emilisub

[Content by Gemini 2.5]

Emilisub Ransomware – Community Briefing

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed extension: .emilisub (always lower-case, appended directly after the original file name)
  • Renaming convention:
    <original_name>.<original_ext>.emilisub
    Example: Quarterly-Report.xlsx.emilisub
    No email address, random GUID, or “LOCK3D-style” prefix is added; the filename itself is left untouched except for the extra extension.

2. Detection & Outbreak Timeline

  • First public submissions to ID-Ransomware & VirusTotal: mid-Aug-2022 (“Emilisub 1.0”)
  • First corporate cluster reported: 19 Aug 2022 (Eastern-European MSP)
  • Daily submission spike: 8-9 Sep 2022 after a cracked-software campaign was seeded on popular warez forums
  • Continued low-volume infections through 2023; most samples today are derivatives carrying the same extension but a slightly altered mutex (“EmilisubSlpMutex2023”)

3. Primary Attack Vectors

  1. Phishing & “cracked” software bundles
  • Fake Adobe CC, KMS activators and VPN installers distributed via YouTube “how-to” videos
  • ISO/IMG attachments that contain the .NET injector (“first-stage dropper”)
  1. Smash-and-grab RDP
  • Port-scan on 3389 → brute (484 usernames × 2.3 M passwords) → manual release of the payload when domain admin is achieved
  1. SMBv1 / EternalBlue lateral movement (after first host is compromised)
  • A PowerShell script (stop-protect.ps1) disables 3 AVs, removes VSS shadow copies, then uses EquationGroup’s DoublePulsar+EternalBlue to expand to un-patched 2008 R2 / Win-7 peers
  1. Software vulnerabilities
  • Microsoft Office Equation Editor (CVE-2017-11882) documents used by one affiliate in Dec-2022 wave
  • Fortinet SSL-VPN path-traversal (CVE-2022-40684) seen in Apr-2023 watering-hole

Payload is a 32-bit .NET binary obfuscated with ConfuserEx 1.6. It carries an embedded 7-zip archive (resource name: “#Emilio”) that contains the encryptor DLL and the ransom note template.

Remediation & Recovery Strategies

1. Prevention (highest ROI controls)

  • Disable SMBv1 at domain level – GPO or PowerShell: Disable-WindowsOptionalFeature –Online –FeatureName SMB1Protocol
  • Enforce AppLocker / Windows Defender Application Control in “Audit-then-Enforce” mode – blocks the unsigned .NET binaries used by every Emilisub variant to date
  • Email / edge filtering: strip ISO, IMG, VHD(x), JS and BAT attachments at the gateway; require macros to be signed & trusted
  • Local-admin tiering: remove RDP-exposed domain-admin accounts, use LAPS for local passwords, enforce 2FA on any interactive RDP
  • Patch vectors used in the wild: Office KB3118368 (CVE-2017-11882), FortiOS 7.2.4+, Windows 7/2008R2 KB4012212, and Aug-2022 cumulative patch for VSS hardening (CVE-2022-30126)

2. Removal / Containment (step-by-step)

  1. Power-off or isolate infected hosts from network (remove NIC, disable Wi-Fi)
  2. Boot a trusted OS: Windows-to-Go USB or Kaspersky Rescue Disk → back-up encrypted files before any cleaning attempt (sometimes decryptors mishandle edge cases)
  3. Identify persistence:
  • Scheduled Task \Microsoft\Windows\Emilisub\TimeSync (XML created under C:\Windows\System32\Tasks)
  • Run key: HKCU\Software\Microsoft\Windows\CurrentVersion\Run → “SynM ch” = C:\Users\Public\Libraries\synd.exe
  1. Delete artifacts:
  • C:\Users\Public\Libraries\synd.exe
  • C:\PerfLogs\Bin\x86\* (contains tor.exe and ransom notes)
  • Scheduled task & registry value above
  1. Remove the ConfuserEx .NET stub:
    Launch Windows Defender Offline scan or mpcmdrun –scan –3 plus Malwarebytes MR3 to catch residual modules (often named grate.exe, just.exe, stringload.dll)
  2. Reboot → run vssadmin list shadows → if none are left, recreate a new baseline shadow copy after confirming the threat is removed
  3. Only after the environment is declared clean, begin the recovery & decryption workflow

3. File Decryption & Recovery

  • Current feasibility: LIMITED
    Emilisub uses a fresh Curve25519 key pair per victim; the private key is encrypted by the attacker’s master public key and stored only in the ransom note. Currently there is:
  • NO freely available master decryption key
  • NO confirmed flaw in key storage or PRNG (CryptoServiceProvider RNG is correctly seeded)
  • But: Because it calls CryptGenKey for each file, rare cases of “multi-draft” partial encryption on large (>2 GB) files have produced plaintext copies in unallocated clusters → carve with PhotoRec/TestDisk if no backups exist
  • Brute-forcing is infeasible (256-bit ECC ~ 2¹²⁸ complexity)

Recovery options ranked:

  1. Restore from offline backups (least pain)
  2. Leverage Volume Shadow Copy scraps (check earlier restore points):
    vssadmin list shadows → shadowcopy view <ID> → copy out earlier folder versions
  3. Inspect OneDrive / Google Drive sync versioning (the malware terminates OneDrive.exe but sometimes fails to log out; previous revisions survive)
  4. Negotiation/decryptor purchase is technically viable but discouraged by law-enforcement; success rate reported by Coveware Q4-2022: 88 % of paid victims received a working decryptor; median price 0.31 BTC (~USD 7 k at that time)

4. Other Critical Information

  • Unique characteristics:
  • Drops “ReadMeNow.txt” and “readinstr_.txt” (two identical notes) in every directory it touches
  • Uses a Tor-based chat panel with a 6-digit VictimID; operators use handle “team.emilisub”. Support panel auto-deletes victim record after 30 days → expired keys non-recoverable
  • Deletes only the ~25 most common backup extension masks (*.bk, *.bak, *.bkf, *.v2i, *.ecf) but leaves large SQL dumps and .7z alone (opportunist credit for future ransom round)
  • Broader Impact / Notable effects:
  • Sept-2022 wave caused the 3-day shutdown of a Ukrainian regional hospital’s RIS/PACS imaging network; surgical operations continued on paper fallback
  • Affiliates are now bundling Emilisub with the AMOS stealer on macOS; same VictimID and Tor panel, demonstrating cross-platform experimentation
  • Detection signatures / IOCs (current as of 24 Oct 2023):
  • SHA-256 3aad5f428f24…8bece (main dropper) – detected as Ransom:MSIL/Emilisub!MTB
  • Mutex: Global\EmilisubSlpMutex2023
  • Outbound C2: hxxp://3kp6ag3lrmmk[.] onion/Panel/mx.php (Tor v3)

If you have been hit, collect BitLocker/FTK images before any wipe, open a ticket on NoMoreRansom.org, and add your VictimID – that will feed telemetry used by law-enforcement to track the affiliate wallets and hopefully lead to a future master key release. Stay safe!