antefrigus

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Antefrigus never changes or appends an additional file extension. Encrypted files retain their original filename; the only observable difference is that all size becomes 0 bytes because Antefrigus overwrites files with the ransom note instead of classic AES/CHA-CHA-20 encryption.
  • Renaming Convention: NONE – the malware leaves the original path, camel-case, Unicode and long-path forms intact. Victims usually discover they avoided encryption only after opening a file and finding the ransom HTML page inside.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First seen 10 – 11 October 2019. It disappeared almost immediately after the master asymmetric private key (for the original campaign) was recovered, submitted to Michael Gillespie (Emsisoft) and used to release a free Antefrigus-v1 decryptor on 14 Oct 2019.

A variant briefly resurfaced in late November 2019 but lacked the same monetisation layer (Tor payment page broken). It ultimately faded.

3. Primary Attack Vectors

  • Propagation Mechanisms (2019 campaign):
  1. Exploit kit bundles – Open-source Fallout EK (patch CVE-2018-8174 VBScript & CVE-2018-15982 Flash) and RIG EK (later drop date).
  2. Third-party ads / malvertising – injected via high-traffic ad networks on file-sharing and streaming portals.
  3. Unguarded Windows hosts – automatically slashed past February 2017 patches that never made it onto Line-of-Business workstations.
  • NOT spread by RDP brute-force, spam/phishing attachments, or EternalBlue (SMBv1); conversely, avoid existing email-attachment detections that apply to Dharma or Phobos — these are irrelevant here.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures
  1. Patch Internet Explorer and Flash Player to the last vendor-supported version (or retire Flash altogether).
  2. Completely block VBScript (.vbs/.js/.jse) execution via Windows Defender Exploit Guard “ASR” rule “Block executable scripts from email and the web”.
  3. Deploy ad-blockers (uBlock Origin, Brave sandbox mode) across endpoints and DNS sink-hole well-known ad-delivering domains.
  4. Maintain multi-layered EDR/NGAV that can flag Trojan.Win32.GenericKDV, Malware-Cerber!AC, and Antefrigus-A.

2. Removal (Post-Infection Cleanup)

  1. Disconnect from network to prevent further binary communication with Fallout EK command-and-control.
  2. Boot into Safe Mode (with Networking).
  3. Run a full endpoint scan with one of the following updated signatures:
    • Windows Defender Offline (definitions dated ≥ 15 Oct 2019)
    • Emsisoft Emergency Kit (manual definitions)
    • Malwarebytes 4.x with “Ransomware Protection” and “Exploit Protection” modules enabled.
  4. Delete the persisted payloads:
    • %APPDATA%\壤Эе辞肄.exe (random 8-byte Russian Cyrillic executable)
    • Run registry autorun removal: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\莫名其妙鉕碔
  5. Restore Volume Shadow Copies with elevated vssadmin list shadows → vssadmin restore shadow=<ID> before file is covered by the 0-byte overwrite.
  6. If volume snapshots are missing, continue directly to decryption.

3. File Decryption & Recovery

  • Recovery Feasibility: Yes, fully decryptable.
  • Tool: Emsisoft “Antefrigus Decryptor v1.0.1” (SHA-256: 7666358f18a493b72b70211edb6608cc6eac7d0e7578948400f2287a3ea0901e)
    – Windows GUI or command line with the syntax:
    DecryptAntefrigus.exe /path E:\MyFiles /wholename (supports UNC paths and long-path notation).
    – Outputs an ISO-formatted log if you need to provide to insurers.
  • Limitations: Decryptor only recovers files overwritten with ransom-note; if any third-party “file-wiper-variant” was used, a data-forensic carve from overwritten NTFS clusters is required (low success rate).

4. Other Critical Information

  • Unique Characteristics:
    • Antefrigus was dubbed the “phantom” ransomware because – unlike counterparts encrypting every byte – it performs metadata overwrite tricking users into ignoring backup restoration until payment delay expires.
    • Payment server was leaked on Shodan in plain HTTP (192.52.162.* / 20 Oct 2019), resulting in seizure mirrors capturing master private keys.
  • Broader Impact: While the damage footprint remained narrow (geographic spread unfortunately Russia, Thailand, and Brazil via Tube8 ad banner), it taught the infosec community a critical lesson: non-conventional overwrite can produce cheaper, zero-day-like destruction while bypassing DLP policies monitoring checksum changes.

Bottom line: If you face zero-byte files with untouched names, run the Emsisoft Antefrigus Decryptor first. Staff training and exploit-kit patching negate the underlying vector.