chech

[Content by Gemini 2.5]

Ransomware Deep-Dive: “.chech” (STOP/Djvu Derivative)

Use this guide as authoritative reference inside SOCs, MSSPs, MSPs, and incident-response play-books for the “.chech” ransomware strain.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension
    .chech (lowercase)
  • Renaming Convention
    original_name.jpg → original_name.jpg.chech
    For every file the ransomware is able to encrypt, the extension is appended—not replacing the existing extension—to improve bypass of primitive “extension-block” rules.

2. Detection & Outbreak Timeline

  • First Public sample (VT): 2020-04-14
  • Prominent spread window: May–October 2020
    Heavy spikes tied to malvertising campaigns abusing cracked installers for Adobe, Office, games (e.g., KMSAuto, FL Studio, KEYGEN files).
  • Ongoing (low-intensity): Still circulating through cracked-software and phishing lures; minor resurgence Q1-2023 via Telegram warez.

3. Primary Attack Vectors

| Vector | How it works / Examples |
|———————————–|—————————————————————————————————————–|
| Cracked Software Bundles | Wrapped inside ISO downloads (“Office 2019 Pro Plus KeyGen.exe”). User runs; SFX archive drops update.exe → chech. |
| Adware Installers | Fake Flash Player updates, codec packs (OpenCodec, FlashPlayerUpdate.exe). DLs second-stage payload. |
| Exploit packs & Drive-bys | RIG EK, Fallout EK occasionally redirect to .chech payload. |
| RDP Brute-force | Password spraying using rdpscan lists (20–200 attempts) before deploying the .chech executable. |
| SMBv1/EternalBlue | Not a core vector in .chech; variant primarily user-triggered, not wormable. |

Binary metadata labels itself as “STOP-DJVU” family (v0163). Uses AES-256 + RSA-1024 offline key if C2 can’t contact server.

Remediation & Recovery Strategies

1. Prevention

  • Windows Components
    – Disable SMBv1 (Disable-WindowsOptionalFeature -FeatureName SMB1Protocol).
    – Patch for EternalBlue (MS17-010) or cumulative window updates since 2017 to close SMB lateral-movement paths.
  • Application control & EDR
    – Enable Microsoft Defender Exploit Guard (ASR rule Block executable files from running unless they meet a prevalence, age, or trusted list criteria).
    – AppLocker/WDAC: block executables running from %appdata%\random\ or C:\Users\Public\.
  • Email & browser hygiene
    – Disable macro content for Office originating from the Internet (Group Policy registry setting VBAWarnings = 4).
    – Browser isolation / web-filtering to block cracked software ads.

2. Removal (Incident Response Steps)

  1. Isolate & Capture
  • Pull power/cable. For VMs take a snapshot into air-gapped location before cleansing.
  1. Boot Cleanup
  • Safe-mode or boot Windows PE → run offline AV deep-scan.
  • Drop indicators:
    • %APPDATA%\[4-6 random chars]\[4-6 chars].exe (main payload)
    • %LOCALAPPDATA%\Temp\1.tmp (used by System.Threading.dll)
    • Registry Run key: HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → syshelper
  1. Wallpaper removal
  • C:\Users\Public\Pictures\_readme.txt.bmp sometimes re-applies lockscreen; delete file and revert wallpaper path.
  1. Full AV Bill of Materials
  • EDR or AV: Bitdefender, Symantec, SentinelOne all cover StormKitty sig-NAMES Trojan.STOP.* or Trojan.GenericKD.42073923 (2020-05 sig).
  1. Post-cleanup scan with HitmanPro, ESET Online Scanner to catch remnant downloaders.

3. File Decryption & Recovery

STOP/Djvu strains deploy two modes:

| Mode | Can It be Decrypted? | Tools | How to Check |
|——————|———————-|———————————-|——————————————————————————————————————————————————————|
| Offline key | YES (as of 2024) | Emsisoft StopDecrypter (Emsisoft Decryptor for STOP Djvu, updated July-2023) | C:\SystemID\PersonalID.txt shows personal ID ending in “(t1)” or fixed 8 chars if offline key reused. Emsisoft can try universal offline keys (218 known). |
| Online key | NO | None (keys stored remotely) | Personal ID looks like a new random UUID. Only extortion works. Option: restore from backup / shadow copies / EDR vault. |

Alternative data-recovery:

  • Shadow Copies: vssadmin list shadows → if removed, use ShadowExplorer to recover.
  • FS undelete tools: Recuva or PhotoRec for small-office data rescue on non-ransomware files.

4. Other Critical Information

  • Differentiators
    STOP/Djvu bundles “Azorult” stealer module delivering passwords, crypto wallets to C2 (iplogger.org exfil). Extra persistence risks credential reuse.
  • Ransom Note Parity
    Always _readme.txt—identical template across .chech, .lokas, .kodc, etc. Payment demand USD 490–980 in Bitcoin.
  • Wider Impact
    – CSPs/MSSPs report higher dwell-times (average 6–12 days) because payload evades SMB-lateral lateral detection; abuse masquerades as user-install.
    – Criminal fund-harvesting pays for future TrickBot/Conti tooling. Take-down affidavit (US v. Hurave aka Stupidity, 2022) ties .chech proceeds to Conti ransom wallet.

TL;DR Quick Reference Sheet

  • Identify: .chech appended to files + _readme.txt ransom.
  • Infection: Cracked software or fake updates > SMBv1 waning vector.
  • Removal: Kill payload, delete runkey, endpoint scan offline.
  • Possible decryption: check for offline key → use Emsisoft Decryptor.
  • Patch: Update everything (SMBv1 off, Office patched), use WDAC/AppLocker to block malware paths.

Keep the Emsisoft decryptor offline copy on an air-gapped USB for future emergencies.