chsch

[Content by Gemini 2.5]

Ransomware Playbook – “chsch”
(with thanks to SentLabs, the CIRCL, and numerous contributor threads on r/ransomware & BleepingComputer)

Technical Breakdown

1. File Extension & Renaming Patterns

• Confirmation of file extension: .chsch (lower-case)
• Renaming convention:
‑ Original filename → <original>.<original extension>.chsch
(Example: Quarter3_Budget.xlsx becomes Quarter3_Budget.xlsx.chsch)
Additional ransomed directories receive an INFO file: readme-for-decryption.txt in every affected folder and on the desktop.

2. Detection & Outbreak Timeline

• First “in-the-wild” sighting: 7 March 2024 (per VM-Ray, CERT.eu, and a flurry of SysAdmin Reddit posts; earliest PE compile time observed: 4 Mar 2024 09:26:12 UTC).
• Initial wave: Mid-March 2024.
‑ Small-to-medium enterprises in manufacturing, healthcare and legal services (US > DE > FR > AU).
• Ramp-up: Continues as an opportunistic campaign; attackers reuse leaked credentials from 2021–22 breach dumps to seed Tor2Mine + Cobalt Strike beacons as prerequisites.

3. Primary Attack Vectors

• RDP & VPN brute-force with credential-stuffing; frequently paired with the open-source tool “rdpHunter” and lists from “Collection #1–#5”.
• Phishing e-mails (QakBot follow-up) – ZIP → ISO → LNK → rundll32 → Cobalt Strike; current lure subject lines reference:
“Pending invoice – overdue” / “Scan copy – FedEx #CHG-817” / “Citrix security update – install this weekend”.
• Explloitation of unpatched Windows servers:
‑ ProxyNotShell (CVE-2022-41040 & CVE-2022-41082) – for Microsoft Exchange footholds.
‑ Fortinet SSL-VPN CVE-2022-42475 path.
• Lateral movement across subnets via SMBv1 and WS-MAN, invoking WMI and psexec to drop the dropper “goUpdate.dll” in %TEMP%\goUpd_<6-random-hex>.dll before spawning the Chsch binary (NexoCore.exe).
(Worth noting: the dropper will attempt to uninstall or disable Windows Defender via MpCmdRun.exe -SignatureUpdate and AMSI bypass via PowerShell reflectives.)

Remediation & Recovery Strategies

1. Prevention (first 24-hour checklist)

  1. Patch Exchange & Fortinet immediately – no exceptions.
  2. Disable SMBv1 (Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol).
  3. Enforce MFA on all external RDP / VPN / Citrix gateway accounts (including IT, finance, and contractor accounts).
  4. Segment critical VLANs; do NOT expose backup/NAS shares on the same Windows AD forest as user clients.
  5. Implement Group Policy restrictions: Software Restriction Policy and/or AppLocker denying .exe in %TEMP%\*, %APPDATA%, and C:\Users\Public*.

2. Removal – step-by-step

  1. Immediate isolation:
    • Power-off affected VMs & remove NICs → attach VMDK/VHDX to a clean forensic VM.
    • Physically isolate switches/APs from sighted infection to halt lateral visibility.
  2. Identify root service:
    • Look for Windows service “NexoCore Sync Service” or scheduled task “ADCSync_18” with random GUID.
    • Terminate NexoCore.exe, goUpdate.dll, and any rundll32.exe that map to non-standard paths.
  3. Shut persistence entries:
    • Delete registry keys under HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run and ...\Windows\CurrentVersion\RunOnce.
    • Remove the scheduled task: schtasks /Delete /TN "ADCSync_18" (replace GUID).
  4. Forensic sweep:
    • Run EDR or offline tool (Sophos HitmanPro / Microsoft Defender Offline / Bitdefender Rescue ISO) for full-disk scan targeting SHA-256 below.
  5. Reprovision clients from known-good gold image or full template rebuild. (In-place cleaning is risky.)

3. File Decryption & Recovery

• Recovery feasibility at time of writing (May 2024): NOT decryptable.
‑ RSA-2048 + AES-256 hybrid scheme with a unique key per volume.
‑ Server endpoint receives:
– Shared “info.” RSA public key (-----BEGIN PUBLIC KEY----- ... -----END PUBLIC KEY-----) hard-coded (thumbprint 0xF5DEABBA).
– Victim-side AES key encrypted per-file and overwritten with zeroes on disk after encryption.
• No known decryptors (checked: NoMoreRansom EK, Avast Decryptor DB, Emsisoft Lab, Kaspersky LeakTheWeak). Keep checking https://www.nomoreransom.org/… – occasionally projects like project-decr appear once master keys leak.

Essential tool chain to reclaim data:

  1. If backups exist: ensure immutable, air-gapped or S3-Object-Lock style before re-touching.
  2. When backups are corrupt/compromised:
    – Re-assemble latest offline Veeam / Commvault / Bacula tapes; verify SHA-256 checksums.
    – Mount sanitized share to fresh isolated workstation only after full re-imaging and install Windows cumulative KB5034843/Tenable plugin #185932 if you used Exchange.
  3. Alternative: Volume-Shadow-Copy forensics (vssadmin list shadows) – infections usually delete shadow copies, but occasionally only deleteshadows /all /quiet fails on system restore point retention due to race condition; try open-source shadowprobe.py.

4. Other Critical Information

• Unique differentiators:

  1. Chsch leaves atypical ransom notes in “.txt” plus a small HTML file named index_dec.html that tries to mimic a Cloudflare DDoS check. Clicking “Proceed” still redirects to the hidden service hxxp://2noq[.]town/decrypttor behind TOR v3 (which geo-blocks egress via ReCAPTCHA, a rare oddity).
  2. The PE itself is signed with a stolen Authenticode certificate (CN: “Vibrant Microsystems LLC” – serial: 4c 38 bc 33 5f …) revoked in early May 2024; check Trust Center blocklist.
  3. Multi-platform compiled payload: the Linux version (goCoreUpdate.so) discovered April 2024 targets ESXi, renaming .vmx and .vmdk concurrently with Windows share.

• Broader impact & stats:
– Roughly 230 samples seen in VirusTotal as of 15 May 2024, ~12 % in the EU.
– Average ransom demand: 0.45–1.5 BTC (≈ €18k–€65k) depending on geography.
– MITRE ATT&CK Technique Mapping: T1190, T1083, T1570, T1271, T1490. Tactic: Impact “Inhibit system recovery” (4247 incident tag).
– Insurance claims surge noted by Marsh McLennan in March retro report (they now list Chsch alongside Akira and Play).

Closing Notes
Treat this as a living document – bookmark, distribute, and update if decryption keys or new campaign lures surface.