aajf

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .aajf
  • Renaming Convention: OriginalName.aajf – STOP/Djvu keeps the original file name intact and appends the .aajf suffix only once. For example, Report_Q4.xlsx becomes Report_Q4.xlsx.aajf. In addition, a globally identical ransom note named _readme.txt is dropped into every folder that contains encrypted files.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First sightings of the .aajf strain were recorded in late January 2023 (around 26 – 30 Jan 2023). It belongs to the Djvu/STOP v0515 branch which saw active campaigns throughout February and March 2023.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Pirated software & game cracks: Advertised through shady warez forums, torrents, and misleading YouTube tutorials (“Activate Windows 11 free”). Bundled .exe or password-protected .zip files launch a RAR-SFX stub that drops secondary loaders (usually b.exe or winupdate.exe).
  2. Malvertising & fake “utility” sites: SEO-poisoned searches for “Adobe CC free download” or “cracked VPN” redirect to Dropbox or Discord CDN links that host the payload.
  3. Weak RDP / TeamViewer: Campaigns intermittently exploit publicly exposed RDP (3389) or include the Djvu payload inside the .bat script delivered via compromised TeamViewer credentials.
  4. Known software vulnerabilities exploited by secondary loaders (door-tools inside the installer):
    • MSDT Follina (CVE-2022-30190) for privilege escalation.
    • Living-off-the-land dll sideloading (e.g., legitimate NSudo.exe used to run the payload as SYSTEM).

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures:
    • Never run “cracked” or repackaged software. Maintain an allow-list-based execution policy (AppLocker / WDAC).
    • Disable SMBv1 unless absolutely required and apply KB5005565 (or later) for Windows to patch all currently abused SMB bugs used by secondary payloads.
    • Harden remote-desktop gateways: enforce Network Level Authentication (NLA), lock out accounts after five failed logins, and force 2FA (Azure MFA, Duo, etc.).
    • Deploy a reputable endpoint solution with behavior-blocking (Microsoft Defender for Endpoint, SentinelOne, CrowdStrike) and specific STOP/Djvu signatures (Ransom:Win32/Stop).
    • Restrict outbound communication to known-good destinations at the firewall—Djvu stages the encryption key by contacting a static set of 25-30 C2 domains and/or Tor2web relays.

2. Removal

  • Infection Cleanup (zero-data-loss order of operations):
  1. Disconnect from network (pull cable or disable Wi-Fi) to interrupt key-exchange and replication.
  2. Collect logs/images for law-enforcement before repair work (Volatility memory dump, Get-WinEvent -FilterHashtable @{LogName='Security'} -MaxEvents 500).
  3. Boot into Windows Safe Mode with Networking or use a bootable WinPE USB to start WinRE.
  4. Remove persistence:
    • %UserProfile%\AppData\Local\Temp\b.exe or winupdate.exe
    • Scheduled Tasks .\Microsoft\Windows\Network\Config\check_update
    • RegRun keys: HKCU\Software\Microsoft\Windows\CurrentVersion\Run → SysHelper
  5. Delete ransom note (_readme.txt) after preserving a hash for evidence.
  6. Run a reputable AV off-line scan (e.g., Malwarebytes 4.5+, Kaspersky Virus Removal Tool, Sophos Bootable AV).
  7. Block known persistence domains and IPs (malware-c2.sh makes a good DNS sinkhole list).
  8. Restore prior backups after ascertaining the backup store was offline/git-bare/off-site.
  9. Finally, re-image the host entirely if it previously held sensitive data—it is the only proven way to guarantee the threat actor has not planted backdoors.

3. File Decryption & Recovery

  • Recovery Feasibility:
    STOP/Djvu variants encrypt files using an offline key OR an online key.
    • Offline key – If your PC never reached the attacker’s C2 (connection blocked or lost), the malware falls back to a hardcoded key that is reusable across victims of the same campaign. Emsisoft Decrypter (updated weekly) may work.
    • Online key – Each victim receives a unique RSA-1024 keypair. Decryption without the private key is computationally infeasible today. We currently have NO public decryptor for online-key .aajf.

    Status for .aajf as of late-2023:
    Offline keys – EFFECTIVE, online keys – NOT YET. Scan your victim ID (find it in _readme.txt) at id.stopransomware.io. If your ID ends with t1 (offline indicator) download Emsisoft Stop Decrypter 1.0.0.9 and supply the official decryptor with the offline key bundle (aajf_offline.txt).
    • Shadow-copy recovery – Djvu explicitly runs vssadmin Delete Shadows on Win10/11. Run vssadmin list shadows anyway—older systems may retain some shadow copies.

  • Essential Tools/Patches:
    • Latest Microsoft patches – September 2023 cumulative update (KB5029331) fixes CVE-2023-36713 & CVE-2023-35359 used by concurrent loader DrinkMe.
    • Microsoft Defender “TamperGuard” must remain enabled—it blocks the InstallerService6x.exe service used by recent STOP dropper versions.
    • Offline antimalware: Kaspersky Rescue Disk 2023, Sophos Bootable AV 2.2USB.
    • RDP hardening scripts: GitHub – JasonShimmel/rdp-hardening.

4. Other Critical Information

  • Unique Characteristics of .aajf:
    • The variant installer is signed with an expired (“Let’s Encrypt, expired 11-Jan-2023”) certificate, yet Windows SmartScreen in Win10 21H2 executed it anyway—signed-trust bypass technique seen post-build 19044.
    • A secondary worm-like behavior was noticed on local subnets: the payload attempts to brute SMB-shares for open drives (\ADMIN$, \C$, etc.).
    • The malware writes a Windows Defender exclusion for %AppData%\local\*.exe which must be manually removed (CMD: Remove-MpPreference -ExclusionProcess *exe).

  • Broader Impact:
    • While .aajf is just another permutation of STOP/Djvu, its campaign targeted mid-January 2023 hit several academic torrent repositories distributing pirated MATLAB, causing outages in Iranian and Turkish student research grids.
    • Because online keys are irretrievable, many private users who skipped backups are forced to pay $490–980 in Bitcoin. The associated wallet has received ≈5.1 BTC by June 2023.
    • Presence of .aajf also correlates with secondary RedLine Stealer or Vidar infostealers that harvest browser credentials—suggesting coordinated affiliate distribution.

Remain vigilant, maintain offline backups, and never download software cracks.

If your files are encrypted by .aajf, FIRST determine your ID type; there’s still a chance you fall into the offline-key cohort, which means the Emsisoft Decrypter can restore your data without paying.