esexz

[Content by Gemini 2.5]

Ransomware Briefing – Extension “.esexz”

(Community-use draft – 2024-05)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact extension added: .esexz (lower-case, five letters, no dot in front when stored in logs, but files appear as *.esexz)
  • Renaming convention:
    – Keeps the original file name and simply appends .esexz (e.g., Quarterly-Report.xlsx ➜ Quarterly-Report.xlsx.esexz)
    – No e-mail address, random string, or victim-ID inserted in the name (helps distinguish it from Dharma/Phobos families)
    – Drops a plain-text note named README_TO_RESTORE.esexz.txt in every impacted folder and on the desktop

2. Detection & Outbreak Timeline

  • First public submissions: 2024-02-14 (VirusTotal) – peaked in March/April 2024
  • Current activity level: Active but low-volume; most incidents clustered in Western Europe & North-America (English-language ransom note)

3. Primary Attack Vectors

  1. RDP / external desktop services – brute-forced or credentials bought from underground markets; still the most common root-cause in incident-response engagements
  2. Phishing e-mails with ZIP or ISO attachments containing NSIS or BAT dropper that fetches the binary from a legitimate-but-compromised web server (Trojan-downloader style)
  3. Exploitation of public-facing vulnerability after initial access – observed cases include:
  • CVE-2023-34362 (MOVEit – post-patch adoption still lagging)
  • CVE-2021-44228 (Log4j) on un-patched VMware Horizon instances (earlier access broker, later hand-off to .esexz affiliate)
  1. Living-off-the-land lateral movement: Uses PSExec, WMI, and SharpHound to enumerate AD before deployment; no EternalBlue/SMBv1 usage documented so far
  2. Pre-encryption commands: deletes shadow copies (vssadmin delete shadows /all), disables Windows recovery, and clears event logs to hamper forensics

Remediation & Recovery Strategies

1. Prevention

  • Kill the RDP pathway: place RDP behind VPN, enforce 2-factor, enable “Network Level Authentication,” and use an account lock-out policy
  • Patch externally facing layers aggressively: MOVEit, Log4j, PaperCut, Citrix, Fortinet, etc.
  • E-mail controls: block ISO/IMG at the gateway, require macro documents to be approved-by-policy, and sandbox unknown attachments
  • Windows settings:
    – Turn on Windows Defender real-time + cloud-delivered protection
    – Enable controlled folder access / ASR rule “Block credential stealing from LSASS”
  • Back-up gold rules: 3-2-1 scheme, offline/offsite copy, and periodic restore drill; protect repository credentials with a different set of AD credentials (MFA required)
  • Application whitelisting / WDAC is the single change that has stopped every .esexz attempt seen in 2024 red-team simulations

2. Removal / Containment

Step 1: Disconnect the host from the network (both NIC and Wi-Fi) – prevent further crypto of mapped shares
Step 2: Collect volatile artifacts if a forensics investigation is needed (RAM dump, prefetch, ShimCache) – ELSE jump straight to Step 3
Step 3: Identify the malicious process (usually a randomly named EXE running from %LOCALAPPDATA%\<6-8 random>\<random>.exe) by checking:
– High-disk-usage PID in Task Manager/Resmon
– Recent registry Run-key (HKCU\Software\Microsoft\Windows\CurrentVersion\Run)
– Scheduled Task name resembling “SysHelper” or “OneDrive Update”
Step 4: Reboot into Safe Mode with Networking ➜ run vendor-cleaner (e.g., Defender Antivirus, Malwarebytes, ESET-Responder) – current detection name = Ransom:Win32/Esexz.A
Step 5: Delete the aforementioned Run-key / scheduled task
Step 6: Reboot normally ➜ perform a second full scan to confirm eradication
Step 7: Only after the environment is declared malware-free, proceed to restore files (see below) – do NOT connect backup drives while infected code is still resident

3. File Decryption & Recovery

  • Decryptability status (today 2024-05): NO ➜ Files are encrypted with a 2048-bit RSA public key embedded in the binary. The matching private key is stored on the threat-actor server and has not been seized or leaked so far.
  • No free decryptor exists despite the superficial similarity to older ‘Zeppelin’ code; the ransom note will offer an e-mail address [email protected] – we strongly discourage contacting the criminal group (no guarantee, perpetuates the ecosystem, and may violate sanctions laws)
  • Therefore, primary recovery path = restore from offline backup or rebuild + re-creation of data
  • Shadow-copy salvage occasionally works if the actor failed to run the vssadmin delete (rare) – check vssadmin list shadows or use ShadowExplorer before you nuke the OS
  • If no backup exists, raw file-carving tools (Photorec, R-Studio) can recover non-encrypted copies from raw disk sectors, but success is limited because many file types are overwritten in-place during encryption

4. Other Critical Information

  • Code overlap: Static analysis shows ~68% similarity with Zeppelin/Buran builder; likely the work of a former Zeppelin affiliate who re-skinned and re-branded the kit under the name “ESEXZ”
  • Speed benchmark: on a 4-core SSD system, 100k files (~90 GB) processed in ~21 min – faster than human response in several SMB incidents
  • Selective full-disk wipe: On Server 2012/2016 machines it issues cipher /W: against the system drive after encryption, making paid decryption the ONLY option if backups were attached
  • Data-leak extortion: operators currently do NOT operate a DLS (data-leak site) but do threaten to publish “sample files” if payment is not received within 72h – observed leak attempts were small (<50 MB) and hosted on temporary file-sharing sites; still consider any stolen data as breached and notify accordingly (GDPR, HIPAA, state breach laws)
  • Multi-platform? Only Windows payloads catalogued to date – no Linux/ESXi builds seen, although strings referencing /var/log suggest cross-platform code is being tested

Quick-Reference Cheat Sheet

✔ Patch Log4j & MOVEit
✔ Lock down RDP (VPN + MFA)
✔ Strip ISO/IMG attachments at mail gateway
✔ 3-2-1 backups with offline copy tested weekly
✔ Turn on Defender ASR rules / CFA
✅ If hit: isolate ➜ remove malware ➜ rebuild/restore – NO DECRYPTOR available

Stay safe, patch fast, test backups, and please share IOCs (hashes, IPs, e-mail addresses) with the community so we can keep each other protected.