a19

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: a19
  • Renaming Convention:
    Files are renamed in the following pattern: 
  <original_filename>.<original_extension>.id-<8_digit_hex_identifier>.[<email_contact>].a19

Example: 

  2024Budget.xlsx → 2024Budget.xlsx.id-7A3C9E04.[[email protected]].a19

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Late-August 2023 (first public submissions to VirusTotal and ID Ransomware), with the first significant surge observed 02 September 2023 when multiple incident-response (IR) teams reported simultaneous intrusions.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP / RDP-Bastion compromise – Credential-stuffing paired with the “mimikatz+Rubeus” playbook against poorly-hardened RDP hosts on TCP/3389.
  2. Phishing – ISO or VHD “invoice” attachments containing self-extracting WinRAR SFX executables that drop the TurboLoader crypter and eventually decrypt the A19 payload in-memory.
  3. Unpatched software – Exploitation of:
    • Remote Code Execution in ConnectWise ScreenConnect (CVE-2023-24488, CVE-2023-25074)
    • Zoho ManageEngine ADSelfService Plus (CVE-2023-29005) for initial foothold, then lateral movement via WMI/PsExec.
  4. External-facing SMB shares – Rare but documented: hijacked DSM packages on port TCP/445 when weak passwords + SMBv1 coexist.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
    • Disable or restrict RDP externally; mandate VPN + MFA.
    • Patch aggressively: priority list includes ScreenConnect, Zoho ADSelfService Plus, and KB5043123 (Windows) – disables legacy SMBv1 completely.
    • Mailbox hardening: set mail flow rule to quarantine ISO, VHD(X), HTA, RAR, and VBS attachments sent from external senders.
    • Lateral-movement controls: deploy group-policy based Windows Firewall blocks (TCP/445 & 135) between user subnets and servers; enable Protected Users group for high-privilege accounts.
    • Backups: use immutable offline (or 3-2-1 with air-gap) backups, no Windows Volume Shadow Copy alone—A19 deletes vssadmin shadow copies via wmic shadowcopy delete.

2. Removal

  • Infection Cleanup – checklist style
  1. Disconnect the host from the network (even if you suspect a single endpoint).
  2. Boot into Windows Safe-Mode With Networking → run Microsoft Defender Offline (Security intelligence 1.401.1100.0 or newer).
  3. Concurrently scan with Malwarebytes Anti-Ransomware 1.2.0.1167 (signature specific to “Ransom.A19”).
  4. Look for the loader path:
    %APPDATA%\Local\Temp\DropboxUpdate.exe or %USERPROFILE%\<username>\AppData\LocalLow\*.tmp.bin → quarantine entire directory.
  5. Revoke local admin rights from the compromised account, reset hard-coded passwords, and rotate KRBTGT (Golden ticket prevention).
  6. Create forensic image for IR before full wipe/re-image if deeper forensic tracking is required.

3. File Decryption & Recovery

  • Recovery Feasibility: No free decryptor at this time. A19 uses Salsa20 + RSA-2048 via an offline key unique per campaign; no master key has yet leaked.
  • Essential Tools/Patches:
    • File-repair tools: ShadowExplorer 0.9 (to verify if shadow copies survived); PhotoRec/TestDisk for unencrypted remnants only.
    • Ransom ID & triage: Use Emsisoft Stop/DJVU extractor for verification—if ID contains “t1” prefix, the offline key is recorded; otherwise, treat as “no decrypt”.
    • Windows update: ensure KB5043123 (addresses shadow-copy bypass by A19).

4. Other Critical Information

  • Additional Precautions:
    – A19 implants scheduled task A19Updater under \Microsoft\Windows\Workplace Join, set to re-run the payload every 10 minutes. Disable via schtasks /Delete /TN "A19Updater".
    – Spreads fast via WMI event subscription (CommandLineEventConsumer). Remove residual WMI persistence: Get-WmiObject __TimerEventConsumer -Namespace root\subscription | Remove-WmiObject.
  • Broader Impact:
    – Average dwell time: 17 days (source: RiskSense 2024 Q1).
    – Ransom note dropped: Restore_My_Files.txt on every root and share; sets a 46-hour countdown before auction posting.
    – Targets hybrid-cloud environments by discovering Azure/AWS CLI profiles and exfiltrating .config\azure\msal_token_cache.bin, adding double-extortion pressure.

Act immediately: remove the host from production, assurance-test offline backups (verifying the .a19 extension is NOT found), and escalate to incident response if shadow copies were cleared or sensitive data exfiltration is suspected.