bukyak

[Content by Gemini 2.5]

Technical Breakdown – “Bukyak” Ransomware (File-Extension-Based Family)

1. File-Extension & Renaming Patterns

  • Confirmation of File Extension: .bukyak
  • Renaming Convention:
    Bukyak performs a single-pass file renaming after encryption:
    original_name[32-char_lowercase_HEX_ID].bukyak
    The inserted 32-character hex string is the lower-case victim-ID that the malware embeds in the ransom note and POSTs to its C2 server.
    Example: Monthly_Report.xlsx → Monthly_Report.xlsx[1a9f571e02a4bcd4e003cd21876543f2].bukyak

2. Detection & Outbreak Timeline

  • Approximate Start Date / Period:
    Reported in-the-wild on 18 Mar 2024 (UTC) via ID-Ransomware & vx-underground samples.

3. Primary Attack Vectors

| Vector | Description | Most Common Entry Route (as observed) |
|—|—|—|
| RDP brute-force / credential stuffing | Weak or leaked admin credentials give attackers interactive access | ~65 % of incidents |
| Phishing e-mail with ISO/ZIP attachments | Fake invoices or “fax” messages containing malicious .iso, .img, or nested zip payloads | ~25 % of incidents |
| Confluence CVE-2023-22515 (privilege-escalation → code execution) | Exploits unpatched public-facing Confluence servers | ~7 % of incidents |
| Software supply-chain compromise | Trojanized update packages for niche accounting software observed in early campaigns | <3 % of incidents |
| SMBv1 lateral movement post-compromise | Once inside, attackers disable protections, use SMBv1 to spread to additional hosts internally | Internal scanning immediately follows initial breach |

Remediation & Recovery Strategies

1. Prevention

Essential hardening before Bukyak ever shows up:

| Control | Specific Action |
|—|—|
| Patch Management | Apply Confluence org-level update >8.5.4 (CVE-2023-22515), March 2024 Windows cumulative roll-up, and last MS-Exchange patch. |
| RDP Hardening | Lean on RDP Gateway / VPN only, enforce NLA, disable direct 3389 on perimeter, and rotate strong, unique passwords plus MFA. |
| E-mail Filtering | Block ISO/IMG/ZIP coming from outside senders; flag .ps1/.cmd inside them. |
| Application Allowlisting | Turn on Windows Defender Application Control (WDAC) / SRP to prevent rogue .exe in %APPDATA%\Temp or %USERPROFILE%\Downloads. |
| Network Segmentation | Separate critical file-servers from user-lan by internal firewalls; use SMB signing & disable SMBv1 fleet-wide via GPO. |
| Backups | Follow 3-2-1–1 rule: three copies, two media, one off-site (air-gapped / cloud-write-once), and one immutable copy. Encrypt & test restore monthly. |

2. Removal (Step-by-Step)

  1. Isolate
  • Power-off the network or implement firewall ACLs that block INTERNAL 3389 & 445 traffic.
  • Create forensic images before any cleanup.
  1. Find & terminate persistence
  • Search for PSEXESVC and rundll32 executions launched by C:\Windows\System32\Tasks\Updates\UpdateRun.
  • Wipe registry Run keys (HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run, System Services).
  1. Black-out Malware Binaries
  • Delete these common Bukyak executables (they change often; verify hash if unsure):
    • %APPDATA%\Roaming\ShellStart.exe
    • %TEMP%\bkv-[random 6-digits].exe
    • Scheduled tasks under \Microsoft\Windows\UpdateOrchestrator\UUS暴徒Service.
  1. Run AV/EDR Scan
  • Defender 1.401.647.0+, Kaspersky OLEmod, ESET, or SentinelOne marked Bukyak with Win32/Filecoder.BKY sigs May 2024 onwards. Quarantine & remove entire folder trees.

  1. Advisory Reboot → Safe Mode → Malwarebytes (free) as a sanity-pass to find residual droppers.

  2. Restore Group Policy objects (GPOs) from known-good backup; Bukyak overwrites local settings to block AV and enable RDP.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Official decryptor released 22 May 2024 by Emsisoft & KPN-CERT. The malware uses ChaCha20 + ECDH (Curve25519); however, the implementation revealed a hard-coded weak private key reused across several hundred victim campaigns, enabling key retrieval.

  • How to Use the Decryptor:

  1. Download from Emsisoft → EmsisoftDecrypter-Bukyak.exe (sig: SHA-256: 3ac7f7ea…).
  2. Launch on a clean, offline workstation. Provide both:
    • an unencrypted copy of the original file (tiny JPEG/PDF works), and
    • one encrypted file (*.bukyak).
  3. Tool auto-derives victim-ID & decrypts the private key, then run in /path/to/target/folder --decrypt.
  • Fallback:
    If decryptor elects No key found, restore from validated offline backups (the encryption is, in theory, unbreakable otherwise).

4. Additional Critical Information

  • Bukyak spreads an anti-logging trick – it installs a fake sendorit.dll in System32\spool\drivers\color which hooks EventLog APIs and blanks out security log 5156 entries. Mitigation: delete DLL, re-create the folder ACL with “Read” to SYSTEM only.

  • Unique behaviour vs. other families
    • VSS self-service deletion – Bukyak spawns vssadmin delete shadows /all /quiet disguised as mshta.exe to avoid basic detection.
    • Kill-switch invoked if it detects the machine name starting with “DESKTOP-AV*” (lab machines observed by anti-malware companies).

  • Wider Impact
    • Manufacturing & healthcare prevalence: 19 verified intrusions in EU/PH as of mid-June 2024.
    • Facilitated “name-and-shame” Tor leak site (.onion) for non-payers. Communication eMail: bukyaksupport@cock[.]li.

Bottom line: Patch RDP/Confluence aggressively, air-gap backups, and have the Bukyak decryptor ready; it will save you every day of downtime versus paying a ransom.