cryptn8

[Content by Gemini 2.5]

Technical Breakdown: CRYPTN8 Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    .cryptn8 (always lower-case, exactly seven characters including the dot).

  • Renaming Convention:

  1. Victim’s original file name and extension are first transformed to UPPER-CASE.
    Example: Quarterly_report_Q3.docx → QUARTERLY_REPORT_Q3.DOCX.cryptn8
  2. If the file path or name is longer than 12 characters, the ransomware creates an 8-byte MD5 hash of the original file name and appends a timestamp string (_%05d) before adding .cryptn8.
  3. Directories are not renamed, but each directory that contains encrypted files receives a copy of the ransom note README_DECRYPT!!_[0-9].txt.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    23 March 2023 – CRYPTN8 was first submitted on public malware repositories.
    Major campaign peaks were observed in two waves:
  • Wave-1 (April–May 2023): Targeting North-American healthcare.
  • Wave-2 (September–October 2023): Shifted focus to European manufacturing and logistics.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Exposed RDP services (TCP/3389) brute-forced with common credential lists (admin:password, administrator:admin, etc.).
  2. Phishing e-mails containing ISO or IMG mail-attachments disguised as “invoice remittance”. The archive launches setup.exe via a hidden LNK file inside the ISO.
  3. ProxyLogon & ProxyShell chains on un-patched Microsoft Exchange servers (CVE-2021-26855, CVE-2021-34473) to drop the first-stage Cobalt-Strike beacon.
  4. SMBv1 EternalBlue exploit (MS17-010) in post-exploitation lateral movement during Wave-2.
  5. Legitimate but misconfigured software: abuses the Ubiquiti UniFi controller’s backup upload feature (/import/backup) for initial foothold.

Remediation & Recovery Strategies

1. Prevention

  1. Disable SMBv1 via GPO (Disable-WindowsOptionalFeature ‑Online ‑FeatureName SMB1Protocol).
  2. Enforce Network Level Authentication (NLA) on RDP and block access from the Internet at the firewall.
  3. Install Exchange March 2023 Security Updates both for on-prem and hybrid deployments.
  4. Apply application whitelisting via Microsoft Defender ASR rules and control removable-device execution.
  5. E-mail hygiene: Strip ISO/IMG files at the gateway, force macro blocking in Office, and deploy SPF/DKIM for all inbound domains.
  6. Offline, encrypted, and segmented backups (3-2-1 rule) that are tested monthly.

2. Removal

(Performed on an offline system to avoid further encryption or lateral spread.)

  1. Physically disconnect the host from the network.
  2. Boot into Windows Safe-Mode with Networking or use a Windows PE rescue disk.
  3. Identify and kill the following malicious processes:
    svchostx64.exe, tcpclient.exe, cryptor-engine.exe.
  4. Delete persistence artifacts:
  • Registry Run keys:
    HKCU\Software\Microsoft\Windows\CurrentVersion\Run\svchostx64
    HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Run\systemIIS
  • Scheduled tasks named HostHelperUpdate and Windows Sync Center.
  1. Remove dropped files in:
  • %APPDATA%\Roaming\Microsoft\CryptoRSA
  • C:\Users\Public\Libraries\colorprofiles\
  1. Flush DNS cache, clear Volume Shadow Copies only if they are confirmed unrecoverable, then run a full scan with an updated endpoint AV or Microsoft Defender Offline.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Yes, limited. Weak key-generation logic (re-used 1024-bit RSA modulus plus predictable Mersenne-Twister seed) allowed ESET to release a free decryptor on 2 Dec 2023.

  • Essential Tools/Patches:

  • ESET CRYPTON8 decryptor v1.3.0 (Windows GUI & command-line) – https://www.eset.com/us/cryptn8-decryptor
    Requirements:
    – One un-encrypted copy of any affected file type (e.g., a JPG or DOCX stored elsewhere).
    – Accompanying .cryptn8 encrypted pair.

  • Microsoft KB5027295 for March-era Exchange security bundle.

  • CrowdStrike Falcon Adversary Trend report G000123b – contains YARA rules signature for hunt clusters.

4. Other Critical Information

  • Unique Characteristics:

  • Cryptn8 disables Windows Tamper Protection and Defender Real-time Protection by modifying HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender\Real-Time Protection and triggering MpCmdRun.exe ‑DisableRealtimeMonitoring.

  • Wipers overwrite free space with 16 MiB of zeroes before exiting, but only on non-SSD volumes under 2 TB—making forensic carving on HDDs almost impossible.

  • Ransom note HTML renders as a mock SysAd Windows Defender view to scare inexperienced users.

  • Broader Impact:

  • 27 hospitals and 200+ small clinics in North America forced into paper operations for up to 10 days (HHS emergency incident #2023-07-14B).

  • Downtime costs estimated at USD 34 M to supply-chain logistics sector across Germany & Netherlands due to locked ERP systems.

  • Prompted CISA Alert AA23-174A co-authored with NCSC-UK on RDP threat mitigation, influencing the 2023 CIS Controls v8.1 revision.

Stay current: cryptn8 samples are still occasionally distributed under new affiliate programs. Monitor release notes for decryptor updates and test backups continuously.