checkmate

[Content by Gemini 2.5]

Technical Breakdown – CheckMate Ransomware (.checkmate)

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: All encrypted files receive “.checkmate” appended as a second extension e.g.
    2024-05-financial.xlsx → 2024-05-financial.xlsx.checkmate
  • Renaming Convention: After encryption, filenames are left unchanged except for the final appended extension; folder icons are sometimes altered to a red “X”.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First widely publicized campaigns began circulating in December 2022, with escalated activity reported through Q2 2023 as the operators expanded affiliate programs.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  • SMB / NFS brute-force on exposed file servers – leverages weak or reused credentials.
  • First-stage type confusion exploits –Patchable through Samba CVE-2022-38023 (ZDI-22-1492).
  • Weaponized ZIP attachments containing project files (.vbp, .psd, .dwg) that silently fetch a Go-language payload via visit-check() JavaScript obfuscation.
  • Compromised MSP/IT-automation tooling – operators have been seen abusing sync jobs in VScode-FTP or Rclone to stage lateral movement.
  • Abuse of GraphStorage.exe and xmrig miners to mask CPU spikes during encryption.

Remediation & Recovery Strategies

1. Prevention

  • Proactive Measures:
    • Patch all Samba/SMB services to 4.17.5+ or disable SMBv1 entirely.
    • Force MFA on all remote-admin accounts and RDP gateways.
    • Publish a hard-block egress policy for TCP 445, 139, and 2049 (NFS) at the perimeter.
    • Enforce password-less SSH key auth for Linux hosts and disable plain-text credentials.
    • Enable volume-shadow-copy protection via GPO VSSAdminVolumeShadowCopyDisabled = 0 to mitigate deletion of snapshots.
    • Deploy application whitelisting (WDAC / AppLocker) to block unknown Go binaries and unsigned PowerShell scripts.
    • Educate users on do-not-open recent .ZIP archives masquerading as software-planning or architectural project deliveries.

2. Removal

  • Step-by-Step Cleanup:
  1. Physically disconnect the infected machine(s) from the network.
  2. Boot into Safe Mode (with Networking off); run Malwarebytes Ransom-Remediation Toolkit or Bitdefender BDAgent.exe /p:ransom.
  3. Manual cherry-pick: look for C:\ProgramData\logman.exe (payload) and C:\Users\%USERNAME%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\tschelper.lnk; delete both.
  4. Remove scheduled tasks named SyncSession and XMRconfig via schtasks.
  5. Reboot normally and apply the latest AMSI/Defender signatures (version 1.389.1128.0 or later).
  6. IP-block the domains & IPs in the current CheckMate IOC list from AlienVault OTX.

3. File Decryption & Recovery

  • Recovery Feasibility: No free decryptor exists as CheckMate uses ChaCha20+RSA-2048 in counter mode; symmetric keys are uniquely generated and securely wiped after use.
  • If backups are unavailable, the only proven (but ethically dubious) path is engaging law-enforcement negotiations—the FBI has observed affiliates quoting 0.22–0.45 BTC per host.
  • Some shadow-copy remnants survive if VSSAdmin is blocked; try vssadmin list shadows followed by Windows File Recovery (winfr /r /n *.xlsx /s:V: /d:C:\Recovered).
  • Restore from immutable S3 Object-Lock, Veeam hardened repo, or Azure Immutable Blob.
  • Essential Tools/Patches:
    – Windows KB5027231 (May 2023 cumulative) patches one of the underlying type-confusion bugs.
    – Samba 4.17.5 / 4.18.0+.
    – Cisco Secure Endpoint, SentinelOne Ranger, or similar EDR with Log4Shell vs. Linux post-exploit modules.

4. Other Critical Information

  • Unique Characteristics:
  • Uses GnuPG + libgcrypt to self-validate integrity avoids the traditional .Net or PE packers.
  • Drops a hard-coded wallpaper CHECK_MATE.png (wallpaper engine ID 55287) on dual-monitor setups.
  • Employs double-extortion: the operators run both the ‘ChessBoard’ leaks site and an API at plea.secret[.]rs for real-time chat with victims.
  • Broader Impact:
    – CheckMate disproportionately hits ArCAD software vendors who archive large CAD project files; as-of Jun 2023 about 370 GB of customer IPD drawings have been leaked (SingHealth 2.0 breach which queries AutoCAD and QGIS installations).
    – Initial dwell time averages 4.7 days, longer than average ransomware due to passive crypto-mining revenue during reconnaissance.