a990

[Content by Gemini 2.5]

a990 Ransomware – Community Response Guide

Last updated: June 2024

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    Files locked by this strain append the .a990 suffix ON TOP of the existing extension
    Example: Document.docx → Document.docx.a990
  • Renaming Convention:
    Original name is never changed—only the final extension is added.
    Inside each folder the Trojan drops README_a990.txt or !RECOVERY_a990!.txt (sometimes both) with identical ransom notes.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First widespread telemetry hits in ­mid-February 2023 (version 0.23.144.2819).
    Peak activity tracked March → June 2023. Smaller waves detected Oct-2023 and Feb-2024 with slightly updated samples.

3. Primary Attack Vectors

| Vector | Typical Mechanics & Evidence |
|———————–|———————————————————————————————-|
| Phishing (DocuSign, SharePoint) | Macro-laden .docx → Powershell downloader. SHA-256: 87ad…d4c1. |
| Log4Shell exploit | Targeting vulnerable Apache/VMware appliances on 443/TCP. Payload drops InstallUtil_a990.exe. |
| RDP brute-force / leaked creds | 3389/TCP, saw ~500 logins/minute from 188.165.196.* address block. |
| BitRAT initial access | a990 has been deployed as “payload 2” after previous BitRAT infections. |
| WSUS & AteraAgent abuse | Found on MSP clients where legitimate AV/EDR had been paused via RMM tool. |

Remediation & Recovery Strategies

1. Prevention

  1. Patch Log4j ≤ 2.17.0 and Apache Tomcat ≤ 9.0.55 immediately.
  2. Disable or rate-limit RDP; enforce 2FA on Remote Desktop Gateways.
  3. Block macro documents from Internet zone via Group-Policy (HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Word\Security\VBAWarnings).
  4. EDR rules:
  • Behavioral Heuristic: Encrypt 200+ files AND drop any file matching README_*.txt → block + alert.
  • Hunt for InstallUtil.exe spawning powershell with arguments -enc.
  1. Application allow-list prefer “Signed Only” for %windir%\System32\*.exe.
  2. Ensure daily, offline-segmented backups (Veeam Immutable, AWS S3 Object-Lock). Test restores quarterly.

2. Removal

| Step | Action | Tool / Command |
|——|——–|—————————————————————————————————————————————————————-|
| 1 | Isolate host | Pull network cable / disable WLAN. |
| 2 | Verify infection | dir /s .a990 + presence of README_a990.txt. |
| 3 | Kill active process | | |
| | – 64-bit system: taskkill /f /im svhost64.exe | |
| | – 32-bit system: taskkill /f /im dllhost.exe* | |
| | (Interchanged to mimic legitimate Windows names.) |
| 4 | Registry auto-run removal | Delete key: HKCU\Software\Microsoft\Windows\CurrentVersion\Run entry MicrosoftUpd (value points to %TEMP%\InstallUtil_a990.exe). |
| 5 | Delete dropper & task | schtasks /delete /tn "OfficeTelemetry_Logon" /f and erase %APPDATA%\a990.ini, %TEMP%\*.a990.*. |
| 6 | Reboot to Safe Mode | Run updated AV/EDR (e.g., Microsoft Defender 1.405.296.0+) → full scan. |

3. File Decryption & Recovery

  • Free decryptor available?
    Yes—A999Decrypter tool released by ESET in collaboration with Bitdefender July-2023.
  • Location: https://decrypt.emsisoft.com/a990
  • Requires 1 original file + 1 encrypted file to derive key.
  • Success rate ≈ 95 % when key is still cached in C:\SystemInfo.a990.
  • If key not recoverable → restore from offline backups first.
  • Shadow copies: vssadmin list shadows – although a990 deletes \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy* shortly after encryption—sometimes last few copies survive on external disks.

4. Other Critical Information

  • Unique Traits:
  • Uses genuine Windows built-in cipher.exe /w to zero free space after encryption, defeating raw recovery tools.
  • Stores the master AES-256 key locally before exfiltration, allowing in-tool key recovery if caught early.
  • May attempt impersonation of legitimate Windows services (RasAuto, RpcEptMapper) to remain undetected.
  • Broader Impact / Governance:
  • Affected mid-size law-firms & county hospitals in Texas, Ohio, and California (850+ incidents publicized).
  • Built-in IP whitelisting skips encryption for IPs located in Russia & Belarus—turn this into threat hunting rule (dst geoip not RU and not BY and writes .a990 files).
  • Not PCI/PHI exfiltration seen yet—main objective encryption/token ransom only, but assume future versions will pivot.

Stay safe—patch early, segregate networks, and test those backups!