asn1

[Content by Gemini 2.5]

Technical Breakdown: asn1 (nominally – actually the BarnOwl strain)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: This ransomware appends .asn1 – always lower-case, no underscore or hyphen.
  • Renaming Convention: \[PROCESS-ID-random_hex\] [_%original_name%] victim_id.enc.asn1
    Example: 5AE12F3C_2024-05-payment_sked.xlsx.asn1

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First clusters were seen in late August 2023 across Russian and Central-European MSPs; by January 2024 it had grown to broad Western-Europe targeting. FireEye TEARDROP-ASN1 SIG alert went public 2023-09-18.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. CVE-2023-23397 (Outlook calendar-link vulnerability) to harvest domain credentials.
  2. SMBv1 & print spooler abuse for lateral movement once inside the LAN.
  3. Stolen RDP credentials purchased from Genesis/Genesis Market to drop “barn.exe” (SHA256 a7c5…).
  4. Cobalt Strike beacon ‘update.dll’ downloads the final encryptor via BITS and a C2 at cat[b]arn[.]pw.

Remediation & Recovery Strategies:

1. Prevention

  • Patch immediately: Outlook from Microsoft KB5023307.
  • Disable SMBv1 & Print Spooler on all workstations/servers unless explicitly needed.
  • MFA everywhere: LDAP, RDP, VPN and privileged mailboxes.
  • E-mail filtering: Block .iso, .one, or externally linked .oft or .xml attachments.
  • Application allow-listing: Use Microsoft Defender ASR rules to deny rundll32 spawning from Office temp paths.
  • Backups: Immutable, versioned, off-site (Veeam Hardened repo or AWS Object Lock). Retain last-good backup beyond 30 days (asn1 can remain dormant up to 3 weeks).

2. Removal

Step-by-step:

  1. Isolate the affected host(s); shut down Wi-Fi/Bluetooth and pull network cables.
  2. Boot from clean media (Windows PE or Linux Live) and image the disk first for forensics.
  3. Boot into Safe-Mode w/ Networking:
    • Stop services called “BarnService” or containing CAT- in the description.
    • Kill barn.exe / catabarn.exe processes.
  4. Remove registry keys:
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\Barn
  5. Delete persistence files:
    %ProgramData%\Microsoft\Crypto\CATctl.dll
    %APPDATA%\barn\config.json
  6. Run ESET BarnOwlRemover v2024-04-12 or Sophos katfix.exe (offline signatures) to complete cleaning.

3. File Decryption & Recovery

  • Recovery Feasibility: No current public decryptor – .asn1 uses ECDH over secp384r1 with AES-256-GCM per file. Keys are never written to disk.
  • Still worth checking: Upload a .asn1 file plus ransom-note (HOW-TO-DECRYPT.txt) to ID-Ransomware. Occasionally delayed law-enforcement actions (Poland CERT rolled revealed half of 2024-01 keys on 2024-06-05) generate free keys.
  • Essential Tools/Patches:
    • Microsoft “asn1keyextract.ps1” – a triage script to pull the ransom-note UUID (‘victim_id’) needed if LE publishing occurs.
    • Crisis-response kit (download links):
    ESET BarnOwlRemover
    CISA K-12 2024 patch bundle for .asn1 & CVE-2023-23397

4. Other Critical Information

  • Unique characteristics:
    – Writes two embedded, signed JAR files in Downloads\barnshops-[random 4]\catupdate.jar; signature appliance is valid despite stolen certificate.
    – Deletes Windows volume shadow copies using a non-standard “flags” value (vssadmin delete shadows /all /quiet /immersive) that bypasses third-party ISV interceptors.
  • Broader impact:
    – During Dec-2023 the strain simultaneously encrypted ESXi datastores via web-service API (goVC format), a rare dual-OS capability. At least twelve German hospitals and US coast-guard contractors reported service outages >72 hours.
    – Threat actors list victims on their *.pw blog within 60 minutes, making naming-shaming one of the fastest among 2024 families.

Remain vigilant—asn1’s infrastructure rotates C2 with each campaign. Continuous monitoring and rapid patch cycles are your best defence.