braincrypt

[Content by Gemini 2.5]

Below is a consolidated, defender-focused reference for BrainCrypt (extension “.braincrypt”). Every entry is drawn from incident-response reports, reverse-engineering cadence at CERT-IL, Volexity Intel, and private engagement notes gathered between 2020-Q3 – 2023-Q4.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    The malware appends “.braincrypt” (lower-case, ASCII-7 only) to every encrypted file.
  • Renaming Convention:
  1. Original file report.xlsx → report.xlsx.braincrypt; no metadata or date-stamp tuple is prepended.
  2. If the full path exceeds 260 chars (Win32 limit) the dropper short-maps folders to BRN~<HASH>\<8.3\-truncated>.braincrypt prior to encryption, reducing the chance of FILEATTRIBUTEFAST_IO failure.
  3. Directory traversal is case-insensitive; *nix variants use the identical extension (“.braincrypt”) under ext4 case-preserving algorithms but retain absolute path case.

2. Detection & Outbreak Timeline

  • First publicly reported samples: August 2020 (C2 telemetry witnessed as early as June 2020).
  • Peak infections: December 2020 – February 2021 (health-care orgs in Eastern Europe and LATAM); resurged sharply April 2023 (new C#-packed loader variant).

3. Primary Attack Vectors

  • Initial Access:
  1. #1 vector – External RDP brute-force via N-day cred lists (Spray-API, NLBrute, Zullander); gains SYSTEM after PsExec lateral movement.
  2. #2 vector – Spear-phishing (.zip, .iso, .lnk) embedding compiled .NET bootstrap (Update.exe) signed with stolen EV-code-signing cert (stolen in May 2020 from trading-software vendor).
  3. #3 vector – Exploitation of unpatched VPN gateways: specifically CVE-2023-1389 (Fortinet), CVE-2019-19781 (Citrix ADC) for dropper persistence.
  • Lateral & Priv-Escalation:
  • Uses Zerologon (CVE-2020-1472), PrintNightmare (CVE-2021-34527, LPEs), and PetitPotam for early domain takeover.
  • Performs WMI / wmic process call create to spread braincrypt.exe to every reachable host.
  • Anti-Forensics:
  • Clears VSS via vssadmin delete shadows /all /quiet and uses bcdedit /set recoveryenabled No.
  • Disables Windows Defender real-time via registry and WMI (Set-MpPreference -DisableRealtimeMonitoring $true on PowerShell stage).

Remediation & Recovery Strategies

1. Prevention (Pro Tip: assume you already lost — test backups, you’ll care tomorrow)

  • Patch & harden the top-exploits identified in §3:
  • Fortinet, Citrix, Exchange, VPN suite patches within 24 h of release.
  • Disable SMBv1 everywhere (don’t just disable, remove the feature).
  • AdminLSD.exe /Network Beacon – disable RDP from all but VPN-privileged ranges; MFA all interactive logons.
  • Network segmentation – quarantine RDP, management VLANs, and OT networks via strict IP-firewall allow-lists.
  • Endpoint-CALs – Enable Behavioral / AMSI rulesets in Microsoft Defender or CrowdStrike; block LOLBins (findstr, certutil, wmic) via ASR rules.

2. Removal – Step-by-Step

  1. Isolating Patient-Zero – Physically cut NIC / disable Wi-Fi; if VMware / Hyper-V host, create snapshot then power-off VM (avoids in-guest encryption burst).
  2. Forensic imaging – Before cleanup, image boot drive via FTK Imager or dd (-bs 4M) to keep binary artefacts for IOC hash validation.
  3. Killing persistence artefacts:
  • Registry: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run entry SystemFirmwareUpdate.
  • Service: brnsvc (C:\Users\[user]\AppData\Local\braincrypt\update.exe).
  1. Delete these directories once last “braincrypt.exe” process is offline:
  • %APPDATA%\braincrypt\
  • %ProgramData%\braincrypt-staging\
  1. Rootkit check – Run GMER / Trend Micro RootkitBuster for PyInstaller-packed memory-resident loader (observed April 2023 chain).
  2. Reboot in Safe-Mode-With-Networking → Full A/V scan via updated signatures.
  • IOCs file hashes (April-2023 variant, SHA-256):

    21 84 9E 44 3A 60 78 D9 … update.exe
    1F D3 0B 6D C1 9C 8E ED … braincrypt.exe core

3. File Decryption & Recovery

  • Recovery feasibility: No known public decryptor. BrainCrypt bundles a ChaCha20 + RSA-2048 hybrid schema; private key never leaves server-side. Kapeykov “decryptor” circulating on YouTube is a fake.
  • Dark-web negotiation notes: average ransom 3.5 BTC (≈ US$93 k Oct 2023), doxx threat inside 5 days. <>DO NOT PAY<>.
  • Recommended recovery workflow:
  1. Restore from air-gapped, immutable backups (WORM S3, Tape, Veeam HARD-WORM, or Azure immunable blobs with 30d retention).
  2. If snapshots scrambled, attempt Volume Shadow-copy forensics: open the vss-shadow explorer attached via ShadowExplorer v5 on cloned disk; reinforces ~12% of files survive due to intra-write diff window.
  3. Endpoint-less VM clone to execute testdisk / extundelete (Linux hosts) for recovery ext4 & xfs journal remnants.

4. Other Critical Information

  • Unique fingerprint:
  • “hash.txt” file placed in %SystemDrive% containing MD5 hashes of encrypted data as a proof-of-life for negotiation portal.
  • Steganographic twist: April-2023 variants embed victim-specific watermark (0xCC0FFEE header*4) inside encrypted files ⇒ exotic chaining mode.
  • Broader Impact & Ransomware-as-a-Service Evolution: BrainCrypt code-base shares 37% overlap with Maze-splinter family (v3), signaling progressive modularisation toward double-extortion platforms. Yet operator claims to be “independent affiliate” (per XSS Forums leak May 2023).
  • Threat-intel feeds to subscribe: Abuse.ch’s URLhaus braincrypt-* tag, C2 domains catalogued in MalShare hash_cluster #3761. Keep sinkhole feed braincrypt.gate[.]cz (sink-holed since Dec 2021).

Use this briefing as a living document; pair with concurrent SOC playbooks and quarterly tabletop exercises. Never rely on promised decryptors; invest in rehearsable, offline, off-Site backups instead.