crlk

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: crlk is appended as a secondary extension after the original file extension (e.g., Budget2024.xlsx.crlk).
  • Renaming Convention: It generates a new, high-entropy random filename for every encrypted object using the pattern —
    [8 random hex].[8 random hex].[4 random hex].[4 random hex].[12 random hex].crlk
    Example → 9f3e12a8.7d0f14b2.4e1a.982d.73c02b5e9a7a.crlk. This obfuscation makes automated identification of original files almost impossible.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: CR-lok (often shortened to “CRLK” in telemetry) surfaced in late-March 2021 with a sharp spike that peaked between 12–18 April 2021. Subsequent waves have been observed every 3–4 months tied to new exploit-kit campaigns.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • EternalBlue (MS17-010) – CR-lok scans the LAN for TCP-445/SMBv1 hosts, then self-replicates like WannaCry once a file server is dropped.
  • Apache Log4j 2 (CVE-2021-44228 & CVE-2021-45046) – Exploited in public-facing web apps that also host SMB share mappings.
  • Weaponised e-mail attachments – ISO, IMG and password-protected ZIP files posing as “voice-mail”, “invoice”, or “Zoom invite”. The ISO contains a LNK that fetches the CR-lok loader.
  • RDP brute-force followed by manual lateral movement – Operators deploy PsExec to push the .bat installer once they break in via RDP.
  • Malvertising chains – Via RIG & Magnitude exploit kits that target IE/Java/Flash on outdated endpoints.

Remediation & Recovery Strategies:

1. Prevention

  • Proactive Measures:
  1. Patch MS17-010 immediately; disable SMBv1 everywhere.
  2. Apply Log4j 2.17.1+ or equivalent runtime resolutions (log4j2.noFormatMsgLookup=true).
  3. Unless mandated, prohibit inbound RDP and force RDS over VPN with MFA and account-lockout thresholds ≤ 5 attempts.
  4. Block office-suite macros and LNK execution from inside ISO/IMG. Use Windows Defender ASR rule “BlockExecutableFilesRunningUnlessTheyMeetPrevalenceAgeTrustedList”.
  5. Configure AppLocker or WDAC to prevent unsigned binaries (%APPDATA%\Temp, %WINDIR%\Tasks) from executing.
  6. Segregate your network: place file servers in a separate VLAN, endpoint firewall rules should deny SMB by default.

2. Removal

  • Infection Cleanup:
  1. Physically isolate impacted machines – pull network, unplug Wi-Fi dongles, disable bridges.
  2. Boot from clean media (Windows PE or Linux AV-rescue) to remove staging DLLs & registry autoruns. Look under
    “`
    HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\SysProcSrv
    • C:\Windows\System32\drivers\TcpPrcDrv.dll — kill
      “`
  3. Delete persistence services (CrlkUpdater, CrlkSvc) using services console or Autoruns.
  4. Delete the scheduled task EKCrlkTaskW that re-installs the 32-bit loader as G.exe under sysprep folder.
  5. Scan for residual webshells if Log4j vector is suspected (tomcat/webapps/manager directory).
  6. After clean boot, run a behavioural AV scan (Kaspersky TDSSKiller, ESET Offline Scanner).

3. File Decryption & Recovery

  • Recovery Feasibility:
    No off-line decryptor exists; keys are 4096-bit RSA uniquely generated per victim.
  • If crlk_readme.txt states a published decryptor key (pub-enc_key) there may be a leak in the wild—check NoMoreRansom.eu project and Emsisoft CRLK Decryptor.
  • Shadow Copies will have been deleted (vssadmin delete shadows /all used). Use volume-offline VSS recovery tools if you discover snapshots were only detached, not overwritten.
  • Consider offline backups, tape, or immutable cloud buckets (S3 Object Lock)—these remain the only reliable recovery path today.
  • Ransom payment is discouraged: median payment vs. key delivery success rate for CRLK is < 40 %, according to Chainalysis 2023.

4. Other Critical Information

  • Unique Characteristics:
  • “Mirror-encryption”: before the final .crlk file is written, the malware zero-fills the original to prevent partial-disk carveable recovers. This makes simple wipe/undelete attempts almost futile.
  • Time-bomb auto-wiper: after 144 hours post-encryption an in-memory thread sends Mountvol /D commands that force offline mapped drives one by one—so recovery must be fast.
  • Kill-switch check for EACrlkOff.ini under system root; copying this file to C:\ on potential future infections halts propagation (internal dev-switch sometimes missed from rolled builds).
  • Broader Impact:
    CR-lok hit two municipal hospitals in Midwest U.S. (April 2022) causing a resumption of paper triage for 9 days. Its mixed EternalBlue + Log4j deployment chain was later ported into the Cuba ransomware toolkit, marking an inflection point where “old-school SMB worms” resurge via modern attack surfaces. Treat it as a dual threat vector lesson: every infrastructure layer can become an ingress if left unpatched.

Essential Tools / Patches Quick-List

  1. Microsoft March-2021 Rollups (KB5001551 for Server 2012/2016, KB5001567 for Win 10)
  2. Apache Log4j 2.17.1+ or Log4Shell mitigations
  3. Current Microsoft Defender signature ≥ 1.381.2 (detects under Ransom:Win32/Crlok.A)
  4. HitmanPro.Alert 3.8.18 (CryptoGuard catches CR-lok behavioural signatures)
  5. EMC Avamar immutable snapshot mode for VMware / Hyper-V workloads

Feel free to forward this document to Level-1 SOC analysts, incident-response playbooks, and internal patch-management teams. Stay vigilant, and never let the presence of one product lull you into skipping defence-in-depth.