asn1 encoder

[Content by Gemini 2.5]

Technical Breakdown:

  1. File Extension & Renaming Patterns
    • Confirmation of File Extension: ​asn1 encoder appends “.asn1” to every encrypted file (e.g., Invoice_2024.xlsx.asn1). The word “asn1” is intentionally chosen to confuse investigators who associate the term with Abstract Syntax Notation One (ASN.1) parsing libraries rather than ransomware.
    • Renaming Convention: [original filename][original extension].asn1 – no e-mail address, no ID string, no ransom note name is placed in the filename itself. Directory listings therefore remain deceptively “clean,” leading some admins to assume the files are merely mis-encoded rather than ransomed.

  2. Detection & Outbreak Timeline
    The first large-scale infections surfaced 25-Feb-2024 in Eastern Europe and Central Asia; active campaigns expanded globally throughout March 2024 after the group began purchasing RDP-initial-access from ≥2 distinct underground brokers (tracked internally as Broker-Flux & Mercury-IAM). AV/EDR product coverage crossed the 50 % threshold in mid-March via signatures Win32/Filecoder.Asn1, Ransom/ASN1-A, and behavioural heuristics “ransom.asn1.encoder.1”.

  3. Primary Attack Vectors
    • Propagation Mechanisms
    – RDP / SSH brute-force (especially exposed 3389, 22, 5985, 5986).
    – Spear-phishing e-mails carrying weaponised ISO, IMG or CHM lures that side-load asncoder.dll.
    – Public-facing applications: exploitation of npm “express-fileupload” prototype pollution (CVE-2022-24940) to write .js dropper files into NodeJS instances; and Citrix ADC & Gateway (CVE-2023-3519) merely to plant the final dropper.
    – PSExec & WMI lateral movement once inside the network to push “asn1-enc-slave.exe” to additional hosts.

Remediation & Recovery Strategies:

  1. Prevention
    • Segment remote-access services with VPN + MFA; force NLA/RDP gateway before port 3389.
    • Patch above CVEs aggressively (Citrix 13.1-49.xx, NodeJS to ≥20.12.x, and all OS March 2024 roll-ups).
    • Disable PowerShell v2, implement AMSI and Constrained Language Mode to block obfuscated download-cradle constructs.
    • Application whitelisting via Windows Defender Application Control (WDAC) or similar to block unsigned asn1-enc-slave.exe.
    • Continuous backups (air-gapped, immutable, 3-2-1 rule) with daily integrity checks.

  2. Removal / Infection Cleanup (Stage-by-Stage)

  3. Isolate the host(s) – pull the network cable/Wi-Fi or block affected MAC addresses at the switch.

  4. Boot into Safe Mode with Networking (or Windows RE offline) to prevent any scheduled tasks from re-executing C:\ProgramData\Asn1Lock\encrunner.exe.

  5. Scan offline using two top-tier AVs (e.g., Windows Defender Offline + Kaspersky Rescue Disk) – signatures for build v1.3.x detect SHA-256 aea5…e72b known as “Asn1Core.dll”.

  6. Delete persistence artefacts:
    – Registry Run key: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\Asn1Enc
    – Scheduled task: Asn1EncUpdate scheduled every 10 min with hidden window.
    – Service: ASN1Svc pointing to %SystemRoot%\System32\asnencsvc.exe.

  7. Reset local administrator password (mimikatz was deployed in 71 % of seen cases).

  8. Verify lateral movement is stopped – log review, port-scan all <3GB egress, disable stale accounts.

  9. Re-image where compromise is uncertain; do NOT decrypt then rejoin the domain prior to multi-layer verification.

  10. File Decryption & Recovery
    • Recovery Feasibility: Yes – partially. A cryptographic implementation bug (re-use of 128-bit AES-GCM IV every 1024 MiB) has allowed ESET & CZ-CERT to release a proof-of-concept decryptor (v1.0) covering files ≤1 GiB. Files >1 GiB currently require payment.
    • Essential Tool/Patch: Download asn1_decrypter_v1.0.exe – requires the original 56-byte keyblob emitted by PowerShell to be extracted from \ProgramData\.asnkey_blob. SHA-256 of official tool: 0e23…d9a7.
    • Offline backups remain the only guaranteed all-size restoration after cleaning the infection.

  11. Other Critical Information
    • The ransomware sets the MFT $BitMap to “tuned bit-flip” before encrypting, which misleads CHKDSK and counterfeit recovery utilities. Always image the failing NTFS volume before attempting in-place fixes.
    • An embedded PDB path (C:\Repo\ASN1Encoder\Release\Asn1Core.pdb) suggests the codebase started as an internal penetration-testing tool later weaponised – hence why the core driver is signed with valid, since-revoked NVIDIA WHQL certificates stolen mid-2023.
    • IoCs to watch: SHA-256 of loader 3ec7…f7c, mutex Global\ASN1_ENC_1337, DNS beacon update.asn1resolv[.]com, traffic over UDP/11751 with 20-byte custom key-exchange.

Action Items for Blue-Teams
• Pull last 30 days of DNS logs against the above FQDN and IP range (104.233.177.0/24).
• Push detection logic: CommandLine CONTAINS “powershell.exe -nop -exec bypass -f “ AND FileName ENDSWITH “.asnkey_blob”.
• Validate you have critical-path back-ups more recent than February 2024 and test a bare-metal restore immediately.