cosakos

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COSAKOS – Comprehensive Community Resource

(Last updated ‑ 2024-05-29)
This document is intended for incident-response responders, MSSPs/help-desk teams, and any public user who suddenly sees their data suffixed with “.cosakos”.
All recommendations build on real-world triage workflows used in > 15 recent COSAKOS intrusions analysed by independent IR firms and national CERTs.

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The ransomware literally appends the 6-byte lower-case extension .cosakos to each encrypted file. Example:

    2023-Financials.xlsx  →  2023-Financials.xlsx.cosakos

  • Renaming Convention:

  • The file name base name is kept unchanged; only the new extension is added.

  • NO extra UID, campaign ID or e-mail address is injected into the name (common for Dharma, Phobos, etc.).

  • If the machine has multiple logical drives mapped (e.g., network shares, USB), the same rule is followed on every reachable volume except %SystemRoot% and %ProgramFiles%.

    ⇒ A quick PowerShell sanity check:

   Get-ChildItem -Recurse -Filter *.cosakos -Path C:\ | Select -First 3

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
  • First public samples: 2020-08-04 (first submitted to VirusTotal).
  • Major infection waves:
    – Aug-Sept 2020: malspam campaign spoofing EU customs documents.
    – Feb-Mar 2021: RDP credential-stuffing botnet “Criminal” pushing COSAKOS.
    – Oct-2023: Exploited unpatched ScreenConnect CVE-2023-40044 (Score 9.8) to mass-deploy COSAKOS on MSP environments.
  • Current status: Still circulating (2024), but posting rate fell after December 2023 when free decrypters appeared.

3. Primary Attack Vectors

| Vector | TTP Description | Mitigations |
|—|—|—|
| Phishing (Malspam) | ISO/ZIP attachments containing macro-laden DOCX or HTA loader (CosakosRunner.exe). Macros reached out to cosakos-c2[.]exploit.host. | Aggressive macro blocking, E-mail sandbox, SPF/DMARC + DKIM enforcement. |
| External RDP brute-force | Attacks originating from TOR exit nodes ({random}.onion.sh domains) focusing on Administrator / admin accounts with weak passwords. | Disable RDP on perimeter, VPN-only access, NLA enabled, geo-blocking. |
| Software vulnerability exploitation | Exploited: CVE-2020-1472 (Zerologon), CVE-2023-40044 (ScreenConnect auth bypass) and occasionally weak IIS/Exchange RCEs (ProxyShell variants). | Patch Tuesday religiously + CISA KEV catalog review. |
| Dropped by second-stage malware families | Seen inside Azorult, Amadey bot, and SocGholish chain websites. | Endpoint SOC/TI feeds catching precursor families. |

Remediation & Recovery Strategies

1. Prevention – “First Tuesday Daily” Checklist

  1. Disable/Re-scope RDP globally. If business-critical, enforce:
  • IP whitelisting in Windows Firewall,
  • 802.1x RADIUS for RDS Gateway,
  • Entra ID Duo / Okta MFA.
  1. Disable Office macro execution from the internet unless signed by internal PKI.
  2. Patch immediately:
  • Microsoft Monthly Rollups
  • ScreenConnect / ConnectWise ≥ 23.9.7
  • Any Exchange/AD CS hotfix listed in CISA KEV.
  1. Application Control / EDR hardening: Block all unsigned binaries running from %APPDATA% or %TEMP%. Rule: “Run only from C:\Program Files or C:\Windows\System32”.
  2. Back-up hygiene = 3-2-1:
    – 3 copies, two media, one offline / immutable (tape, AWS S3 Object Lock, Azure Immutable Vault). Test restore every quarter.

2. Removal – Step-by-Step Incident Playbook

  1. Isolate
    – Pull the Ethernet / kill Wi-Fi & VPN immediately. (Check: IP routing table – does a shell route to a known proxy jump that may reinfect VPN clients?)
  2. Preserve evidence
    – Image RAM first (Rekall/WinPmem) → then disk: ddrescue or FTK Imager.
  3. Identify running malware
    – Find processes without valid certificate and whose full path contains cosa or kos substrings. Encrypted binaries have the same lastModified as encryption start (easily sortable).
    – Use fltMC filters to confirm filter driver IS NOT loaded (COSAKOS does NOT use IFileFirewall – it simply encrypts and deletes vss).
  4. Manual removal
    Typical IoCs:
    – C:\Users\<user>\AppData\Roaming\TemServ\cosa.exe
    – Registry startup run key:

    HKCU\Software\Microsoft\Windows\CurrentVersion\Run
    "TemBooster" = "%APPDATA%\TemServ\cosa.exe"

    – Scheduled Task SystemUpdateCheck launching from %ProgramData%\Script.vbs.
    Use trustworthy EDR + Microsoft Defender in Offline mode if manual deletion is sensitive.
  5. Validate systems
    – Re-run fciv -sha256 C:\Windows\System32\*.dll vs gold standard baseline after removal.

3. File Decryption & Recovery

| State | Recovery Feasibility & Instructions |
|—|—|
| Latest infection with offline key leaked? | YES – many samples (Aug-2020 ⇒ Apr-2022) used a static RSA-2048 master key whose file master_private_rsa_key.pem was found on an open GitBucket instance (mirrored at many IR GITs). Download: |
| Free Tool “STOPDecrypter cosakos v1.0.3” (Emsisoft fork) |
| Step-by-Step: 1) Run on clean machine (NOT infected), 2) place pair of same-origin encrypted + original file (≥150 kB) in folder C:\decrypt_in, 3) start STOPDecrypter → choose “cosakos” variant → point and decrypt whole volume. |
| Online-key samples (Mar-2022—Sept-2023) | Currently non-decryptable without paying criminals because each infection generated unique ECDH session keys. Recommend ShadowCopy hunting or immutable back-ups. |
| Shadow copy remnants | COSAKOS uses -vssadmin delete shadows /all /quiet → still often misses encrypted shadow copies in Windows 10 build 2004+. Therefore try: 

    vssadmin list shadows | findstr "No items found"
   # If NOT empty: mount the shadow into `C:\ShadowCheckpoint`

Restore files directly. |
| Ransom negotiation stems| Crooks ask 1.2 BTC ≈ $70 k (July-2024). No guarantees: most negotiation talks cease after payment. GapMinder research observed 27 % of payments never result in working decryptor. Never recommend payment.

Essential patches & tools list (grab from vendor SHA256 page):

  • MS KB5004442 – fixes Zerologon re-use.
  • ScreenConnect Upgrade: https://docs.connectwise.com/en-us/connectwise_control/Patch/ReleaseNotes/2023.9.9
  • Malwarebytes Nebula / SentinelOne repair kit (free for IR non-commercial)

4. Additional Critical Information

  • Unique characteristics
    – COSAKOS uses user-mode cryptographic primitives only (CryptGenRandom → AES-CBC 256; PEM private key bundled but encrypted). No kernel driver ⇒ simpler to remove but also simpler for AV heuristic detection.
    – After encryption it injects Readme_Restore.txt inside every encrypted folder containing ransom note with e-mail cosakosrestore@mailtor[.]net. The note formatting never changed, letting pattern-matching via YARA rule: /COSAKOS[._-]RESTORE@[^@]+.\w+/i.
  • Broader impact
    – Estimated 19 k victims globally since August 2020. Primary sectors: dentistry offices, SMB MSPs, and small legal firms (< 20 seats).
    – Notable multi-victim: A French Managed Service Provider infecting 1,200 downstream endpoints during the ScreenConnect campaign (Oct-2023).
    – IOC feed provided to abuse.ch’s URLhaus and MISP Cerberus community for continuous tracking.

Final Word

COSAKOS is largely defend-and-prepare ransomware: keep systems patched, privileged access restricted, and backup immutable. Use the free STOP decryptor if your files were encrypted before May-2022; otherwise, rely on versioning backups and post-incident hardening to prevent re-encryption.