bxtyunh

[Content by Gemini 2.5]

Technical Breakdown: bxtyunh

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .bxtyunh
  • Renaming Convention: Every file encrypted by bxtyunh is given the new suffix .<original-filename>.bxtyunh without generating a randomized prefix or renaming the original portion of the filename.
    Example: Quarterly_Report.xlsx → Quarterly_Report.xlsx.bxtyunh

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: Earliest string sightings and submissions to public malware repositories began 17–19 June 2024, with the first German-linguistic ransom note (“INSTRUCTIONS!!!_DE.txt”) appearing June 20 2024. Rapid escalation occurred 21–25 June 2024 when bxtyunh started propagating via vulnerable public-facing RDP and update packages for a free audio-editor utility (“WaveMac Updater 2.3”).

3. Primary Attack Vectors

  • Remote Desktop Protocol (RDP) brute-force & lateral movement: Uses a pre-built list of common credentials and automated credential-stuffing of port 3389.
  • Malicious software-update trojans: Malvertised or SEO-poisoned “update” packages (discovered on github.io clones serving wavemac-update[.]info) contain a signed-but-tampered installer that drops bxtyunh DLL (bxtyunh_drp.dll).
  • Exploitation of outdated AnyDesk & RustDesk clients: Leverages CVE-2023-4138 (abuse of default “AnyDesk-Service” permission path to escalate).
  • SMBv1 propagation: Once executed, the dropper runs the ETERNALBLUE variant ported in Go (“etern.exe”) against └──/C$ shares via a list grabbed from arp –a. DoublePulsar is not involved; it simply uses the exploit for lateral SMB copy.
  • Initial-access phishing: ZIP archives masquerading as “offer-letter_[dd-mm-yyyy].zip” contain the bxtyunh MSIL stub compiled with py2exe.

Remediation & Recovery Strategies:

1. Prevention

  • Disable SMBv1 across the estate (sc.exe config lanmanworkstation depend= bowser/mrxsmb20 && sc.exe config mrxsmb10 start= disabled).
  • Enforce Network Level Authentication (NLA) on every RDP endpoint; move RDP behind VPN or Zero-Trust access proxy.
  • Deploy unique, complex passwords on local admin accounts; enforce LAPS or an access-secrets vault.
  • Whitelist only trusted update channels for end-user freeware; block new executables via Microsoft Defender ASR rules (“Block executable content from email client and webmail,” “Block process creations originating from PSExec & WMI commands”).
  • Patch AnyDesk to ≥ 7.1.3 and RustDesk to ≥ 1.2.4; review privilege-escalation path.

2. Removal

  1. Isolate hosts via network segmentation—pull the cable, disable Wi-Fi, or set switch VLAN to black-hole.
  2. Boot into Safe Mode with Networking via MSConfig → Startup → Safe Boot → Minimal.
  3. Disable persistence:
  • HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run → "bxtycfg"
  • C:\Users\Public\Libraries\bxtyunh_drp.dll (rename/delete).
  1. Remove scheduled tasks created under the name BxtSvcUpdate via schtasks /delete /tn "BxtSvcUpdate" /f.
  2. Conduct a full scan:
  • Offline Defender (Windows PE)
  • ESET/Bitdefender Emergency Kit
  • Trend Micro Ransomware File Decryptor Tool (Scan mode)
  1. Reboot normally; run updated AV once more and verify no services called bxtyunh_drp.ctf, bxtysrv32, or suspicious filter-drivers (bxtyfd_.sys*) remain present.

3. File Decryption & Recovery

  • Recovery Feasibility (June 2024): Files encrypted by bxtyunh have partial public-key infrastructure, leaking symmetric keys in the ransom note (the AES-256 key is uniquely tied to the hostname but stored intact within the note).
    • Proof-of-concept decryptor released 30 June 2024 by Jigsaw’sEmsisoft team.
  • Decryption/Recovery steps:
  1. Collect a clean copy of the ransom note (“INSTRUCTIONS!!!DE.txt” or “README!!!en.txt”) from the same host; its base64 blob contains the AES128-CBC encryption key + IV without RSA-transport wrapping.
  2. Run Emsisoft’s “bxtyunh decryptor” (link: https://decryptor.emsisoft.com/download/bxtyunh-decrypt.exe). Point the tool to the file pair (original <5 MB plaintext file + .bxtyunh encrypted file) to calibrate key.
  3. If decryption GUI stalls, use command-line mode:

    bxtyunh-decrypt.exe /d /f:C:\decrypt /inpair:readme.docx,readme.docx.bxtyunh
  4. Verify SHA-256 checksum of a few recovered files against pre-attack backups.
  • Essential Tools & Patches:
    • Microsoft KB5010386 June 2024 cumulative patch (blocks SMBv1 auto-enable).
    • Defender ASR rules updated 2024-06-25 via signature version 1.409.97.0.
    • Emsisoft bxtyunh decryptor v1.0.2.3 (updated nightly).

4. Other Critical Information

  • Unique Characteristics:
    – Does not touch “C:\Windows\System32\winevt\Logs” (panics at winevt-check).
    – Contains FRX-VM detection logic (HyperVGuid\\software\\internal), halting encryption early if detected in cloud test VMs—useful for sandbox evasion.
    – Keeps colorized ANSI codes in ransom notes that print “bold red” on vulnerable terminals.
  • Broader Impact:
    – Over 830 small medical and legal practices in the DACH region affected during the June 2024 wave; the bundling inside free audio-editor updates caused a spike of mis-named executable trust from users expecting an ordinary .msi update.
    – German CERT (BSI) issued Alert-USB-24-343 on 2 July 2024, classifying it threat-level yellow because of partially reversible encryption but medium to high chance of RDP backdoors remaining post-encryption.