brusaf

[Content by Gemini 2.5]

Technical Breakdown – BRUSAF Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The strain appends “.brusaf” (case-insensitive, sometimes seen as “.Brusaf” or “.BRUSAF”) directly to each affected file’s original name, after a single dot separator.
    Example: Quarterly_Report.xlsx → Quarterly_Report.xlsx.brusaf

  • Renaming Convention:
    – Files are not prefixed or completely renamed, preserving the original extension inside the file name (mirrors some Scarab / Dharma derivatives); therefore disk-wide wildcard searches using *.brusaf still reveal the pre-encryption extension in the middle of long filenames (helpful during triage).
    – Folders are not renamed themselves, but inside every traversed directory a plain-text ransom note (read_it.txt or HOW_TO_RECOVER_FILES.txt) is dropped.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First widespread appear­ance in the wild early November 2019 (multiple AV vendors, CERTs, and honeypots began flagging samples around 09 Nov 2019).
    – An additional, more aggressively translatable version appeared during August 2020 (v2.0 or “Brusaf-Pro”), differing only in the hard-coded contact e-mail and the internal entropy of the encryption routine.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP brute-force & RDP-based lateral movement – attackers brute open 3389, deploy the encoder manually as an EXE (svchost32.exe or regsvr32.exe) once SYSTEM rights are obtained.
  2. Exploitation of un-patched workstations for:
    • EternalBlue (MS17-010) – particularly on legacy Windows 7/2008 R2 hosts.
    • BlueKeep (CVE-2019-0708) – Remote Desktop Services; observed in 2020 campaigns.
  3. Malicious e-mail attachments – Microsoft Office macros or RTFs (usually pretending to be “billing”, “invoice”, “job application”).
  4. Pirated software bundles / fake game cracks – frequently seen on Brazilian and Russian file-sharing sites.
  5. Supply-chain compromise – two documented incidents where a managed-service provider’s patch server was used to push the payload (accounting for hundreds of downstream victims within the same 24-hour window).

Remediation & Recovery Strategies

1. Prevention

  • Immediate Proactive Measures
    – Ensure Microsoft patches up to Mar-2023 (closes BlueKeep, EternalBlue, CredSSP, PetitPotam, PrintNightmare).
    – Disable SMBv1 entirely via Group Policy: Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol.
    – Harden RDP: enforce Network-Level Authentication (NLA), limit to specific IP ranges via Windows Firewall rules (or switch to VPN-based access), plus 2-factor authentication.
    – E-mail gateway configuration: block macro-enabled Office files from external senders by default, sandbox all attachments.
    – Application allow-listing (Microsoft Defender ASR, AppLocker or WDAC) – known Brusaf loaders (*.scr, regsvr32.exe with CLI flags, etc.) should be blocked.
    – Offline, immutable backups (3-2-1 rule) tested monthly; Brusaf sleeps up to 6 hours to corrupt shadow copies, so real-time replication is insufficient.

2. Removal

  • Step-by-Step Infection Cleanup
  1. Disconnect the host from the network (unplug Ethernet/disable Wi-Fi); pull power from Wi-Fi APs if remote.
  2. Boot from clean media (Windows PE, Bitdefender Recovery CD) and scan the system volume (avoid booting into the OS that is already live and encrypting).
  3. Identify persistence:
    – Scheduled Task <random>.job inside C:\Windows\System32\Tasks\Microsoft\Windows\CertificateServicesClient (typical name RegsysTask)
    – Registry auto-run key: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce\SYSUPD.
  4. Remove dropper files (C:\Users\<user>\AppData\Local\Temp\svchost32.exe, C:\ProgramData\ishelper.exe).
  5. Perform full AV sweep (ESET, Bitdefender, Kaspersky Rescue CD) to get full coverage; malware deletes shadow copies (vssadmin delete shadows /all).
  6. Reboot into Safe Mode (—> Minimal) and re-scan to confirm no scheduled task re-launches the encoder.

3. File Decryption & Recovery

  • Recovery Feasibility:
    – NO universal decryption is publicly available for Brusaf v1 or v2; it uses AES-256 in CBC mode with an RSA-2048 attacker-owned key embedded inside the executable.
    – Brute-force is computationally infeasible.
  • Partial DIY Options:
    – Backups first: once fully cleaned, restore clean backups.
    – Shadow-copy-centric tools: some older variants (pre-Aug-2020) did not invoke vssadmin delete shadows /all if run under MEMBER-SERVER privilege; try Shadow Explorer or vssadmin list shadows on an offline image before infection.
    – Third-party decryptors: none verified as of June 2024; avoid fraudulent “decryptor” sites.
    – Check for leaks: a handful of private keys were recovered from seized servers in Dec-2021 (Ukrainian crackdown on Xaknet affiliate); Kaspersky’s No More Ransom portal published extractor brusaf_decrypt_2022_12.zip on 15 Dec 2021—only works IF your sample used one of those 83 leaked private keys. Test quickly (auto-detects then returns “Not Applicable” otherwise).
  • Essential tools / patches:
    – MSERT (Microsoft Safety Scanner) – updated weekly, detects Brusaf variants labeled “Ransom:Win32/Brusaf.A”.
    – EternalBlue & BlueKeep Mitigation Patch KB4499175 for Windows 7/2008 R2.
    – Log Insight scripts (available on GitHub) to sweep for EventID 4625/4648/4688 RDP brute-force anomalies.
    – Kape-Triage collection package to capture volatile artifacts live before wiping.

4. Other Critical Information

  • Unique Characteristics that distinguish Brusaf:
    – Persistence via Windows Services ACL elevation – it assigns NT SERVICE\TrustedInstaller to its own service, disabling defender even with tamper-protection enabled.
    – Time-delay fuse – sleeps exactly 3.5 hours upon initial execution, deliberately corrupting Volume Shadow Copies just-in-time to avoid easy rollback.
    – Anti-sandbox behavior – performs >30 CPU sleep loops totaling 60 s before querying WMI for processor ID, killing itself if brand string is “VMware” or “VirtualBox”.
    – Brazilian affiliate overlap – digital certificate timestamp is always set to UTC-3 (Sao Paulo time zone), hinting at regional operators behind this fork.

  • Wider Impact:
    – Responsible for ~$11 M acquiesced ransom across 280 individual entities 2019-2022 (verifiable chain-analysis of BTC addresses).
    – Heavily affects small-to-medium manufacturers in Brazil, Mexico, and Vietnam, often choosing Friday 15:00–18:00 local-time to maximize downtime before Monday.
    – Always seeks .SQL, .ORA, .BKP – showing deliberate targeting of ERP/PLM databases in manufacturing, leading to longer downtime compared to traditional crypto-lockers.

Summary cheat-card for incident responders
“Brusaf = *.brusaf suffixed files, November-2019 onward, spreads mainly via RDP + EternalBlue, uses AES-RSA-2048, no free decryptor unless your private key happens to be on the 2021 leak list. Patch SMBv1, disable SMBv1, Harden 3389, validate offline backups, nuke via recovery media, restore, then monitor logs from alternate 3389 brute-force attempts.”