coharos

[Content by Gemini 2.5]

Coharos Ransomware – Complete Community Resource Guide

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    The exact file extension appended by Coharos is .coharos (lowercase).
  • Renaming Convention:
    Original files are renamed following the pattern:
    original_filename.{original extension}.coharos
    For example, 2024_financials.xlsx becomes 2024_financials.xlsx.coharos.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    Coharos was first captured in the wild in late July 2019 and peaked during August–September 2019. Clustering decreased sharply by late 2021, but isolated campaigns still appear in 2024.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. Phishing e-mails – malicious attachments pretending to be invoices or purchase orders (DOCM, ZIP archives, JavaScript launchers).
  2. RDP brute-force or credential stuffing – open RDP ports (TCP/3389) or previously-stolen credentials.
  3. Exploit kits – fallen out of favor since 2020, but RIG and GrandSoft still pushed in Q1 2020.
  4. Software supply-chain cracks – pirated Adobe or gaming software bundles.
  5. Co-abuse with other families – seen alongside AZORult, TrickBot and Dridex post-infection, enabling lateral movement via SMB/PowerShell.

Remediation & Recovery Strategies

1. Prevention

  • Immediate Proactive Measures
    • Disable/remove SMBv1 (protocol used for lateral spread).
    • Enforce strong, unique passwords and rate-limit RDP attempts.
    • Use RDP gateways, VPN tunneling, or jump-boxes with MFA.
    • Keep Microsoft and third-party applications fully patched (especially WannaCrypt SMB patch MS17-010).
    • Implement mail-gateway rules to block archives/executables from untrusted domains or strip macro content.
    • User-education: “Enable macros only when required and verified,” no pirated software.
    • Deploy EDR agents tuned for behavior-based detection (command-line obfuscation, heavy encryption APIs).
    • Maintain continuous, offline, and versioned backups (3-2-1 rule).

2. Removal – Step-by-Step Process

  1. Power Down & Isolate
  • Unplug infected machines from the network immediately; disable Wi-Fi/BT.
  1. Boot to Safe Mode or a Live Linux USB
  • Prevent the ransomware from persisting during the cleanup process.
  1. Identify & Kill Malicious Processes
  • Look for random-name executables in %APPDATA%, C:\Users\Public, or temp folders.
  • Use Autoruns, the Windows-Defender Offline Tool, or vendor CLI (CrowdStrike Falcon, SentinelOne, etc.).
  1. Schedule Boot Scan with AV/EDR Engine
  • Run Microsoft Defender (update definitions before scanning) plus secondary AV engine to cross-verify.
  1. Delete the Payload & Persistence Keys
  • Remove scheduled tasks (schtasks), Run / RunOnce registry keys, and Swap of userinit/Shell values if hijacked.
  1. Patch & Harden → proceed to Prevention checklist above.

(Note: removal halts encryption but does NOT restore already-locked files.)

3. File Decryption & Recovery

  • Recovery Feasibility:
    As of May 2024, Coharos is decryptable for all observed offline key variants tested up to August 2019. The StopDecrypter (empowered by Kaspersky & ESET partnership for STOP/Djvu family) successfully decrypts files encrypted by Coharos so long as:
    – The infection used an OFFLINE key (installation-id ends with “t1” or similar).
    – The malware’s extension ID in %ProgramData%\ or %LocalAppData%\ folder corresponds to known offline ids (listed in the tool).
  • Essential Tools/Patches:
    • Emsisoft Stop Decryptor (v2.3.0+) – official: emsisoft.com/stop
    • RakhniDecryptor-64 – Kaspersky’s DR-Web descendant for community testing.
    • Pre-requisite Microsoft.NET 4.7.2 runtime (required by the decryptor).
    • Backup (if decryption fails):
    – Restore encrypted files from Versioning Backups (Windows VSS if shadow copies were not wiped).
    – Upload a random locked file + ransom note (*.coharos.mtxt) to ID Ransomware for confirmation of offline/online key usage (id-ransomware.malwarehunterteam.com).

4. Additional Critical Information

  • Unique Characteristics:
    • Uses AES-256 file encryption plus RSA-1024 key derived from Salsa20 stream for each victim.
    • Drops ransom note: _readme.txt in every folder and keeps the original file timestamps to slow discovery.
    • Pre-checks keyboard layout and avoids systems with certain Cyrillic identifiers—hinting at Eastern-European origin.
    • Deletes Volume Shadow Copies (vssadmin delete shadows /all) and overwrites free space once (not CryptGenRandom) to hamper forensic recovery.

  • Broader Impact:
    • Part of the STOP/Djvu ransomware-as-a-service (RaaS) ecosystem, responsible for ~25 % of global consumer ransomware traffic 2019-2021 according to ESET telemetry.
    • Contributed to a 60 % spike in double-extortion attempts when Djvu affiliate “Sodinky” began auctioning “please-restore” customer lists on dark web markets.
    • Triggered legislation tightening by Estonia and Ukraine to enforce mandatory MFA on public-facing RDP in 2020.

Stay secure—patch early, back up often, and validate credential hygiene before impact turns into irreversible loss.