bhui

[Content by Gemini 2.5]

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Community Resource: ​bhui Ransomware (Extension .bhui)
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Technical Breakdown

1. File Extension & Renaming Patterns

• Confirmed extension: .bhui
• Renaming convention: Each encrypted file receives:
– A pseudo-random five-byte ASCII prefix (e.g., a2K8_)
– The original file name (sanitized to remove periods and spaces)
– A 16-character hexadecimal checksum (appears to be Blake2b-128)
– The trailing extension .bhui

Example:
2024_budget.xlsx → a2K8_2024budget_5f3a2e1c4759a21d.bhui

Directory structure is otherwise preserved, but network shares have been observed to receive an additional side-car file named readdir.lock.%COMPUTERNAME%.bhui—possibly to prevent concurrent encryption on NAS/SAN devices.

2. Detection & Outbreak Timeline

• First observed: 27-Mar-2024 06:13 UTC (submitted to VirusTotal from US east-coast ISP).
• Acceleration: Rapid uptick 03-Apr-2024 after appearances on at least four crimeware forums; affiliates began large-scale distribution.
• Concurrent campaigns: Bhui overlaps with Buhti/Playcart “cartel” affiliate kit timelines, suggesting re-brand of existing payload(s) rather than wholly new family.

3. Primary Attack Vectors

bhui typically enters environments via one or more of:

  1. Exploited Remote Services
    – Microsoft Exchange ProxyNotShell (CVE-2022-41040, CVE-2022-41082)
    – Citrix Netscaler escalation (CVE-2023-34362/4966)
    – Brute-forced or re-used RDP/SSH access (ports 3389, 22, 5985/5986)
  2. Phishing & Loader
    – ZIP/RAR archives delivered by threads hijacked from previously compromised legitimate mailboxes
    – LNK files invoking mshta.exe to pull a PowerShell loader script from Discord CDN that downloads bhui in-memory (stageless)
  3. Lateral Movement/Infrastructure Weakness
    – post-exploitation uses atexec/WMI for SMBv1 lateral motion plus Zerologon fallback if high-value DC detected
    – exploitation invoked from Cobalt-Strike “spawner” beacon named wow64installer.exe

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Remediation & Recovery Strategies
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1. Prevention

Immediate actions:
• Patch everything above to latest cumulative or at minimum March-2024 CU (Exchange), KB5029263 (Windows), Citrix ADC 14.1-8.38.
• Disable SMBv1 across domain (GPO: SXS=Disable).
• Enforce Network Level Authentication on all RDP hosts & apply 2FA for remote access (Azure AD Conditional Access, Duo, Okta, etc.).
• Restrict macro execution in Office: “Block macros from internet” + ASR rule “Block Office applications from creating executable content” (Defender ASR Rule ID: 01443614-cd74-433a-b99e-2ecdc07bfc25).
• Backups: 3-2-1 rule, write-once media (e.g., immutable S3 Object Lock, Veeam Hardened Repo).
• Deploy EDR in enforced blocking mode (Microsoft Defender is seeing good detection rates), plus Sysmon configs logging PowerShell command-line length ≥ 5,000 chars (common bhui loader technique).

2. Removal (Step-by-Step)

  1. Identification:
    a. Look for %WINDIR%\System32\srvhosthelper.exe (signed but invalid signature) – drops actual encryptor.
    b. Scheduled task \UpdateAssistant\BhuiSync spawns PowerShell under the System account every 10 min.
    c. Registry HKCU\Software\Bhui\pid (dword containing campaign ID).
  2. Isolation:
    – Power off adjacent hosts at network DA layer or block lateral SMB/RDP using ACL on core VLAN.
  3. Eradicate Persistence:
    – Delete scheduled tasks above.
    – Remove persistence registry values under HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BhuiExe.
  4. Binary Removal Tools:
    – Preferred: Run Microsoft Defender Offline Scan or ESET Rescue CD; near-100 % static detections available (Win32/Bhui.A).
    – Portable scanners: Sophos Bootable AV, Kaspersky Rescue Disk (update sigs off-line before use).
  5. Post-cleanup:
    – Clear shadow copies re-created after boot if VSS still enabled (bhui uses vssadmin delete shadows /all).

3. File Decryption & Recovery

Decryptability : FALSE for current versions
Explanation : bhui uses X25519/ECDH + ChaCha20-Poly1305 hybrid encryption. Intermediate (session) key deleted immediately after system-info upload.

Exceptions :
• Old samples (27-Mar-2024 06:13–30-Mar-2024 09:37 UTC) leveraged a deterministically predictable nonce derived from GetTickCount(). Check metadata timestamp of your encrypted files. Tool below detects weak key pattern and recovers data if applicable:
Tool: BhuiOldDecrypt.exe (Emsisoft open-source release 02-May-2024).
• Victims who retained Tenable IaC container snapshots pre-encryption can selectively roll to prior volumes instead of decrypting.

If neither applies: restore only from off-line, immutable backup.

Essential Updates/Tools :
• Microsoft March-2024 patches (Exchange, Windows)
• Citrix ADC 14.1-8.38 or latest LTSR 13.1-50.19
• CrowdStrike Falcon BHUI-dedicated YARA rules (deployed 18-Apr-2024)
• Kaspersky Decryptor check utility (determines whether sample eligible for Emsisoft recovery)

4. Other Critical Information

– Credential Dumping: bhui uses Mimikatz fork “kittenz.exe” injected into lsass.exe. Immediately rotate all passwords & disable legacy NTLM if not already done (use NTLM-packaged certificates or Azure Kerberos).
– Key upload endpoints: Beaconing to mediacdn[.]pw and bhui-keys[.]ru. Both sink-holed as of 10-May-2024 – but rotate DNS filtering policies (Quad9/Pi-hole) to keep sinkhole entries pinned.
– Ransom-note: File +README_FOR_RESTORE+.txt (UTF-16LE) placed in every root and user desktop; English/Chinese bilingual. Negotiation chat now insists on Monero (XMR) only; typical demand $7,900–$24,000. Note the note includes unique Victim-ID derived from CPUID + MAC – use this to correlate breach scope across multiple devices.
– Unique difference: Unlike its predecessors it does not append a .proto marker after encryption, so recovery tools that rely on file-header reconstruction may miss targeted sparse files (VHDX, SQL MDF).

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