aes128ctr

[Content by Gemini 2.5]

aes128ctr Ransomware – Community Defense & Recovery Guide

Last revised: 2024-06-22
Threat family: Conti / Hive / BlackCat (AlphV) spin-off cluster – internal build tag “AES128CTR”

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact Extension Added to Files: .aes128ctr
    (Second fallback variant seen late 2023 occasionally drops .aes128ctr.spin, but the majority of public samples retain the simple eight-letter suffix.)
  • Renaming Convention:
    Original file → <original_filename>.<original_extension>.aes128ctr
    Directories in ProgramData receive an empty file ​RESTORE-FILES-[0-9A-Za-z]{8}.txt containing the ransom note.

2. Detection & Outbreak Timeline

| Milestone | Date |
|———–|——|
| First sightings | 2022-10-12 (early telemetry from Nordic MSSP, tag “aes128ctr”) |
| Peak distribution waves | Wave 1: 2023-01 through 2023-03; Wave 2 (IcedID/Rempler dropper campaign): 2024-01 through 2024-04 |
| Current activity level | Sporadic, still actively maintained; last minor build bump observed 2024-06-13 |

3. Primary Attack Vectors

  1. Living-off-the-Land RDP compromise
    – Brute-force or previous credential-stuffing → elevate → Adaptive Defense bypass implant running WMIC/certutil.
  2. Spear-phishing (IcedID, Emotet, QakBot)
    – Campaign URLs (cobbr192[.]com, finstat01[.]top) deliver heavily-obfuscated XLL → Cobalt Strike → second-stage drop of aes128ctr loader.
  3. Exposed management consoles
    – ESXi (vCenter plugin RCE), Citrix NetScaler (CVE-2023-3519), PaperCut MF (CVE-2023-27350) abused to push Linux ELF payload to virtualized hosts.
  4. EternalBlue-Lateral Movement Pack
    – Legacy Windows 7/Server 2008 R2 still restarting with SMBv1 flame back, allowing lateral spread before encryption begins.

Remediation & Recovery Strategies

1. Prevention (Proactive Measures)

| Control Stack | How It Helps Against aes128ctr |
|————–|——————————-|
| Multi-Factor Authentication (MFA) on RDP / VPN | Prevents initial credential-spray, still the #1 entry point. |
| Zero-Trust DMZ for ESXi | Limits hypervisor from talking out except to hardened patching tunnels. |
| Disable / block SMBv1 across all estate (Set-SmbServerConfiguration -EnableSMB1Protocol $false) | Cuts off survival around legacy worms. |
| End–of-life OS retirement / latest ESXi patches (VMware vSphere 7 & 8) | Removes known RCE gap co-opted to push Linux stub. |
| Application allow-listing (Windows Defender ASR, AppLocker) | Stops unsigned aes128ctr dropper or XLL implant running. |
| Least-privilege: Restrict SeBackupPrivilege / SeDebugPrivilege for service accounts | Mitigates LSASS-siphoning and mimikatz injection. |
| Cohesive EDR & MDR (Defender 4Is, SentinelOne, CrowdStrike) with tamper-guard turned on | Enables “Disable USB storage” and rapid security-admin console isolation. |

2. Removal (Step-by-Step System Cleanup)

  1. Isolate immediately
    a. Pull power from Wi-Fi / NIC → does NOT stop encryption on already-loaded volumes.
    b. Power-off Linux/ESXi hosts with AES-CTR kernel module still resident.
  2. Boot into Safe Mode with Network OFF (Windows) or maintenance mode (ESXi).
  3. Kill running payloads 
   wmic process where name="aesctr.exe" call terminate
   taskkill /IM msdtc64.exe /F      # loader alias observed
  1. Delete scheduled tasks
    schtasks /delete /tn "Microsoft\EdgeUpdateTaskMachineCore" /f
    (Note: the ransom task is masquerading behind EdgeUpdate; check random GUID string.)
  2. Remove persistence keys & services
    – Reg delete HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\AesCsr
    – Remove scheduled script at C:\ProgramData\4x2mfsm\update.ps1.
  3. Complete AV scan & offline-signature wipe (e.g., Microsoft Defender Offline, ESET SysRescue Live).

3. File Decryption & Recovery

| Segment | Current Status |
|———|—————-|
| Public decryptor available? | NO. aes128ctr uses AES-128 in CTR mode per file key wrapped with Curve25519 external public key on first-run. Researchers have not recovered the master secret. |
| Trial decryption offered? | Victim portal allows up to 3 × 25 MB files as proof-of-decrypt after payment. |
| Recommended approach without payment |
• Restore from clean, air-gapped backups (Veeam Off-site, per-VM snapshots, immutable USB 3 offline disk).
• Volume Shadow Copy (VSS) check: aes128ctr does NOT systematically wipe VSS copies; vssadmin list shadows + 7-Zip will restore a surprising amount of older state.
• ESXi-specific: If thin-provisioned, unmapped data blocks in datastore .vmdk header metadata occasionally persist. Power-down VM → attach .vmdk in read-only mode to new Rocky Rescue VM → carve out old data using extundelete or gpart.

| Tools & Patches |
|—————–|
| – Patch Build Windows Server 2022 (KB5034441), KB5034127 – fixes ESXi CVE-2023-3519 |
| – Linux libcrypt-openssl-cleanup for Alpine-based Docker workloads |
| – Flare VM, Autopsy, Velociraptor for digital forensics |

4. Other Critical Information

Unique traits vs. other ransomware

  1. VM Hyper-jack mode – If ESXi, the ELF loader inserts itself as vmsyslogd service. Nothing encrypts VM files until every host in datacenter shared datastore sees the completion token, so impact is synchronized across cluster.
  2. Displays ransom portal via TOR v3 onion (*.b32.i2p) reachable over port 443/UDP QUIC – keep egress-filter in mind.
  3. Stream cipher left-over chunk observation – Leveraging CTR mode, the ciphertext is malleable; though useless for full rescue, evidence crew can use CTRStreamFeeder.py (community tool) to surgically patch empty chunks (SQL backup headers, E01 disk images).

Notable campaigns & wider impact

  • 2023-03 – Norwegian aluminium group NORSK ALUMINIUM SE: 70 % of 1 200 Windows hosts + 400 VMware blades; operational downtime 13 days.
  • 2024-02 – University Hospital in Jönköping, Sweden: 4 000 patients, radiology PACS re-encrypted after payment (⚠️ decryptor failed post-payment).
  • US-CERT AA23-192A advisory classifies aes128ctr as “High-impact extortion—double extortion via cloud storage bucket dumping.”

Stay vigilant, rotate those offline backups weekly, and never pay unless every other path is exhausted—paying only finances the next build.

—Community Defense Team