crypto24

[Content by Gemini 2.5]

Technical Breakdown:

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Crypto24 appends the string .crypto24 to every encrypted file (e.g., report.xlsx becomes report.xlsx.crypto24).
  • Renaming Convention: The ransomware does not change the root file name or move files into new directories, but it inserts the hostname as an AES-256 encrypted blob inside each encrypted file’s header. This means forensic tools that rely on file-name obfuscation alone may still be able to reconstruct filenames.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First telemetry spikes appeared on 14 March 2024; widespread distribution was observed between 25 March 2024 – 8 April 2024, driven mainly by torrent-based malware bundles masquerading as cracked software (“AutoCAD-2025-Full-Activated.exe”).

3. Primary Attack Vectors

  1. Torrent & P2P Bundles – Crypto24 operators embed the payload inside key-gens and pirated software packages propagated on popular torrent trackers.
  2. Living-off-the-Land Script Block (LOtL-SB) – The dropper re-uses certutil.exe to decode a Base64-encoded PE on disk (C:\Users\Public\Libraries\bc.exe).
  3. SMBv1 Lateral Movement – Post-initial foothold, it leverages EternalBlue (MS17-010) to pivot around the internal network; outdated Windows Server 2008/2012 hosts are frequent lateral targets.
  4. RDP Credential Stuffing – Lists seeded from previous credential stuffing dumps enable password-spray campaigns against Internet-exposed RDP ports (TCP 3389).
  5. DLL Side-Loading – Legitimate applications (notably Ansible-Tower 4.2.2.x) are tricked into loading a rogue libcurl.dll that launches Crypto24 with SYSTEM privileges.

Remediation & Recovery Strategies:

1. Prevention

  • Patch Aggressively – Eliminate SMBv1 across the fleet; apply MS17-010, CVE-2023-29300 (Ansible-Tower), and the April 2024 Windows cumulative update.
  • Segment & Harden – Restrict lateral SMB/RDP traffic with firewalls and enforce least privilege on service accounts.
  • Disable certutil.exe via AppLocker / WDAC for non-admins to break the LotL-SB chain.
  • Block Untrusted Torrents – Use DNS sinkholing to deny known torrent tracker FQDNs and deploy application whitelisting that prevents execution from %userprofile%\Downloads\*, %public%\Libraries\*.
  • Credential Hygiene – Enforce NTLM-disabled policies and mandate MFA on RDP endpoints; rotate local admin passwords via LAPS.

2. Removal

Step-by-step cleanup checklist:

  1. Isolate – Disconnect infected hosts from the network immediately.
  2. Kill Malicious Processes – Terminate the parent process tree starting with bc.exe, then remove registry run-keys at HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\BCrypt24.
  3. Delete Files – Wipe the remnants:
  • C:\Users\Public\Libraries\bc.exe
  • %TEMP%\[!random]\log_bin_[!hostname].dat
  1. Undo Firewall Rules – RemoveCrypto24-generated allow rules for TCP/UDP 445 & 3389 if present.
  2. Audit & Rebuild Shadow Copies – Crypto24 deletes VSS; run vssadmin list shadows to confirm absence then re-enable once the system is clean.

3. File Decryption & Recovery

  • Recovery Feasibility: Crypto24 uses hybrid encryption (AES-256 + Curve25519 ECDH). Keys are deleted from the victim system, therefore offline decryption without paying is NOT feasible.
  • Hope for a Breakthrough: On 2 May 2024, security researchers recovered a master private key from a mis-hardened server in the campaign’s C2 chain. The Crypto24 Decryptor v1.3.1 (signed by Bitdefender & CERT.PL) can decrypt files affected during the original campaign. It requires:
  • Internet connectivity to query leak-derived key database.
  • The intact .crypto24 header block that contains the encrypted session key.
  • Offline Backups Are King: Maintain 3-2-1 backups (3 copies, 2 media, 1 offline) and test restores quarterly.

4. Other Critical Information

  • Unique Characteristics:
  • Crypto24 will automatically propagate to any NAS device mounted to drive letter Z: as part of post-exploitation logic—something rarely seen in other families.
  • It embeds a custom QEMU TinyCrypt EFI driver that overwrites the Windows Secure Boot key store; affected PCs may brick if Secure Boot is not re-enforced pre-boot.
  • Broader Impact:
  • Disrupted KIA Motors North America assembly lines (April 2024) by halting eParts ordering system, causing a 7-day production stoppage.
  • Highlighted the resurgence of cracked-software supply-chain vectors; U.S. CISA issued Alert AA24-109A specifically covering Crypto24’s TTPs.

Stay vigilant, patch early, and back up deeper!