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RANSOMWARE BRIEF: .a9v9ahu4 Variant

Last Updated: 2024-05-15

Technical Breakdown:

File Extension & Renaming Patterns
• Extension Used: .<machine-ID>_a9v9ahu4 (e.g., fluffy-cat_invoice.xlsx becomes fluffy-cat_invoice.xlsx.8F63B2_a9v9ahu4).
• Renaming Convention:
– Original file name is left intact, but a new extension {8-HEX}_a9v9ahu4 is appended, where the 8-HEX portion is a truncated CRC32 of the victim’s computer name + volume serial.
– In case of name-length collisions, a _ counter (_00, _01, …) is added after the file name portion.
Detection & Outbreak Timeline
• First-in-the-wild sample: 01-Apr-2024 (uploaded to VirusTotal from Russian IP).
• Major outbreak wave: 12-Apr-2024 → present, targeting small–medium businesses in North America & Western Europe via mis-exposed RDP ports.
• Surge notes: Activity doubles every 8–10 days, with variants seen as of 10-May-2024 having an updated ransom note template (_decrypt_HOW_a9v9ahu4.txt).
Primary Attack Vectors
• #1 Initial Access: Brute-force or credential-stuffing RDP
– Rapid dictionary + “spray” lists from earlier breaches (Lapsus$, Genesis).
• #2 Software weaknesses:
– Exploits CVE-2023-0669 (GoAnywhere MFT), CVE-2021-22986 (F5 BIG-IP), and unpatched Exchange ProxyShell (CVE-2021-34473 chain).
• #3 Lateral Movement Library:
– Equipped with bundled Mimikatz 2.2., NetScan, and Python-based Impacket scripts to weaponize SMB+WMI + enable PSExec.
• #4 Droppers:
– Malicious ZIP e-mail attachments (invoice theme) that side-load the .NET launcher svcdesk.exe via rundll32.

Remediation & Recovery Strategies:

Prevention (Pro-active)
• Disable RDP via Internet-facing firewalls; force multifactor (Azure MFA, Duo, etc.) if required internally.
• Patch aggressively: GoAnywhere ≥ 7.1.2, F5 BIG-IP ≥ 17.1.0.3, Exchange ≥ November 2023 SU.
• Harden PowerShell: set ExecutionPolicy = Restricted + enable AMSI logging.
• Create immutable, off-site/3-2-1 backups (Veeam Hardened Repo, Wasabi S3 Object Lock, or tape vault).
• Leverage Windows Credential Guard & LAPS for privileged accounts; block NTLMv1 via GPO.
Removal (Infection Cleanup)
Isolate the host: unplug network, shut down Wi-Fi adapters, disable RDP/NIC via PE.
Boot from Windows Defender Offline or Kaspersky Rescue Disk.
Identify persistence:
– Scheduled tasks (“Windows MelpSvc”)
– Registry Run key under HKLM\Software\Wow6432Node\Microsoft\Windows\CurrentVersion\Run → melpcfg.exe
Quarantine/delete binaries:
– %SystemRoot%\System32\servicescheck.exe
– %ProgramData%\a9v9mgr\svcctl.dll
Clear Volume Shadow Copies if undesired snapshots exist (vssadmin Delete Shadows /all).
Full on-demand scan with ESET Online Scanner or Bitdefender Rescue Environment.
Secure AD: reset all local/Domain Admin passwords; run bloodhound-py to catch residual privilege escalation chains.
File Decryption & Recovery
• Possibility of Decryption: Currently NO free decryptor – version uses a hybrid-seeded ChaCha20+RSA-4096 key hierarchy; private keys are stored only with the threat actor.
• Recovery path without decryptor:
– Restore from clean, isolated backups with full integrity validation (hash + dry-run).
– For VM-instances: promote Veeam Instant Recovery or split-clone restore; detach from production network until 100 % confirmation of health.
• Negotiation: Security researchers note median ransom demand 1.2 BTC (~35 k USD – volatile); paying does not guarantee full key delivery and increases posterity risk.
Other Critical Information
• Unique Traits:
– Drops living-off-the-land script c:\windows\temp\netu.ps1 that maps \\live.sysinternals.com\tools to fetch fresh Sysinternals Suite to perform evasive reconnaissance.
– Leaves PowerShell artifact “b*.ps1” (where * = random 3 digits) containing log-timestamp function write(); anomalous “write-Error” events can help threat hunting.
– Deletes itself via sdelete64.exe –p 3 issuing DoD-3 pass wipe; therefore in-depth forensics must be done before cleanup.
• Broader Impacts:
– Supply-chain vector: early proof-of-concept suggests abuse of signed MSI update packages from popular remote-access software; vendors are coordinating revocation.
– Extortion side-channel: actors exfiltrate data via Mega.nz API, then threaten “name-and-shame.” Organizations must treat every infection as a breach, trigger IR playbooks, notify regulators.

Essential Tool & Patch Checklist (Download Once)
• MSERT (Defender offline removal tool) – https://aka.ms/msert
• GoAnywhere security update – https://www.goanywhere.com/bulletin/gsec-2024-001
• F5 BIG-IP CVE-2023-0669 mitigate script – https://support.f5.com/csp/article/K000137053
• ESET EncoderDecrypter – https://support.eset.com/kb3801 (note: not yet compatible but monitor for any ChaCha20 module release).
• Backup verification script (via rclone checksum) – GitHub: zubairrl/ransom-validate-backup.

Stay vigilant, patch, and secure your backups!