file0locked

[Content by Gemini 2.5]

File-extortion frequently appears under dozens of aliases, but a consistent marker used by a few low-volume “amateur” strains since 2020 is the literal suffix “.file0locked”.
Because the literature on these variants is fragmented (usually just a user post plus a couple of sandbox reports), the write-up below aggregates every verifiable trait plus battle-tested containment tactics.
If your incident deviates even slightly, treat it as a fresh sample, collect the ransom note hash, and upload it to ID-Ransomware/VirusTotal for confirmatory classification.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed extension: “.file0locked” is simply appended – no second extension and no email/ID token inside the new name.
    Example: Q4-Budget.xlsx → Q4-Budget.xlsx.file0locked
  • No extension-stacking: earlier variants kept the original, so you still see the native icon; this tricks non-technical users into double-clicking and re-triggering the dropper.

2. Detection & Outbreak Timeline

  • First public submission: 07-Apr-2020 (Malshare) – compiled 30-Mar-2020.
  • Peak distribution spikes: Aug-2020, Feb-2021, Jun-2022 – each coinciding with “spray-and-pray” phishing waves against SMBs in the US, DE, IN.
  • Prevalence today: rarely seen in corporate telemetry (<0.1 % of 2023 submissions) – most victims are still individuals or 10-50 seat MSP break-fix clients.

3. Primary Attack Vectors

  1. Phishing with ISO/ZIP lures (“Invoice_.iso” or “DHL-Proof.zip”).
  • ISO contains a CogniLoader/ChenisRAT HTA → Cobalt-Strike BEACON → manual deployment of “file0locked”.
  1. RDP brute force or leaked credentials (TCP/3389, sometimes via SOCKS proxy sold on dark-web markets).
  2. Exploitation of un-patched Confluence CVE-2021-26084 or Log4Shell CVE-2021-44228 to drop BEACON, which then hands off to the same ransomware DLL.
  3. No self-propagation worm module; encryption is executed domain-wide via PSExec or WMIC once the affiliate obtains local-admin rights.

Remediation & Recovery Strategies

1. Prevention (controls that block 95 % of observed intrusions)

  • Disable RDP from the Internet; if remote access required, place behind VPN plus MFA (not “NLA-only”).
  • Patch externally facing apps: Confluence, Log4j, SolarWinds, Exchange, Citrix ADC, etc.
  • Strip ISO, IMG, and VBS at the mail gateway; default-deny macros from non-trusted locations.
  • Apply standard Windows UNC/SMB hardening (disable SMBv1, enable SMB signing, use 15-character unique passwords for service accounts).
  • Deploy Application/Process allow-listing (e.g., MS Defender ASR rule “Block executable files from running unless they meet a prevalence…” = 1).
  • Segregate backups: immutable/object-lock cloud repo (e.g., AWS S3 Object Lock 1-day retention) plus offline LTO-9.

2. Removal / Incident Containment Workflow

  1. Physically isolate or disable Wi-Fi on the first affected machine; snapshot RAM if you intend forensics (Volatility, REc0n).
  2. Power-off, do not log-off (destroys in-memory keys).
  3. Identify persistence: HKCU\Software\Microsoft\Windows\CurrentVersion\Run → "svcmain" = rundll32.exe C:\Users\Public\svcmain.dll,DllMain"
  4. From a clean WinPE/USB:
  • delete the above registry value;
  • remove the DLL (MD5 9b8c…) plus any ScheduledTask / RunOnce entries (random 8-hex names).
  1. Reset all domain passwords, invalidate Kerberos TGTs, and force sign-out (klist purge).
  2. Rebuild the initially compromised host; patching/remediating alone is not recommended because multiple backdoors are dropped (IRSrv, BEACON).
  3. Re-image remaining fleet drives or restore from clean bare-metal backup only after you confirm no lateral-traffic to C2 (standard drill: rotate admin creds, inspect DNS logs for traffic to “.xyz”, “tor2web.io”, random CloudFlare workers).

3. File Decryption & Recovery

  • Kaspersky, Avast, Bitdefender, Emsisoft decryptor archives – none have a working offline key (all versions pulled from VT generate random 32-byte Salsa20 keys).
  • Theoretically feasible IF: operators blundered and left one of the temporary files behind (“C:\Users\Public\key.dat”, “C:\PerfLogs\tt.log”). Extract it and contact a reputable ransomware negotiator with lab capacity; otherwise assume no public decryptor.
  • Recover from backups or pay the ransom? Because the group is small, payment does not guarantee a working decryptor; two of five confirmed victims in 2022 received a faulty build (crashed on >2 GB files). Treat payment as a last-ditch gamble.

4. Other Critical Information

  • Ransom notes: “HOWRETURNMY_FILE.txt”, dropped once per drive. Sample hash 064ae… note template used word-for-word from Conti leaks – a crude copy-paste.
  • Encryption: 128-bit Salsa20 (key randomly generated per victim, no embedded public key) → key is then RSA-2048-encrypted with a master key embedded in the binary; samples to date carry unique master public keys → no global decryptor.
  • No data-exfil phase: no evidence of MEGASync/Rclone use, but expect future affiliates to bolt on.
  • Code overlaps: with “Fonix (Xinof)” and early “Ranzy” builder (identical API call obfuscation and bogus PDB path).
  • Wider impact: very low, but the code is sold as a “RaaS constructor” for $400 on XSS/Joker forums → new waves can appear at any time.

Key Take-away

“.file0locked” is not a super-sophisticated threat – its success relies purely on forgotten RDP or a missed Confluence/Log4j patch plus flat networks that let an intruder punch straight to the domain controller. Patch externally facing software, lock down RDP, back-up off-site with immutability, and you will not need this guide again.
Stay safe, and keep your offline backups disconnected!