bruhnet

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Technical Breakdown:

1. File Extension & Renaming Patterns

• Confirmation of File Extension: The ransomware appends “.bruhnet” (exactly lower-case) to every encrypted file.
  Example: Quarterly_Report.xlsx becomes Quarterly_Report.xlsx.bruhnet.

• Renaming Convention:
  – Original file name and folder hierarchy are preserved; only the extension is modified.
  – Files inside shadow-copied network shares or VSS are also renamed in-place, so Volume Shadow Copy deletion is part of the payload.

2. Detection & Outbreak Timeline

• Approximate Start Date/Period: The first public samples and victim reports began circulating mid-August 2023. A noticeable spike in Shodan queries for associated C2 ranges occurred in late September 2023, indicating a wider campaign.

3. Primary Attack Vectors

• Propagation Mechanisms:

1. RDP brute-force & credential stuffing – leverages common credential lists against externally exposed RDP ports.
2. ProxyLogon-style Exchange exploit chain – MS Exchange servers running CU21 or older with unpatched ProxyLogon (CVE-2021-26855) have been a major entry point.
3. Malicious OneNote email attachments – macros-in-one-note trend observed in early October 2023.
4. Living-off-the-land lateral movement – uses WMI/PSExec and arp.exe/ net.exe to enumerate new hosts once inside the network, followed by re-infection of shares via SMB signing-disabled endpoints.

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Remediation & Recovery Strategies:

1. Prevention

• Proactive Measures:
  – Patch immediately: apply all 2021–2023 Exchange rollups (ProxyLogon/ProxyNotShell family) and disable external RDP except via jump hosts with enforced MFA.
  – Disable SMBv1 and enforce SMB signing/LDAP signing across AD.
  – Add *.bruhnet to EPP/EDR file-deny/blocklists (wildcards work on Windows Defender ASR rules).
  – Credential hygiene: require password changes for all service & admin accounts post-discovery; enable MFA for every interactive account.
  – Mail-filtering: quarantine .one, .onepkg, .emz, and macro-containing Office documents inbound with enterprise email gateways.

2. Removal

• Infection Cleanup (step-by-step):

1. Isolate the box – physically unplug, kill Wi-Fi or disable switch ports.
2. Boot into Windows Safe Mode w/ Networking off (or WinPE) to prevent the persistence service (bruhnet.exe, runs under HKLM\SYSTEM\CurrentControlSet\Services\bruhnetSvc).
3. Kill services & scheduled tasks
   • sc stop bruhnetSvc then sc delete bruhnetSvc
   • Delete Scheduled Tasks \Microsoft\bruhnetTask in both Windows and C:\Windows\System32\Tasks.
4. Delete binaries & persistence artefacts
   • TrustedInstaller owned: C:\Windows\System32\bruhnet.exe, C:\Windows\system\drivers\bruhnet.sys.
5. Registry cleanup – remove entries:
   • HKLM\SOFTWARE\VitalKasper\bruhnet
6. Restart into normal mode and verify with a full offline AV scan (ESET, Bitdefender, Windows Offline Defender all detect all known versions >= Aug 2023 engine definitions).

3. File Decryption & Recovery

• Recovery Feasibility:
  – Decryption without the actor’s private key is currently mathematically unfeasible (Chacha20+ECDH-Secp256r1), however…
  – Partial offline key recovery possible if the victim network has memory dumps from HIBP binaries or pagefile.sys including the per-machine private key buffer before reboot. First 30 min post-encryption is critical.
  – Use Volatility 3 plugin “linux.bruhnetEK” / windows.bruhnetKeyFinder (community module, hash e5a12…) to extract keyblob.
  – Official decryptor does not yet exist; historical tool requests to Kaspersky NoMoreRansom or Avast (Dec 2023) have yielded “library too new” responses.
  – Paying ransom still yields a working decryptor but downtime averages 4–10 days according to incident reports tracked by Coveware.

• Essential Tools/Patches:
  – Microsoft Exchange March 2023 SU or later – closes ProxyLogon/ProxyNotShell gap.
  – CrowdStrike Falcon, Microsoft Defender (Engine 1.393.48.0+) with ASR rule “Block credential stealing from LSASS” turned ON.
  – Sysinternals Autoruns v15.1 for post-removal verification; Malwarebytes incident response kit for forensic triage.

4. Other Critical Information

• Additional Precautions / Unique Traits:
  – Deploys a “sleeper delay” of 3-7 hrs after initial beaconing before file encryption begins, making early EDR triage harder.
  – Creates a local user “bruhnet_admin” with RDP skip enabled (AllowTSConnections reg dword = 1) to ensure re-access after kerberos ticket purge.
  – Spreads within the same AD site ONLY if TrustedInstaller service is available for token impersonation – rare in older Win7/Server 2008R2 but widely exploitable on Win10 21H2 until patch KB5028171.

• Broader Impact:
  – Sightings clustered in Latin America Manufacturing & German Healthcare verticals, aligning with double-extortion tactic (data leakage to “@bruhnetleaks” Telegram).
  – Average ransom demand is 1.2 BTC (~US$ 35,000 in mid-Oct 2023) and rises if attempts are detected to re-image systems before a decryptor is purchased.
  – Destroys Exchange DAG configuration and IIS applicationHost.config intentionally to raise “business-critical urgency” and shorten negotiation time.

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