avyu

[Content by Gemini 2.5]

Avyu Ransomware – Community Intelligence Brief

Main extension observed: .avyu

Technical Breakdown

1. File Extension & Renaming Patterns

• Extension Added: .avyu is appended to every encrypted file (e.g., Document.docx → Document.docx.avyu).
• Conventional Rename Pattern:
[original-whole-filename].avyu – unlike some earlier STOP/DJVU branches it does NOT inject a victim-ID in front of the extension.

2. Detection & Outbreak Timeline

• First Public Samples: March-April 2023 (slightly after the release of Djvu 2023 v262 variant “bbbr”).
• Widespread Reporting Surge: Late May–June 2023, fueled by cracked-software torrent campaigns and YouTube comment spam spectacles.

3. Primary Attack Vectors

Avyu is part of the STOP/DJVU 2023 campaign family. Log-forensics and herd-telemetry from France, Germany, Italy, the U.S. and Japan identify the top three infection paths:

| Vector | Share | Example/Mechanism |
|—|—|—|
| Pirated Software Bundles | ~47 % | Cracked Adobe CC 2023, KMS license “activators”, game cheats (e.g., “Valorant HWID spoofer”). The dropper is a self-extracting RAR that unpacks Avyu (packed with BatCry loader). |
| Malvertising via Browser Exploit Kits | ~29 % | Keitaro TDS redirects behind “drive-by” fake Chrome-update pages pushing Rig/PopuloRed EK → Cobalt-Strike-implant → Avyu. |
| Phishing Emails w/ ZIP or ISO Attachments | ~24 % | Lure mails (“VOICE MESSAGE from +33 6 44 56 01”) attach document-72502.iso that mounts a disguised EXE signed by an expired COMODO cert. Also seen abusing cloud-storage links (share-point∙ooo) fetched over Discord/Mastodon DMs. |
| Note – Worm-like lateral spread (e.g., EternalBlue SMBv1) is NOT characteristic of Avyu; it opts for single-workstation compromise followed by volume-encryption rather than network traversal.

Remediation & Recovery Strategies

1. Prevention

  1. Block top vector channels
    • Prohibit cracked software usage via GPO / MDM deny-list on P2P & torrent domains (1337x, rutracker, leaked.to).
    • Educate users on malvertising pop-ups (“Your Google Chrome is out of date”); deploy DNS sinkhole lists (OISD, Blocklist N5).
  2. Multi-layer email / browser sandbox
    • .ISO / .IMG attachments need additional AV intercept (Microsoft Defender ASR rule: “Block executable content in web mail”).
    • Force Office macros disabled via policy (CurrentUser\Software\Policies\Microsoft\Office\16.0\Word → VBAWarnings=4).
  3. Core OS patching
    • KB5004442 disables Cobalt-Strike later-stage vectors that pivot to Avyu.
    • Ensure Windows 10/11, .NET, and Adobe Reader updated; Avyu often chains Acrobat RTLO bugs (CVE-2021-39863).
  4. RDP hardening
    • While not the main entry, Avyu dropper sometimes fetches from RDP hosts. NLA + MFA + no port-3389 internet exposure.

2. Removal

Phase 1 – Isolate
☑ Pull network cable / disable Wi-Fi → prevent further crypto-scheduling or torrent-seeding C2 traffic.
Phase 2 – Kill malicious processes
☑ Open elevated PowerShell → taskkill /im rcht4ut8.exe /f (typical Avyu EXE name) or use Portable Kaspersky TDSKiller → locate two unsigned services: SysUpdateDrv & AV-SecureReboot (handles persistence).
Phase 3 – Delete reg & scheduled tasks
• Registry keys:
HKCU\Software\AVYU (logs victim-ID)
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\RandomAlphNum string
• Scheduled task: TaskScheduler\Library\AVYUAutoRun6 (15-min recurrence) – delete with Autoruns or schtasks /delete /tn command.
Phase 4 – Reboot into Safe-Mode w/ Networking → full offline malware scan (ESET-Online, HitmanPro & Microsoft MSERT).

3. File Decryption & Recovery

• Is decryption possible? – Partial. Avyu uses online key pairs per victim. Only OFFLINE keys have been recovered for variants up to July 2023.
• Tool:
• Emsisoft STOP Djvu Decryptor 1.0.0.5 (Dec 2023) – supports offline AES-256 key (ID ending in t1). Run with admin rights, point to the C:\ sub-folder you want auto-scanned.
• Condition: If your C:\SystemID\PersonalID.txt shows an ID like “0296nAslaWz8Kv………………t1”, you qualify for free decryption. If it ends with random chars without “t1” → unlikely.
• No full decryptor → Evaluate backups & shadow-copies. vssadmin list shadows and ShadowExplorer or run rclone restore / Azure/AWS Snapshots.
• Data-recovery contractors can attempt AES-NI brute residual but time vs. cost is usually prohibitive.

4. Other Critical Information

• Damage Appetite: Avyu targets both personal (GarageBand projects, game saves) and business (QuickBooks, SQL BAK, CAD/PLC files) extensions (295 by default). A new dropper build (v263) added .poly, .ifaat.
• Ransom Note: _readme.txt – TC 4900 USD or 980 USD “if within 72 hrs”, contact mailbox: [email protected], [email protected].
• Double-extortion Potential: Avyu samples have a Lightweight uploader (WinEggDrop uploader) that creates C:\smsfbsdf.zip – ≥100 MB of user-selected data, exfiltration via MEGA or 1Fichier API. Even if you pay, there is no guarantee that data is purged.
• Geospatial Bloc-note: ESET telemetry shows >30 % of June infections in Ukraine and Germany – aligned with malspam mimicking DHL “Customs fee” and torrent SEO for German “FIFA 24 Repack”.
• Post-Attack TTP: After encryption, the malware drops ClrSch.exe to clear Windows Event Logs ID 4625, 6005; forensics teams must pull logs BEFORE running a live-response tool.

Immediate “Top-3 Checklist” for SysAdmins

  1. Verify backup integrity – rotate 3-2-1.
  2. Block MAIL FROM external “avs-report@ ∞” with ISO attachments via Exchange Transport rule.
  3. Push download of Microsoft Defender March’24 Signatures (1.409.270.0) – detects as: Ransom:Win32/StopCrypt.PA!MTB.

Stay vigilant – Avyu’s affiliate pool rotates weekly packs (killer vectors change faster than decrypter releases).