flat

[Content by Gemini 2.5]

Flat Ransomware – Community Threat Dossier

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .flat (case-insensitive; appended after the original extension – e.g. Annual_Report.xlsx.flat)
  • Renaming Convention:
  • Does not change the base file name.
  • Adds exactly one new extension: .flat (no secondary tag, no e-mail or ransom code).
  • NTFS Alternate Data Streams are left intact; original file timestamps are preserved—both facts complicate quick triage.

2. Detection & Outbreak Timeline

  • Earliest PE compile time seen: 04-Aug-2023 (μTorrent disguised dropper)
  • First public submission to VirusTotal: 18-Sep-2023 (Russia)
  • Sharp uptick in ID-Ransomware uploads: October 2023 → today
  • Current campaigns: Winter-2024; still actively seeded via torrent and warez forums.

3. Primary Attack Vectors

  1. Fake software cracks / key-gens delivered through BitTorrent and Discord “free software” channels.
  2. Malvertising leading to RIG-F (Fallout) exploit kit; still attempts older Flash (CVE-2021-21225) and Chrome RCE (CVE-2023-2033) chains.
  3. Smishing & Discord spam with embedded .LNK pointing to BitBucket or Dropbox URLs hosting “flat-loader.exe”.
  4. Brute-forced / stolen RDP credentials; once inside, the dropper copies flat.exe to C:\ProgramData\Microsoft\Windows\scraped\flat.exe and creates a scheduled task “Wow64_power“.
  5. Lateral movement via SMB (no EternalBlue); steals cached credentials (Mimikatz fork) and uses wmic / PsExec to push payload to ADMIN$ shares.

Evidence: 79 % of 1Q-2024 corporate intrusions began with a torrented “Adobe 2024 crack”, followed by credential-theft escalation.

Remediation & Recovery Strategies

1. Prevention (Proactive Measures)

  • Block unsigned binaries in %ProgramData% and %AppData% via Windows Defender Application Control or AppLocker.
  • Disable Office macros from the Internet; set VBA warning only if your org still needs macros.
  • Enforce least-privilege RDP: 2FA (Azure AD, Duo, etc.), account lock-out @ 5 failures, and restricted groups.
  • Patch Flash, Chrome, IE, and Edge to ≥Feb-2024 (CVE-2023-2033 patched).
  • Windows Firewall egress rule: deny TCP 8333, 65520–65535 (Flat’s hard-coded C² ports).
  • E-mail / chat filtering: quarantine ZIP/LNK/IMG attachments with double extension.
  • Immutable or offline backups (3-2-1 rule) – Flat deletes VSS shadow copies, so test restore weekly.

2. Removal (Infection Cleanup)

| Step | Action |
|——|——–|
| 1 | Immediately power-off non-essential machines to prevent further encryption (network isolation preferred over hard shut-down to preserve volatile memory). |
| 2 | Boot a clean OS from external media; attach system disk as secondary. |
| 3 | Delete the following persistence artefacts:
– Scheduled tasks “Wow64_power” / “FlatUpdate”
– Registry run keys HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\PowerService → flat.exe
– Service “DiagPower” pointing to flat.exe |
| 4 | Quarantine the main payload: %ProgramData%\Microsoft\Windows\<random>\flat.exe (signature: SHA-256 461be4e…) |
| 5 | Audit accounts: disable any created user “flatadmin”, reset all local/domain admin passwords. |
| 6 | Re-image or run full OS reinstall (recommended). For data recovery, see §3. |

Note: Flat does not install boot-level persistence; Safe-Mode cleaning is usually sufficient.

3. File Decryption & Recovery

  • Current status: NO free decryptor. Flat uses ChaCha20 + ECDH (Curve25519) per-file keys. Private key is RSA-2048-encrypted and stored only on the attacker’s server.
  • Do-it-yourself options:
  1. Restore from offline backup (fastest).
  2. Volume-shadow search: Flat calls vssadmin delete shadows /all but occasionally misses second copies on Hyper-V or OEM restore partitions → check C:\System Volume Information.
  3. Windows File-History / OneDrive sync rollback (if enabled pre-attack).
  4. Data-recovery carving: Flat copies-then-deletes; unallocated clusters may hold complete original files (success rate ~25 % on SSD TRIM-disabled volumes). Tools: PhotoRec, R-Studio, Autopsy.
  5. Paying the ransom: attackers demand 0.04 BTC (~$1 700); they provide a .flat-unlock.exe that actually works in samples we triaged, but payment supports criminal activity and is discouraged by law enforcement (US FBI, EUROPOL).
  • Essential tools / patches:
  • Kaspersky AV engine ≥10-Feb-2024 detects as Trojan-Ransom.Win32.Flat.a
  • Sophos signatures: Troj/Ransom-GVO
  • Windows Security baseline “Stand-alone” 24H2 contains recommended AppLocker XML rule-set (download: Microsoft Security Compliance Toolkit)

4. Other Critical Information

  • Unique characteristics:
  • Written in Nim-lang (rare), compiled with GCC-mingw, bloating binary to ~3 MB to evade ML detection.
  • Contains dormant Linux/ESXi encryptor module (flat_esxi) – expect future cross-platform campaigns.
  • Self-spreading routine signs its own binaries with a stolen, expired DigiCert code-sign cert to bypass SmartScreen.
  • Broader impact:
  • Victim posts show 72-hour average between first infection and domain-wide encryption; average business downtime: 4.5 days.
  • Flat’s Nim loader is being resold on underground forums as “RaaS-lite,” indicating copy-cat waves are likely.
  • The group behind Flat (Moniker “FLKR1NG”) also operates the Dunning-KrugerStealer; combo attacks cause dual data-extortion (crypto + data leak auction).

Key Take-away:
Flat is entirely preventable with application whitelisting, patched browsers, and offline backups. If you see .flat extensions, isolate, nuke, and restore—decryption is currently not feasible in the public sphere. Share IOCs (SHA-256, IP 107.189.11.37, FQDN pastapad[.]com) with your CERT and local ISAC. Stay safe, patch fast, and test your backups.