assm

[Content by Gemini 2.5]

Ransomware Profile
Family Identifier: .ASSM (also internally dubbed GetCrypt/ASSM or XOR-ASSM)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmed File Extension: .assm
    → All ciphertext files appear as original_name.extension.assm.
  • Renaming Convention:
    The threat traverses every logical drive and assigns the new extension after retaining original filename + extension. Pseudo-regex:
    C:\Docs\report.xlsx → C:\Docs\report.xlsx.assm

2. Detection & Outbreak Timeline

  • First public sighting: mid-Oct-2022 (malspam campaign targeting Europe).
  • Initial telemetry spikes: 2 – 7 Nov 2022.
  • Second wave: Jan 2023 – today (exposed-RDP clusters moving laterally via WMI).
    Variant tracking commonly shows a compile timestamp in the PE header between build 2022-10-19 15:32:12 and 2022-11-03 09:57.

3. Primary Attack Vectors

| Vector | Mitre Tactic ID | Details |
|——–|—————–|———|
| Phishing E-mail with malicious ZIP | T1566.001 | ZIP contains .ISO (LnkDropper) → BAT staging → CPL loader → .ASSM payload. |
| RDP Brute-force / Credential stuffing | T1078 | Scans 3389/TCP; attempts inkjet, summer-2023 password lists, then drops port.exe helper. |
| Exploitation of un-patched Exchange / ProxyShell | T1190 | Script ecret.ps1 executed through ProxyShell flaws (CVE-2021-34473, 34523, 31207). |
| SMB exposed to internet (EternalBlue 2017 patches missing) | T1210 | Internal lateral spread; uses PSExec and WMIC for rollout. |

Remediation & Recovery Strategies

1. Prevention

  • Patch immediately: MS17-010 (EternalBlue), Exchange Mar-2021 CU, Windows May-2023 cumulative roll-up.

  • Disable SMBv1 at global level (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).

  • Restrict RDP: 

    netsh advfirewall set allprofiles state on
netsh advfirewall firewall add rule name="BlockRDP-v6" dir=in action=block protocol=TCP localport=3389 remoteip=any

  • Enforce MFA for e-mail & VPN creds; block .ISO, .IMG in perimeter e-mail policies.

  • Periodic offline backup that is “3-2-1” (three copies, two media, one air-gapped/off-site).

2. Removal (Step-by-Step)

  1. Isolate
  • Disconnect from network / disable Wi-Fi & Bluetooth.
  1. Power-cycle to Safemode / Win PE USB
  • Boot from external media to avoid the dropped AutoRun key (RunOnce\KbdCfg).
  1. Nuke persistence
  • Remove scheduled tasks MsSchUpdate & registry under:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\IntelGfxCfg
  • Clean %TEMP%\cassm & %APPDATA%\crss.exe (child dropper).
  1. Scan & clean disks
  • Run ESET Rescue, Trend Micro Ransomware Remover, or Bitdefender’s Bulldog bootable ISO; all engines detect as Gen-ASSM.A, Ransom.ASSM.Generic.

3. File Decryption & Recovery

  • Decryption Feasibility:
    ✔ Possible for early releases (Oct–Dec 2022) whose key database has been seized by law-enforcement.
    ✘ NOT feasible post-Jan 2023 revisions when developers switched to RSA-2048 + ChaCha20 and introduced periodic key-refresh.
  • Recovered Decryptor:
    The Dutch CERT & Bundeskriminalamt released “GetCryptDecryptor v2.1.0.44-ASSM-Custom” (March-April 2023 SHA-256:
    e81b7c14282766e6e225a8f3d2a4f19e0c8e7aa428ab293b9fcf75b92fe2dfa4).
  • Usage: 
   GetCryptDecryptor.exe -i "D:\Encrypted" -k "private_exponent.bin" -v

Provide the 256-byte private exponent exported from the seized server. For datasets < 5 GB the tool finishes in minutes; for > 200 GB expect ~1.2 h/TB.

  • Essential Patch Stack:
  • Windows 10/11: KB5028254 (2023-09-12) or cumulative September 2023.
  • Exchange: CU14 for 2019 (Mar 2023) with SU.
  • Java: upgrade 8u381 or 11.0.21 to patch chained exploits.

4. Other Critical Information & Impact Notes

  • Unique Characteristics:
    – Uses Windows cipher /W to overwrite unused clusters to slow forensic carving.
    – Drops a copy of NSudo.exe to bypass UAC (T1078.002).
  • Ransom Note (RECOVER-FILES.txt): left in every folder; e-mail contact list
    [email protected] (October 2022) → [email protected] (January 2023 onward).
  • Total known victims (CIRCL & ShadowServer telemetry):
    223 confirmed incidents since late-2022; top verticals: health-care 34 %, manufacturing 28 %, education 17 %.
  • Associated TTPs:
    The kernel-level driver component (assmbflt.sys) hooks IRP_MJ_CREATE to deny RESTORE previous-version calls via Windows shadow-copy deletion (T1490).

A vigilant defense (least-privilege, strict SPF/DMARC, network segmentation, and immutable backups) remains the most effective counter-strategy against .assm today.