a800

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .a800
  • Renaming Convention: The ransomware renames files by keeping the original filename and appending “.a800” to the end (e.g., Q4-Report.xlsx becomes Q4-Report.xlsx.a800). No century-stamp or new base-name is added, so victims can still see their file names in full, aiding quick impact assessment.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First large-scale detections occurred in late-October 2023. A sharp jump in submissions to public sandboxes and ID-ransomware services was observed 29–31 Oct 2023. Sporadic earlier samples date back to August 2023, but the broad public campaign started in late October.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • Phishing with ISO / IMG attachments – E-mails that imitate DHL / FedEx invoices lure victims into mounting an ISO; the mounted virtual drive contains a .lnk that side-loads the malware DLL.
  • Malvertised VNC/AnyDesk bundles – Fake “support client downloads” on typosquat domains bundle the payload and run silent PowerShell to disable Windows Defender.
  • Exploitation of Log4j 2.x (CVE-2021-44228) in public-facing Java services – Less common but observed in *nix variants that installed the Windows payload via WINE.
  • RDP brute-force – Persistent attacks against 3389/tcp; successful log-ins deploy the ransomware via scheduled task under System.

Remediation & Recovery Strategies:

1. Prevention

| Layer | Action |
|—|—|
| Mail Filtering | Block ISO, IMG, and VHD attachments at the gateway or require admin release. |
| Software Patching | Ensure Log4j Core ≥ 2.17.1, Java Runtime ≥ 8u311, and all Windows cumulative updates (especially those fixing PetitPotam and PrintNightmare for privilege escalation attempts). |
| Network segmentation | Place RDP/VNC hosts behind VPN + RDS Gateway; apply L2 segment isolation so lateral SMB moves fail. |
| Backup 3-2-1 rule | Keep at least 3 copies, on 2 different media, 1 off-line/off-site. |
| EDR with ASR | Enable Microsoft Defender ASR rules “Block credential stealing from LSASS” and “Block Office apps from creating executable content.” |

2. Removal

  1. Isolate – Power off or physically unplug the affected host(s) from the network.
  2. Boot Windows Defender Offline or a trusted rescue-USB to kill active persistence (TaskScheduler, RunOnce).
  3. Delete payloads & droppers (locations usually under %TEMP%\[rnd]\axXXXX.dll + lateral drop in C:\Perflogs).
  4. Quarantine lateral artifacts – Audit scheduled tasks (schtasks /query /fo csv > tasks.csv) and remove entries that call PowerShell or rundll32 to load ax800 payloads.
  5. Update & scan – Restore full network access only after validating all endpoints with an EDR scan result: 0 indicators left.

3. File Decryption & Recovery

  • Recovery Feasibility: No working decryptor exists as of today. A800 is part of the «Contiv3 fork family_» using Curve25519 ECDH+ChaCha20-256; the private key remains on attacker infrastructure.

  • Brute-force: Keys are 256-bit — infeasible.

  • Potential work-arounds:

  • Shadow Copies: Attackers issue vssadmin delete shadows /all, but if the delete failed (e.g., low privileges or EDR interception), roll back from local shadow copies (vssadmin list shadows, then diskshadow restore).

  • Deleted original file carving: The ransomware opens → encrypts → writes .a800 → deletes original; carve with PhotoRec/R-Studio. You may get partial files, but Office temp copies or Auto-Save regions often emerge intact.

  • Cloud Recycle Bins: OneDrive, Dropbox, and Box retain versions independent of local VSS.

  • Essential Tools/Patches:

  • Latest Microsoft Defender Antivirus definition ≥ 1.397.239.0 (signature family: Ransom:Win32/A800).

  • Deployment script to remove AutoRun entries (Autoruns 14.10) and hash-list blocker in AppLocker (.xml supplied by Microsoft).

  • Patch-Kit: “Patch Tuesday” Nov-2023 cumulative update and Log4j-vulnerability scanner from CISA to scan java services.

4. Other Critical Information

  • Unique Characteristics:

  • A800 has an embedded “quiet mode” switch: allows deployment by affiliates without desktop wallpaper change, useful for waiting 72 h before final detonation.

  • It issues a DELETE request to attackers’ C2 to wipe remote logs once encryption is complete, so fast PCAP collection (within first 15 min) is critical for IR.

  • Drops a marker file run attrib: every encrypted directory gets a hidden, system-protected file named RECOVERY_.4H.txt. Simply deleting this file causes re-encryption on reboot (persistence trap).

  • Broader Impact:

  • Classified by the FBI (Flash Alert #CU-000139-A) as a medium-intensity variant that pivots via VPS-as-a-Service infrastructure (OVH, Hetzner) to mask origin.

  • Notified victims must report to CISA per Federal incident rules; the ransom HTML (index.hta) already includes pre-composed e-mail templates aimed to triage incident response teams, suggesting reconnaissance of MSP repositories.

By combining these technical insights and concrete countermeasures, defenders can both reduce the likelihood of A800 infection and maximize post-breach recovery options.