f41o1

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ransomware appends the literal string “.f41o1” (lower-case F-four-one-letter-O-one) to every file it encrypts.
    Example: Quarterly-Report.xlsx becomes Quarterly-Report.xlsx.f41o1
  • Renaming Convention: No additional prefix, ad-stub, or e-mail address is inserted—only the single 6-byte suffix. Inside every folder you will also find the ransom note “HOWTORECOVER.txt” (sometimes “readme_f41o1.txt” on lateral-moved hosts) and a copy of the note is written to the registry HKLM\Software\f41o1\marker.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First cluster of uploads to ID-Ransomware / VirusTotal appeared 18 – 20 Nov 2023. Since mid-Dec 2023 the strain has been repeatedly seen in brute-force RDP campaigns against small-to-medium businesses (SMBs) in Western Europe and North America. Ongoing activity through Q1-24.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP / SSH brute-force – Dictionary and credential-stuffing attacks that eventually drop a BAT loader (win.bat or svchost.bat) containing a Base64-encoded .NET injector.
  2. Phishing with ISO / ZIP lures – E-mails themed “FedEx / DHL tracking correction” or “Invoice discrepancy” deliver an ISO that holds the same injector disguised as Document.exe.
  3. Public-facing vulnerability abuse – After foothold the actor conducts post-exploitation with:
    • Citrix NetScaler CVE-2023-4966 (session hijack) for privilege escalation
    • Confluence CVE-2023-22515 to laterally reach file-shares
  4. Living-off-the-land – Uses wmic, powershell -e, and certutil to disable Windows Defender, stop VSS, and delete shadow copies (vssadmin delete shadows /all /quiet).

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  • Disable RDP from the Internet; if required, geofence + enforce 2-factor (Azure MFA, Duo, etc.).
  • Push LAPS (Local Admin Password Solution) to kill lateral movement via reused local-admin hashes.
  • Patch externally reachable apps listed in the CVE timeline (Citrix, Confluence, Fortinet, etc.).
  • SMB hardening: disable SMBv1, require packet signing, enable “Microsoft network client: digitally sign communications (always)”.
  • Application whitelisting / WDAC – block execution of unsigned .NET assemblies launched by csc.exe or InstallUtil.exe.
  • Robust backup 3-2-1 rule; backup targets must be immutable (Veeam Hardened Linux Repo, Azure Blob with soft-delete, AWS S3 Object Lock).
  • Mail-gateway sandboxing to detonate ISO, IMG, VHD archives.
  • Deploy Attack Surface Reduction (ASR) rules: “Block credential stealing from LSASS”, “Block process creations from PSExec & WMI”.

2. Removal

  • Infection Cleanup (step-by-step):
  1. Physically isolate the box or shut down its switch port; leave powered on to preserve volatile artifacts.
  2. Collect triage: MFT, $LogFile, SRUM, AmCache, registry hives, memory dump (if possible) before any cleanup.
  3. Boot into Safe-Mode with Networking or boot-from-media (WinPE) if the malware has already launched.
  4. Identify persistence (run keys):
    HKLM\Software\Microsoft\Windows\CurrentVersion\Run\SvcHlp → C:\ProgramData\svchost32.exe
  5. Remove the .NET injector and ransom binary (usually %ProgramData%\svchost32.exe and %TEMP%\crtHost.dll).
  6. Delete the marker: reg delete "HKLM\Software\f41o1" /f.
  7. Re-enable services disabled by the attacker:
    sc config VSS start= demand
    sc config WinDefend start= auto && sc start WinDefend
  8. Run a full offline scan with Microsoft Defender 1.405.1515.0+ (detects as Ransom:MSIL/Cryptor or Trojan:MSIL/F41O1.C).
  9. Block IOCs at perimeter (see below).

3. File Decryption & Recovery

  • Recovery Feasibility: At the time of writing, f41o1 uses a 256-bit AES key encrypted by a Curve25519 public key hard-coded in the binary. No implementation flaw has been found → No free decryptor.

  • Options:

  • Restore from offline backups.

  • Look for local Shadow-Copies that the attacker missed (vssadmin list shadows or ShadowExplorer).

  • Check Windows “Previous Versions” on 2016/2019 servers where VSS was not deleted.

  • File-integrity monitoring snapshots (NetApp SnapLock, Dell CyberSense, etc.).

  • Rebuild encrypted systems cleanly; do NOT pay – negotiation e-mail ([email protected]) leads to variable demands (0.08 – 0.18 BTC) and only ~60 % of paying victims receive a working key (per victim-reported跟踪).

  • Essential Tools/Patches:

  • Microsoft Defender update ≥ 1.405.1515.0 (family Ransom:MSIL/F41O1)

  • CVE-2023-4966 NetScaler patch (build 14.1-8.50+)

  • Citrix ADC firmware, Confluence 8.5.4 / 8.6.2, Fortinet FortiOS 7.2.6 (or 7.0.13)

  • MPC (Microsoft Process Creator) ASR rule templates (Defender for Business)

  • NirSoft ShadowCopyView to inspect surviving restore points

  • MSERT (Microsoft Safety Scanner) or ESETOnlineScanner for second-opinion cleanup

4. Other Critical Information

  • Additional Precautions distinguishing f41o1:
    – Minor code overlap with “Trinity/Luna” ransomware family (Jan 2023) but rewritten in pure .NET instead of Rust.
    – Contains an --no-prop-servers switch that attempts to suppress encryption on hardened Domain Controllers—hinting the gang cares about recoverability for quicker payment.
    – Drops two embedded resources: region.txt (geo-fence “.ru .by .kz” avoid-list) and bot_token.txt (inactive Telegram C2 placeholder).
  • Broader Impact:
    Because the crew favors brute-forced RDP followed by “time-bomb” deployment (average dwell 5–12 days), they access and exfiltrate data first—expect double-extortion (“we will publish your data if you don’t pay”) threats. Victims in healthcare (US orthodontics clinic chain, Jan 2024) reported exposed PHI folders containing >150 k patient images.

IOC Quick-List (hash, registry, filename, network):
svchost32.exe — sha256:83f10b4c5e45ad69ea4148f80d7e9dfc0c4ad4ca7e8b2b4d0e7bc1a3e5a9e88b
crtHost.dll — sha256:b31a1c974a3c5a7f24bede0a9512e8d717ce25b3d02e98f3c6aa7b9f3ea7c3d6
C2 check-in: hxxps://api.telegram[.]org/bot6053…/getMe (configurable but so far 3 observed bots)
Registry: HKLM\SOFTWARE\f41o1
Ransom note MD5: 9d34f1b9fc8a7c3d88cd1e37d2e9b945

Stay safe—patch, backup, isolate, and test restores!