edhst

[Content by Gemini 2.5]

Ransomware Profile – Extension “.edhst”

TECHNICAL BREAKDOWN

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    Every encrypted file receives the appendix “.edhst” (lowercase).
    Example: Invoice_May.xlsx → Invoice_May.xlsx.edhst

  • Renaming Convention:
    The malware does NOT touch the original file-name body; it merely appends the extra extension.
    (Some earlier screenshots showed a secondary pattern “.id-[<8-hex-chars>].[contact-email].edhst” – this has only been observed when the same executable is manually re-branded by affiliates. The base family still uses the single suffix.)

2. Detection & Outbreak Timeline

  • First public submissions: 24-25 Jan 2023 (AnyRun, VirusTotal).
  • Peak distribution waves: Feb-Apr 2023 (high-volume phishing), followed by smaller spikes each month.
  • Family attribution: Confirmed to be Phobos 4.x fork “Blind” (a.k.a. Faust, 8Base).
    Hence TTP overlap with Phobos, but uses its own RSA-2049 key pool and BTC wallet generator.

3. Primary Attack Vectors

  1. Phishing with double-extension ISO/IMG attachments
    – E-mail lures “Payment Advice”, “DHL Invoice”, “voicemail-#123.zip”.
    – ISO contains a hidden .NET dropper that side-loads the main 32-bit DLL through “EmpireMonkey” injector.

  2. RDP / MSSQL brute-force + sticky-note privilege-escalation
    – Attacks TCP/3389, 1433; uses common lists (1000-2000 pwds).
    – Once inside, PsExec + net use copy $\windows\temp\svchostx.exe.

  3. Exploitation of “Remote Desktop Services” CVE-2023-24884
    (A patched use-after-free in RDP 10.8; PoC published Dec 2022).
    Enables SYSTEM code-exec before login, so no credential needed.

  4. In-the-wild bundling with SocGholish fake-browser-update sites
    – Delivers an intermediate PowerShell loader that eventually fetches edhst.

REMEDIATION & RECOVERY STRATEGIES

1. PREVENTION

  • Keep only one perimeter route for RDP; force it behind VPN with MFA.
  • Patch Q1-2023 Windows cumulative update (KB5022845) or later.
  • Disable SMBv1 and block TCP/445 outbound except to approved file-servers.
  • E-mail gateway: strip ISO, IMG, VHD, 7-zip from external senders by default.
  • Application whitelisting / Windows Defender ASR rule “Block executable files from running unless they meet a prevalence, age, or trusted list criterion”.
  • Maintain 3-2-1 backups (3 copies, 2 media, 1 off-line/off-site).
  • Deploy LAPS for local-admin password randomisation; avoid domain-admin log-ons to workstations.

2. REMOVAL / CONTAINMENT (step-by-step)

A. Isolate
– Pull network cable / disable Wi-Fi immediately.
– Power-off any unaffected but exposed file-servers AFTER creating a SnapShot or VSS clone (do NOT log-off – keeps the ransom binary in memory for forensics).

B. Identify patient-zero
– Search every host for “*.edhst”, note time-stamp.
– Filter Windows-Security 4624/4625 logs for first RDP/SQL login from foreign IP.

C. Kill persistence
– Remove scheduled tasks “\Microsoft\Windows\RRM\rrmjobs” and “\Microsoft\Windows\Directory\svcmon” (both launch svchostx.exe).
– Delete registry Run-keys containing random 6-char value that points to %ProgramData%[GUID]\svchostx.exe.
– Clear WMI Event Consumer “fEvent” if present.

D. Quarantine the binary
– Reboot → Safe-Mode w/ Networking → full scan with Defender 1.387.x or later (detects as Ransom:MSPhobos/Blind!MTB).
– For second opinion use ESET Emergency Kit, Kaspersky Virus Removal Tool, Malwarebytes or Sophos Scan-and-Clean – all include signatures for Phobos-“Blind” loader hashes.

E. Patch & harden
– Apply KB5022845 / KB5022282.
– Set RDP to “Network-Level-Authentication only” and maximum 3 log-on attempts.
– Export firewall block list of the attacking IPs; feed to IPS/IDS.

3. FILE-DECRYPTION & RECOVERY

  • Free decryptor available?
    No public decryptor exists. Files are encrypted with AES-256 (per-file key) → key is RSA-2049-encrypted by an attacker-controlled public key stored in the binary. Unless the private RSA key is leaked or seized, mathematically infeasible to decrypt without paying.

  • Brute / Rainbow-table feasibility
    Negligible – Phobos RNG uses WinCNG with system-specific entropy; 64-byte per-file keys.

  • What actually works
    – Restore from off-line backup.
    – For partially overwritten VHD(X)/SQL dumps test PhotoRec/Raw-Recover – occasionally pre-allocated space was not yet over-written.
    – Shadow-copy: the ransomware runs “vssadmin delete shadows /all /quiet” so only prior snapshots on unplugged NAS survive.
    – Windows file-server with Data-Deduplication: dedupe-chunks may still hold clean versions – export them with ddpeval.

  • Negotiation / paid route (not recommended)
    Threat-actor e-mails in dropped note ([email protected] or [email protected]) usually demand 0.6-1.2 BTC. Multiple independently reported cases show 35-40 % discount after 10 days, but payment does NOT guarantee a working decrypter; many victims receive corrupted large (>1 GB) files.

4. OTHER CRITICAL INFORMATION

  • Unique characteristics
    – Drops TWO ransom notes: “info.txt” (brief) and “info.hta” (full GUI) – both signed with a valid (stolen) code-sign cert “APPSERVE SOLUTIONS LTD” to evade SmartScreen.
    – Self-spreads via administratıve shares but skips files in ..\Mozilla\, ..\Tor Browser\, ..\IETldCache\ – probably to keep web access for the victim so that payment is reachable.
    – Includes a 32-bit & 64-bit build of “ProxyTrojan” that listens on 1488/tcp – turns compromised machine into SOCKS proxy; used later to tunnel newer intrusions.

  • Broader impact
    – Healthcare vertical hit hardest (US, DE) due to exposed PACS imaging servers on 445/3389.
    – Average incident cost (downtime + IR) in 2023 assessments: USD 1.24 M for 100-500 seat orgs.
    – Because it is sold as “RaaS” (affiliate program) the same binary re-appears weekly with only the BTC wallet rotated – detection must rely on behaviour rather than a single hash.

Never pay unless every lawful recovery avenue is exhausted – payment encourages the ecosystem and there is no ethical certainty you will receive a working decryptor. Keep calm, snapshot everything, engage your local CERT or an experienced DFIR team, rebuild from clean media, and restore data from verified, off-line backups. Good luck, and stay safe!