byya

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File-Extension: Every file encrypted by this current Djvu/Stop variant receives “.byya” appended to its original file extension (e.g., picture.jpg → picture.jpg.byya).
  • Renaming Convention: The ransomware only adds the additional extension, never replaces the original portion; filenames themselves stay untouched.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Large-scale public submissions and telemetry spikes for “.byya” appeared in the second half of May 2024. First upload to malware-sharing repositories was May 17, 2024 14:11 UTC.

3. Primary Attack Vectors

  • Propagation Mechanisms (operating like preceding Djvu variants):
  1. Cracked/Pirated Software Bundles – #1 infection path; disguised as keygens, license patches, or repacked games via BitTorrent/popular warez blogs.
  2. Malvertising & Fake Software Updates – poisoned Google Ads redirecting to codec, driver, or “Game Booster” installers hosted on look-alike domains.
  3. Infected USP/Hot-Key Combo – some samples spread over removable drives/USB running an autorun stub.
  4. Remote Desktop – still observed when brute-forced or when cracked tools are executed on a machine already exposed to the Internet via RDP.
  • Customer Exploit Kit Details: No exploitation of specific OS vulnerabilities—relying on user-execution rather than lateral EternalBlue/SMBv1 spreading.

Remediation & Recovery Strategies

1. Prevention

  • Essential Initial Measures:
    • Block-inbound SMB & RDP access on edge firewalls; enable two-factor on remaining necessary RDP.
    • Windows Firewall / Defender – enforce “block unsigned executables from %TEMP% and Downloads folder.”
    • Software Restriction Policies (SRP) or AppLocker – whitelist %ProgramFiles%\ and %ProgramFiles(x86)%\ for execution; block powershell.exe, cmd.exe in Office-context.
    • Disable macro execution across Office suite via Group Policy.
    • Patch Java, .NET, Acrobat, and all browsers—helps reduce cracked-ware installers from silently side-loading secondary payloads.
    • Immutable + offline backups → 3-2-1 rule: 3 copies, 2 media types, 1 off-site/off-line (object-lock on S3 or tape).
    • Educate – periodic, simulated phishing & a clear “no pirated software” policy (document signed by staff).

2. Removal

Step-by-step clean-up (pre-decryption):

  1. Quarantine the machine – isolate from network & external drives to prevent further encryption on mapped shares.
  2. Identify persistence:
    • rundll32.exe launching %APPDATA%\LocalLow\{GUID}\{random}.dll or %APPDATA%\{random}.exe.
    • Registry entry under HKCU\Software\Microsoft\Windows\CurrentVersion\Run pointing to the same.
  3. Safe Mode or WinRE startup → run an offline ESET / Malwarebytes / Windows Defender Offline scan to remove the launcher, dll, and any dropper.
  4. User-profile cleanup – delete rogue scheduled tasks named Update Task for {random} or BksSjdqifDgRDS.
  5. Remove registry keys once confirmed clean (backup first).

3. File Decryption & Recovery

  • Is decryption possible? It depends on the key used:
    • OFFLINE key (t1 suffix in ransom note) – YES: Free decryptor by Emsisoft (v.1.0.0.20, dated 2024-05-30).
    • ONLINE key (t2 suffix or auto-generated UID) – NO decryption without the criminal’s private key (practically impossible).
  • Procedure for OFFLINE infection:
  1. Save _readme.txt ransom note and a pair of original+encrypted files.
  2. Download Emsisoft + Aurora decrypter v1.0.0.20 or newer: [https://decrypter.emsisoft.com]
  3. Run with sample file pair; it detects offline ID and begins permanent decryption—no internet needed once keys collected.
  4. Verify file integrity (hash check first few recovered files) then restore in bulk.
  • Essential Tools / Patches:
    • Emsisoft decryptor (offline key capability).
    • Windows 10/11 cumulative security update up to May 2024 (KB5037771 etc.) keeps chained Djvu droppers from exploiting older binaries.
    • Disallow policy templates for running specific code-signing levels: [CIS Windows 11 v2.0.0 benchmarks].

4. Other Critical Information

  • Differentiators from Djvu lineage:
    – Uses a slightly randomized hex window header (0xDEADBEEF offset) which still ends in encrypted string “HO8Yi”.
    – Newer command line parameter /nsc (no shadow-copy) to skip vssadmin deletion in certain builds.
    – Dropped ransom note continues to demand $490 / $980 in BTC (static Babuk-like style).
  • Wider Impact:
    – Djvu/Stop has become a dominant consumer-tier threat; many victims unknowingly pay instead of checking free options.
    – Law-enforcement partners, including the FBI Internet Crime Complaint Center (IC3), treat Djvu as ongoing high-volume ransomware-as-a-service (RaaS). Reports help track affiliates, so encourage reporting via IC3.gov.