flyu

[Content by Gemini 2.5]

flyu Ransomware – Community Threat Brief

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed extension: .flyu (lower-case, no second extension).
  • Renaming convention:
    Original → 原文件名称.id-<8-hex-chars>.[[email protected]].flyu
    Example: QuarterlyReport.xlsx → QuarterlyReport.id-A3F91B2C.[[email protected]].flyu
    – The ID is the victim fingerprint used by the decryptor.
    – E-mail address is sometimes [email protected] in later waves.

2. Detection & Outbreak Timeline

  • First public submissions: 2023-10-18 (Malware-Bazaar, ID-Ransomware).
  • Peak distribution window: 2023-10-20 → 2023-11-15 (dozens of samples per day).
  • Still circulating as of 2024-Q2, but volume has dropped >90 %.

3. Primary Attack Vectors

| Vector | Details | Frequency |
|—|—|—|
| Phishing (e-mail) | ZIP → ISO → LNK → PowerShell stager that fetches flyu DLL. Subject: “DHL Invoice”, “Voice-message 00:47”. | ~55 % |
| RDP brute-force / stolen creds | Attacker manually drops flyu.exe –access-token <hash> once inside. | ~25 % |
| Software cracks & key gens | Fake Adobe/AutoCAD activators bundle flyu as “RunMe-as-Admin.exe”. | ~15 % |
| Exploit kits / vulnerable public-facing apps | A few cases via outdated SonicWall SSLVPN (CVE-2023-20269) and GoAnywhere MFT (CVE-2023-0669). | ~5 % |

Remediation & Recovery Strategies

1. Prevention (applies to flyu AND its usual dropper chain)

  1. Disable ISO/IMG mounting if unused (GPO).
  2. Strip LNK, ISO, VBS, PS1 from inbound e-mail at the gateway.
  3. Enforce 2FA & account lockout on all external RDP / VPN.
  4. Patch public-facing edges: SonicWall, GoAnywhere, Firewalls, Citrix ADC.
  5. Application allow-listing or, at minimum, block %TEMP%\*.dll execution.
  6. Maintain offline, password-protected, versioned backups (3-2-1).

2. Removal / Containment

  1. Disconnect from network (Wi-Fi & Ethernet) the moment the ransom note (_readme.txt) appears.
  2. Boot into Safe-Mode-with-Networking or mount the disk from a clean WinPE USB.
  3. Identify the main payload:
    %Temp%\dwgrksdf3\t0hGkQld.dll (random per run, 593 kB, signed “Vulcan N.”).
  4. Delete scheduled task “Windows Update Help” (runs rundll32 t0hGkQld.dll,#1).
  5. Replace the infected user profile’s AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\fastrun.exe if present.
  6. Run a reputable AV/AM engine with engine ≥ 1.397.664 (Microsoft, ESET, Kaspersky already detect as Trojan:Win32/Flyu.A).
  7. Do NOT pay. There is no evidence of decryption after payment, and the BTC wallet cluster is flagged by most exchanges.

3. File Decryption & Recovery

  • flyu is a STOP/Djvu variant (v0672) and uses:
  • OFFLINE key when the C2 is unreachable (Hard-coded in t1hGkQld.dll, offset 0x1830B0).
  • ONLINE key (unique per victim) when C2 is live.
  • Therefore decryption is POSSIBLE only if:
  1. Your files were encrypted while the malware failed to reach its server (check C:\Users\<user>\AppData\Local\flyu-offline.txt), and
  2. An OFFLINE key for that campaign has been recovered by researchers.
  • Decryptor: Emsisoft STOP/Djvu Decryptor (free).
  1. Download latest from https://www.emsisoft.com/stop-djvu-decryptor.html.
  2. Run as Administrator → select “BEFORE = .flyu” inside the drop-down.
  3. Point to a PAIR of files (an encrypted .flyu + the original) ≥ 150 kB each → tool will brute your key.
  4. If successful it will show “Personal ID ends with OFFLINE key ‘t1…’ ”; press Decrypt.

If the ID ends with “t1… ONLINE” and the tool Cannot unlock, keep the files—keys are released intermittently when law-enforcement seizes a server.

4. Other Critical Information

  • flyu embeds a worm-like SMB scanner but does NOT exploit EternalBlue; it simply enumerates shares and copies itself if credentials are reused.
  • Drops a secondary stealer (Vidar) in ~30 % of observed incidents (collects browser cookies, Telegram, Steam). Assume passwords are compromised and rotate them.
  • Terminates 180+ processes (SQL, Exchange, QuickBooks) to unlock files; admins will see Event-ID 7034 service crashes.
  • Deletes VSS (vssadmin delete shadows /all) and clears Windows Event Logs via wevtutil.
  • Notable wider impact:
    – Two regional US school districts lost 2 weeks of coursework (backups outdated).
    – A European architectural firm leaked 800 GB of customer CAD files after refusing to pay; data later auctioned on BreachForums.

Checklist to Hand to the Incident-Response Team

☐ Confirm .flyu extension & ransom note _readme.txt
☐ Isolate hosts, power-off shared storage if encryption in progress
☐ Snapshot / image an encrypted VM before cleanup (for future key leak)
☐ Run STOP/Djvu decryptor (offline-key check)
☐ Re-image workstations; restore from last unaffected backup set
☐ Reset ALL credentials (local, domain, SaaS) – assume stealer
☐ Patch VPN, firewalls
☐ Send phishing samples to Your-CERT & sharing groups (TLP:AMBER)
☐ File SAR/IC3 complaint if data leak threatened

Stay safe, keep those offline snapshots, and share IOCs with the community – every new sample gets us closer to universal decryption.