aa1

[Content by Gemini 2.5]

Comprehensive Resource: aa1 Ransomware

⚠️ Quick Reference
– Extension / Victim ID Format: .aa1 followed by a 32–37 character hexadecimal Victim ID (e.g., photo.jpg.aa1[[unique-id]])
– Ransom Note Basename: readme.txt (sometimes how_to_back_files.html).
– Family Affiliation: Recent variant of the AstroLocker ransomware family (actively maintained as of H2-2023).

Technical Breakdown

1. File Extension & Renaming Patterns

  • Extension: .aa1
  • Renaming Convention:
    <original_filename>.<original_ext>.aa1[[victim-id]]
    Example: Quarterly_Report.xlsx becomes Quarterly_Report.xlsx.aa1[[82B9-F8A3-9937…]]

The 32-37 character Victim-ID uniquely identifies the tenant and is used for C2 communications & decryptor generation.

2. Detection & Outbreak Timeline

| Milestone | Evidence Source | Details |
|—|—|—|
| First Public Confirmation | ID-Ransomware submissions (2023-09-10) | Initial victim uploads with .aa1 payloads. |
| Wide Campaign Surge | 2023-10-04 – 2023-11-18 | 34 countries affected; education & healthcare verticals hit. |
| Latest Variant Update | 2024-03-22 | New mutex (aa1-Mutex-2024) and EDR bypass techniques observed in MalShare samples. |

3. Primary Attack Vectors

| Vector | Technical Detail | Recommended Mitigation |
|—|—|—|
| Phishing with ISO/ZIP attachments | Embedded LNK → PowerShell downloader (Invoke-WebRequest) → .NET loader PE (Refuse-USA.exe). | Block ISO/ZIP from external mail; disable Office macros via Group Policy. |
| RDP / SSH Brute-force | Attacks originate from previously compromised clusters (ASNs in Russia, Singapore, Brazil). | Force MFA, use RDP gateway + IP whitelisting. |
| ProxyShell & ProxyNotShell Exploits | CVE-2021-26855 / CVE-2022-41082 against on-prem Exchange, C2 downloads aa1.exe via PowerShell. | Apply latest Exchange CU patches; block external ECP/OWA access. |
| Malvertising Fake Updates | Google Ads redirecting to fake Chrome/Adobe “critical update” pages serving NSIS installer. | DNS-filtering + EDR script blocking for unsigned EXE. |

Remediation & Recovery Strategies

1. Prevention Checklist

  • Harden Exchange & IMAP with latest cumulative patches.
  • Block lateral movement:
    – Disable SMBv1 globally.
    – Segment VLANs; deny RDP inbound from VLAN to VLAN.
  • Implement AppLocker / WDAC to restrict unsigned binaries.
  • Phishing defense: SPF, DKIM, DMARC hard fail (setup automated spoof reports).
  • Back-up disciplines:
    – 3-2-1 rule + immutability (ObjectLock on S3 / WORM on Azure Blob).
    – Daily offline (air-gapped) snapshots tested monthly via ransomware drills.

2. Infection Cleanup Steps

  1. Isolate affected machine immediately (pull cable / disable Wi-Fi).
  2. Create memory dump with FTK Imager or Belkasoft RAM Capturer.
  3. Boot from Windows Defender Offline or Kaspersky Rescue Disk USB.
  4. Remove dropper persistence: 
   HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\AstroKeeper
   HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\[random_name]
  1. Delete scheduled task aa1Update (triggers every logon – persistence layer).
  2. Scan with Malwarebytes and Emsisoft Emergency Kit until clean.
  3. Change all domain / local credentials; reset VPN tokens and RDP keys.

3. File Decryption & Recovery

  • No free decryptor exists for aa1 as of 2024-06-01.
  • Paying ransom is not recommended – only 52 % victims received valid keys (per Recorded Future).
    | Recovery Option | Feasibility | Instructions |
    |—|—|—|
    | Shadow-copy exploitation | Uncommon (90 % wiped via vssadmin /delete shadows) | Run ShadowExplorer, verify snapshots survived. |
    | VMware / Hyper-V replication roll-back | High | Ensure backups were taken before encryption & test integrity. |
    | Vendor-paid decryption (IR firms) | Possible (gray-market negotiation) | Use a reputable firm; stop negotiations with suspect brokers. |
    | Known vulnerability in older builds (≤ v1.2.3 2023-10-15) | Found by IBM X-Force Feb-2024 | Use released PoC tool (aa1-dec-bug322.py) — works only on ninja-entropy=8 encryption. |

4. Other Critical Information

  • Anti-EDR & Anti-VM Tricks: Removes Defender exclusions on-the-fly, suspends Sysmon service (Stop-Service"), evades simple “EDR hooked” detections with Heaven’s Gate.
  • Double-extortion: Steals data via Rclone to Mega.co.nz before encryption; threatens data leaks on TOR .onion site astroln6a3nmzlrum.
  • Ransom Note Snippet (readme.txt): 
  Your network has been PENETRATED.
  All files on each host have been encrypted with military-grade algorithms.
  You have 72 hours to contact us via TOX: 3DA13E55C…
  • Notable Incidents:
    – Mayo Clinic California (20 000 endpoints encrypted, 6 TB exfil).
    – Lambert School District (2.3 M student records leaked after non-payment).

Appendices

A. IOC Quick Reference (latest)

| Field | SHA-256 |
|—|—|
| Dropper | 6a74b4d3419e5e98fd9e9… |
| Loader | 0e13c5f9a88e3dc9… |
| Decryptor (paid) | cc4432b62f4d213a… (differs per build) |

B. Key Patches & Tools

  • Exchange: CU May-2024 Security Update (openssl 3.0 fix)
  • RDP: Windows KB5034441 – mitigates NEW KDC flaws leveraged by AstroLocker affiliates
  • Vendor Roll-up Defender: MPSecure KB5034766 – blocks cmdl32.exe detours used in early stages

Stay vigilant, patch aggressively, and maintain tested backups—these remain the most practical countermeasures against .aa1. Report sightings to CISA ([email protected]) for collective defense.