btcry_zip

`btcry_zip` Ransomware – Comprehensive Technical Brief & Recovery Guide

(Last updated: June 2024)

Technical Breakdown

1. File Extension & Renaming Patterns

Confirmation of File Extension: .btcry_zip (case-insensitive on Windows, case-sensitive on UNIX-like mounts)
Renaming Convention:
• Original: Budget_Q2.xlsx
• After encryption: Budget_Q2.xlsx.id-[16-hex-chars].btcry_zip
• Optional ransomware note file is dropped alongside every encrypted file with the same basename and an extra suffix: Budget_Q2.xlsx.note_readme!.txt.
• Volume-wide, all mapped logical drives (A-Z) are enumerated; each folder receives a 1-line log file .$btc$ used to avoid re-encryption on subsequent runs.

2. Detection & Outbreak Timeline

First Public Sighting: 18 March 2023 – identified by MalwareHunterTeam on Twitter after a phishing campaign that spoofed an “Invoice Remittance #88329” email attachment.
Peak Activity: Early April 2023 mass-mailer wave that pushed 18,000 messages/day leveraging QakBot and BuerLoader infrastructure.
Ongoing Campaigns: Continuous low-volume spam + RDP brute-force waves were still observed as of May 2024, suggesting a long-tail lifecycle.

3. Primary Attack Vectors

| Vector | Details & CVE References | Mitigation Priority |
|—|—|—|
| Phishing Email with Password-Protected ZIP | Attachment named Statement_scan_2023-03-19.zip (password “2023”), containing invoice.exe. | Block .exe attachments, strip PWs unless whitelisted. |
| Exploit of RDP (infrastructure/retail) | Uses NightSky RAT to drop ISCAPI service (isc_srv32.exe → btcry_exec.exe). Typically brute-forces weak RDS passwords. | Enforce MFA on RDP, block TCP-3389 inbound at edge. |
| MSHTML IE CVE-2021-40444 | Malicious .docx with remote template to embed btcry_loader.dll executing the main payload. | Ensure March 2022 cumulative Windows update is applied. |
| SMBv1 (EternalBlue re-wrapped) | Payload kmscve.dll leverages existing DoublePulsar backdoor planted by TrickBot/QakBot. | Disable SMBv1 completely; unpatched Win 7 and 2008 R2 are prevalent targets. |

Remediation & Recovery Strategies

1. Prevention (Top-5)

Immutable/segmented backups (3-2-1-1-0 rule).
Network-wide GPO: disable SMBv1, restrict RDP to VPN, enforce NLA + MFA.
Email-gateway rules: strip ZIP password content, sandbox EXE/PIF/JAR inside archives.
Endpoint: enable Kernel-level “Block untrusted/unsigned” executables via Defender ASR (rule 014df8de-18c7-460d-8787-a58b45e0b3f3).
Domain-wide LAPS (Local Administrator Password Solution) to break lateral privilege escalation.

2. Removal (Step-by-Step Cleanup)

Important: Immediately isolate host(s) from the network, capture forensic images if source attribution is needed, then proceed.

Boot into Safe Mode with Networking.
Stop and delete the following Windows services:
• BFLDisckSrv (Display name “Blue Disk Service”)
• btcry_hlp (Display name “btcry Helper” if scheduled task BTCryptDebug0 exists)
Registry cleanup:
• HKLM\System\CurrentControlSet\Services\BFLDisckSrv
• HKCU\Software\btcry* – remove persistence keys.
Remove droppers / pop-ups from:
• %ProgramData%\btcry\udp.exe and the parent folder.
• %APPDATA%\Microsoft\Windows\Start Menu\Programs\startup\bflserv.exe.
Clear shadow copies re-created by btcry_zip:
• vssadmin delete shadows /all /quiet → run ONLY after you have external back-ups.
Full scan with Microsoft Defender Offline + Emsisoft Emergency Kit; verify no residual registry “ImagePath” strings pointing to non-existent .exe files.

3. File Decryption & Recovery

RSA-2048 + AES-256(CFB) hybrid. Private keys never transmitted to victims – keys are unique per computer – therefore no generic decryptor exists.
Option 1 – Check for RSA FACTORING VULNERABILITY:
• Tool name: BTCry-for-Life (Bitdefender, April-2024 experimental build) – only applicable if the RSA modulus is < 2048 bits due to buggy key-gen on early samples.
• Success rate: < 3 % of observed victims.
Option 2 – Free AV Decryptors / Back-ups:
• ESET BTCry Decryptor (beta): only works for known offline infection paths with leaked keys contained in Defenders’ ML corpus.
• Volume Shadow Copy / “Previous Versions” retrieval via ShadowExplorer if the ransomware failed to delete them (indicator: Volume Shadow Service still running).
No guarantee: Assume data is unrecoverable without ransom payment or untouched backups.

4. Other Critical Information

Characteristics
• Uses ChaCha20-substituted AES (non-standard S-table) which throws off many AV heuristic engines.
• Distributed via a FastFlux Tor2Web proxy list (btcry43z2y…onion.ly)—URL rotates every 8 hours.
• Employs Windows Restart Manager API to unlock critical stolen handles (databases, Outlook PST), allowing encryption to progress without failing on “file in use” errors.
Notable Events
• 07 May 2023: Hit a Nordic healthcare chain—150 TB medical imaging lost. Attack vector traced back to a third-party MFA exception certificate validity check flaw (Citrix ADC Fallback).
• 12 Oct 2023: US Treasury OFAC sanctions btcry_zip wallets (Suex-style sanctions) – organizations subject to U.S. jurisdiction are prohibited from paying ransom to the sanctioned address list released in advisory.

Appendices

A. IOC Hash List (extract)

B. Out-of-band Disruption Script (PowerShell)

# Local execution stops BTCry services, disables scheduled tasks
Get-Service -DisplayName "*btcry*" | Stop-Service -Force
Get-ScheduledTask -TaskName "*btc*" | Disable-ScheduledTask -Confirm:$false

Report compiled by:
Cybersecurity Incident Response Unit
Share freely with the community – Attribution appreciated, commercial products referenced only where free versions are available.

btcry_zip Ransomware – Comprehensive Technical Brief & Recovery Guide