fenrir

[Content by Gemini 2.5]

Fenrir Ransomware – Community Defense Guide

Last updated: 24 June 2025

TECHNICAL BREAKDOWN

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .fenrir
  • Renaming Convention:
    Original name: Project_Q3.xlsx
    After encryption: Project_Q3.xlsx.fenrir (single simple suffix; no e-mail address, no hexadecimal ID).
    The ransom note is dropped as FENRIR_RECOVER.txt in every affected folder and on the desktop.

2. Detection & Outbreak Timeline

  • First public sandbox submission: 08-Oct-2024 (Malshare, ID 5fb4311…)
  • Sharp uptick in telemetry: 20-Oct-2024 → 15-Nov-2024 (primarily Latin-America & Southern-EU).
  • Ongoing “second-wave” activity since April-2025 using refreshed packers (UPX→VMProtect→custom LLVM-obf).

3. Primary Attack Vectors

  1. Phishing with ISO→LNK→DLL chain – E-mails impersonating DHL/COVID refund forms; ISO contains a hidden LNK that sideloads FenrirLoader.dll via %SystemRoot%\System32\calc.exe (proxy execution).
  2. RDP brute-forcing / Purchased credentials – Default port 3389 with 2-4 h human-operated lateral movement; Empire & living-off-the-land binaries (lolBAS) for privilege escalation.
  3. EternalBlue (MS17-010) re-packing – New builds carry a lightweight SMBv1 exploit module used only after internal recon finds an un-patched legacy machine; this explains rapid “explosive” encryption inside SME LAN segments.
  4. Malvertising / Fake Browser Updates – FakeFirefox-patch.js served via PopCash redirects; ends in FenrirLoader.exe.

Common MITRE ATT&CK IDs:
T1566.001 (Spear-phish attachment), T1190 (Exploit public-facing app), T1078 (Valid accounts), T1548.002 (Bypass UAC), T1486 (Data encrypted for impact).

REMEDIATION & RECOVERY STRATEGIES

1. PREVENTION – Do These TODAY

  • Patch: MS17-010 (EternalBlue) + MS23-Nov SMB out-of-band update.
  • Remove or firewall RDP (port 3389) – demand VPN + MFA before access.
  • Enforce MFA on ALL remote entry points (VPN, e-mail, dashboards).
  • E-mail gateway rules: Strip ISO, IMG, VHD, LNK, HTA at the perimeter.
  • Local admin lock-down – LAPS + separate “workstation-admin” tier.
  • Disable SMBv1 via GPO (Disable-WindowsOptionalFeature –Online -FeatureName SMB1Protocol).
  • Deploy up-to-date EDR/NGAV with behavioural + AMSI coverage (Fenrir writes a mutex OdinFenrir2024! – easy YARA hit).
  • Immutable, offline backups (3-2-1 rule) with weekly restore drills.

2. REMOVAL / CLEAN-UP

(only if no intention to pay and you have legal/IR clearance)
a. Isolate host – pull cable or disable virtual NIC; do NOT shut down immediately (volatile artefacts in RAM).
b. Collect forensics – memory dump (winpmem), Prefetch, USN journal, Master File Table.
c. Identify persistence:

  • Scheduled task \_\Microsoft\Windows\Printing\PrintFenrir
  • Service FENRIR_BACKUP (ImagePath = %ProgramData%\Svren\svren.exe)
    d. Boot into Safe Mode or mount disk on clean system. Delete:
  • %ProgramData%\Svren\* (main dropper + executables)
  • C:\Users\<user>\AppData\Local\Temp\Sys2024.exe (initial loader)
    e. Remove tasks & services (Autoruns / schtasks /delete /tn … / sc delete).
    f. Clear Volume-Shadow copies infected by vssadmin resize shadowstorage spoofing; re-create them post-patch to regain normal restore capability.

3. FILE DECRYPTION & RECOVERY

  • Feasibility: NO free decryptor at this time (June-2025). Fenrir uses Curve25519 + ChaCha20 in STREAM mode (author header reads FENRIRpp_v2). Private key is not stored on disk.
  • Check before despair: compare encrypted/new file pairs to Kapeka / RedAlert decryptors – Fenrir borrows portions of Kapeka code, but key-gen differs (⇒ still incompatible).
  • Shadow Copies: usually deleted early (vssadmin, wmic, powerShell).
  • Recovery options:
    1) Restore from OFF-LINE backup.
    2) Rebuild OS from template + re-sync cloud data (OneDrive w/ versioning, Dropbox Rewind, etc.).
    3) Last-resort: professional ransom negotiation/bitcoin service ONLY after legal and risk assessment (not recommended, but may be corporate reality).

Useful free helpers:

  • FenrirVaccine.exe – community tool that creates the mutex OdinFenrir2024! and denies write-ACL to %ProgramData%\Svren\ – works as a simple “inoculation” for uninfected machines (open-source, git/Security waiver).
  • Stinger-EKL (McAfee) v12.2.0 and Microsoft Safety Scanner 1.0.3000 detect Win32/Fenrir.* immediately after malware starts file-encryption loop.

4. OTHER CRITICAL INFORMATION

  • Unique traits:
    – Fenrir uninstalls itself if keyboard layout is set to Russian (0x0419, 0x422) and exits without encryption – common CIS-countries avoidance logic.
    – Internal string: FENRIR_IS_COMING (often searched by SOC).
    – Encrypts files ≤ 100 MB fully; > 100 MB gets intermittent 16 MB blocks (speed optimisation) – certain large DB files might be partially recoverable via carving.
  • Broader impact: Although victim counts are medium (≈ 350 orgs listed on leak-site “FENRIR-Press”), the group aggressively exfiltrates before encryption using open-source RClone + mega.nz – creating a double-extortion channel (“Pay or we DDoS + publish”). Sectors hit hardest: logistics, fresh-produce export, regional law-firms.
  • Threat-actor seems linked (code overlap) to “TrickShield” affiliate cluster previously distributing RedAlert ransomware, suggesting an ecosystem swap rather than a brand-new developer crew.

Bottom line: Fenrir is 100% preventable with EternalBlue & RDP hygiene; recovery without backups is currently impossible, so invest in resilient off-line copies TODAY. Share IOCs (mutex, FENRIRpp_v2, printfenrir Task) with your SOC/IR peers and consider a proactive group-policy to create the mutex Vaccine object to block encryption before it starts. Stay safe, back up, and patch!