fileslack

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .fileslack
  • Renaming Convention: Victim documents are encrypted and their original filename is appended with “.fileslack” (no extra suffix, prefix, or email address). Example:
    invoice_04_2025.pdf → invoice_04_2025.pdf.fileslack

2. Detection & Outbreak Timeline

  • First publicly submitted samples were captured in late-May 2019 and peaked through June 2019. The campaign never reached the same volume as families such as Ryuk or Phobos, but clusters of infections have resurfaced sporadically in SMB/SME networks through 2023.

3. Primary Attack Vectors

  • Exploited RDP brute-forcing – open 3389 or 3389/UDP redirected via NGROK/CLOUDFLARE tunnels.
  • “Cracked” software bundles – fake Adobe, CAD, and game piracy installers include tiny NSIS dropper that silently pulls the C++ loader.
  • Credential-stuffing via SMB shares – uses Mimikatz output to hop laterally (but does not leverage EternalBlue; no exploitation of SMBv1 vulnerability).
  • PPI (Pay-Per-Install) service – small affiliate installer pushed by RIG EK and Fallout EK malvertising chains (2019 only).

Remediation & Recovery Strategies:

1. Prevention

  • Network: Block/restrict TCP 3389 ingress (GEO-filter if feasible), enforce multi-factor VPN-before-RDP rules.
  • Hardening: Disable SMB v1/v2 unless absolutely required; use signed SMB v3, limit lateral-movement user rights, enforce LAPS for local admin passwords.
  • User education: Warn about “cracks”, keygens, and unexpected ZIP attachments—typical lures attached to “order/invoice/waybill” phishing mails.
  • Patch discipline: Keep OS, browsers, and any RDP-gateway software patched; loader often runs PowerShell to fingerprint missing KBs as a precursor.
  • Defensive stack: Maintain reputable EDR/NGAV with behavioural analysis (fileslack loader spawns cmd⟶powershell⟶.exe from %TMP% – very noisy).
  • Immutable backups: 3-2-1 rule, offline copy plus S3 object-lock or WORM-tape. Credential isolated from production AD.

2. Removal (high-level assuming “offline” first responder)

  1. Disconnect NIC / shut Wi-Fi; power-off any obvious lateral-movement hosts.
  2. Boot infected workstation from a clean USB–WinPE (or detach HDD) and create a full bit-stream image for forensics.
  3. From WinPE:
    a. Delete attacker persistence (Scheduled Task named “ServiceHub”, Run-key “FileSync”, and WMI Event Consumer “SystemSync”).
    b. Kill/delete binaries: %APPDATA%\Microsoft\servicehost.exe, %TEMP%\slack svc.exe, C:\ProgramData\slsvc.dll.
    c. Remove ransom note (“FILESLACKRECOVER.TXT”) – but preserve hash/note in forensics zip.
  4. Boot into Safe-Mode-with-Networking; run current AV/EDR “rescue” scanner to quarantine residual artefacts (usually two packed DLLs injected into svchost).
  5. Reset all local & domain credentials that logged onto the box via RDP in last 30 days.
  6. Patch/upgrade RDP stack, disable any rogue port-forward rules at perimeter.

3. File Decryption & Recovery

  • Decryptable? No. Fileslack uses Curve25519 (asymmetric) + ChaCha20 stream cipher. Private key never leaves the attacker’s server. No known implementation flaw or leaked master key has surfaced.
  • Free decryption therefore requires:
    – (a) a valid ransom purchase and receiving the per-victim decryptor, or
    – (b) locating a recent, clean, offline backup.
  • File-recovery tools without the private key can only carve unencrypted deleted copies from shadow copies that survived (vssadmin is not wiped by the strain but old copies are often overwritten).
  • Proven data-carvers: PhotoRec / Scalpel / Kroll ShadowProtect MFT scan for old Office or PDF temp files.

4. Other Critical Information / IOCs

  • Unique Mutex = “FileSlackMainMutex1337” (x86) and “FSMainMutex64” (x64).
  • Ransom note (“FILESLACKRECOVER.TXT”) is NOT dropped in every folder – only in %userprofile%, Desktop, and the root of any mapped drive it encrypted; this saves time during triage.
  • Some builds contact pstorage.space (or mirror pstorage.cdn) for key exchange; more recent re-skins use Slack-style emoji in HTML ransom page to spoof legitimacy.
  • Warning: Several fake “free decryptors” circulating in blogs for this family; they are trojanised info-stealers—warn users to rely only on official law-enforcement/Emsisoft or NoMoreRools repositories.
  • Insurance & legal: Because Fileslack payloads sometimes exfil before encrypting, treat every incident as both ransomware + data-breach and analyse proxy/DNS logs for large file transfers to mega.nz or pcloud.com. Report to local authorities (e.g., FBI IC3, UK NCA) – small family tracks help map affiliate infrastructure.

By combining the above technical knowledge with disciplined containment and verified backups, the community can eliminate Fileslack infections without feeding the ransom ecosystem. Stay patched, segment your networks, and protect those 3-2-1 backups!