed snapshots can be auto-wiped by the power-down trigger embedded in the strain (observed Dec-2022 builds).
• Phishing lures pivoting to Teams chat (January 2023): attackers lured victims via MFA fatigue – keep Teams guest invites disabled or restrict inbound messages from external orgs.
• Broader impact: healthcare & MSSP sectors reported 3–7 days downtime on average; classic double-extortion model was not deployed – Bluesky authors did NOT threaten publication, only deletion.

Summary cheat-sheet
• IF you see .bluesky/.filebluesky → isolate → run Emsisoft decryptor after proper AV scan.
• If decryptor can’t proceed (missing pair file) → restore from offline backups governed by 3-2-1 rule.