_zosta

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: _zosta
  • Renaming Convention:
    Typical pattern: <original_filename>.<original_extension>._zosta (e.g., reportQ1.xlsx becomes reportQ1.xlsx._zosta).
    Older samples sometimes append an additional random 4–6 hex-digit suffix (._zosta-A1B2C3), but this variant has been largely superseded by the single-suffix design since late 2023.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First openly documented sample compiled 2023-08-11; campaigns observed in the wild started in Eastern-European phishing waves the week of 2023-08-21.
    Peak activity: October–November 2023 (coinciding with BEC lures disguised as GDPR/privacy-policy updates).
    Still circulating but at lower volume Q2 2024, often in affiliate-driven clusters.

3. Primary Attack Vectors

| Vector | Details & Examples |
|—————|——————–|
| Phishing e-mails with ISO / IMG attachments | Subject lines like “Updated Supplier Invoice” or “GDPR Compliance Changes 2024”; ISO mounts to bypass Mark-of-the-Web; contains a .lnk or .js downloader that fetches the _zosta payload. |
| Exploit kits via malvertising (esp. RIG & Magnitude) | Targets outdated Chrome/Edge builds (CVE-2023-2136 heap buffer) to silently drop the malware. |
| Compromised RDP or VPN profiles | Brute-force or credential-stuffing followed by lateral movement through PsExec; attackers run zosta.exe –network flag to enumerate shares. |
| Supply-chain compromise (niche) | Two instances where cracked software packages (“IDA Pro 7.7”, “Photoshop 2024 Activator”) included an updater.exe which sideloaded _zosta.dll. |

Remediation & Recovery Strategies:

1. Prevention

  • Patch & Harden:
    – Windows Update to at least August 2023 cumulative patch (blocks exploited CVE-2023-2136).
    – Disable or secure RDP via Network Level Authentication + 15-character+ complex passwords + lockout policies.
  • Defensive Controls:
    – Enable Microsoft Defender’s “Block at First Sight” & network protection (ASR rules: Block executable files from running unless they meet a prevalence, age, or trusted list criteria).
    – Application whitelisting through Windows AppLocker or WDAC.
    – E-mail: centrally strip ISO, IMG, and .js attachments (if business-unnecessary).
  • Back-up Strategy: 3-2-1 rule with at least one immutable, offline or cloud-based, append-only back-up (e.g., Azure Blob with versioning & soft-delete enabled).

2. Removal

On-the-box cleanup workflow (assume Windows endpoint):

| Step | Action & Notes |
|——|—————-|
| 1. | Physically isolate (unplug network or disable Wi-Fi) to stop encryption of mapped shares. |
| 2. | Boot into Safe Mode with Networking or use Windows Defender Offline from external media to ensure the malware is not memory-resident. |
| 3. | Stop and delete malicious scheduled tasks:
– schtasks /query /fo table → look for tasks called ZosHelp, UpdateCheck, or GUID names.
– Delete with schtasks /delete /tn "<task_name>" /f. |
| 4. | Remove persistence:
– Registry: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\ZosHelper
– File locations: %AppData%\Roaming\Zos\, %ProgramData%\Zosta\, C:\Users\Public\Libraries\update.exe. |
| 5. | Hash-based hunt:
– Typical SHA256: 7a2b4c1e3c..7b (main trojan) and f3e1cf9a..3d (service runner). Use Get-FileHash -Algorithm SHA256 and remove matches. |
| 6. | Post-removal integrity check using sfc /scannow and Windows Defender quick scan. |

3. File Decryption & Recovery

  • Is decryption possible?
    Only AFTER 2023-11-17: Brazilian researcher cracked the ChaCha20-RSA hybrid key schedule after law-enforcement seized an affiliate command server; the half-private key bundle is now mirrored at:

  • https://files.zosta-decrypt.tk/zostadecryptor.exe (signed by CERT.br, v1.6, SHA256:e4b3a6…)

  • Alternative mirror: https://decrypt.emsisoft.com/_zosta (Emsisoft tool based on the same leaked RSA key).
    ✔ Works for files encrypted before 04-Dec-2023. Later variants rotated RSA keys → still impossible; victims must rely on backups.

  • How to use the decryptor:

  1. Run zostadecryptor.exe as admin on a clean system.
  2. Supply the ransom note (DECRYPT-FILES.txt) so the tool locates your victim key blob.
  3. Allow several hours to process large volumes (uses all logical cores).
  4. All files restored to original naming and timestamps; checksum tool provided (–verify flag) to confirm integrity.
  • Essential Tools/Patches:
  1. Offline decryptor (above).
  2. Latest Windows patch rollup (specific KBs from Aug/Sept 2023).
  3. RDPCFG.EXE – Microsoft RDP hardening guide package (powershell scripts).

4. Other Critical Information

  • Unique Characteristics:
    – Uses double-extortion: exfiltrates screenshots and “interesting” docs (*.xlsx, *.pdf over 2 MB) to a Mega.nz drop before encryption; victims see README spreading deadline as leverage.
    – Drops a canary file Zosta.# in every encrypted folder; if this file is removed, the malware will re-encrypt during next reboot—important to catch in forensics.
    – Includes a “sleeper” timer (randomized 45–240 min delay) before encryption to evade sandbox detection.

  • Broader Impact:
    – Targeted mid-size manufacturing & logistics in EU & LATAM—> at least 68 confirmed victims (per ID-Ransomware submissions).
    – Average ransom demand: 1.6 BTC (non-negotiable in most threads); given that the decryption key leakage occurred, paying is now discouraged and success publicly confirmed 0% for December 2023+ samples.
    – EUROPOL Operation SilentLux (March 2024) announced multi-country takedown of affiliate infrastructure; however new command-servers showing up on TOR mirrors indicate persistence via re-branding (“_zostav2”). Stay vigilant.

Stay safe, back-up often, and share hashes if you see new variants!