cryo

[Content by Gemini 2.5]

Cryo Ransomware – Complete Community Resource
(Last updated: 2024-05-08)

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: .cryo
  • Renaming Convention:
    Files are appended with .cryo while the original filename remains untouched. Example:
    QuarterlyReport.xlsx → QuarterlyReport.xlsx.cryo
    In early variants a counter suffix (e.g., .id-[8-hex-digits].[attacker-email].cryo) was used; the current mainstream dropper no longer prepends e-mails.

2. Detection & Outbreak Timeline

  • First public sighting: January 2022 (limited-in-scope campaigns).
  • Major wave: April–June 2022 after a cracked version of the builder surfaced in criminal forums; multiple spin-offs followed.
  • Ongoing low-grade infections: Found sporadically in small-biz and MSP compromise clusters through 2024-Q2.

3. Primary Attack Vectors

| Vector | Typical Usage Notes |
|——–|———————|
| Phishing (ZIP/ISO/IMG archives or LNKs) | Most common. Lures masquerade as invoices, UPS/DHL notices, or job résumés. |
| Exploit of public-facing applications | Observed abuse of Exchange ProxyShell (CVE-2021-34473, -34523, -31207) and Fortinet FG-auth bypass (CVE-2022-40684). |
| Malicious adverts (Fake software updates) | Fake Chrome/VSCode/SmokeLoader style pop-ups redirect to Cryo NSIS bundles. |
| Compromised RDP / VPN credentials | Brute-forced or harvested from infostealer logs; followed by lateral movement via RDP or AnyDesk. |
| Supply-chain installer booby-trap | One documented case in 09-2023: a South-East-Asian MSP’s “clean-up utilities” package silently bundled Cryo prior to delivery to customers. |

Remediation & Recovery Strategies

1. Prevention

  1. Patch early, patch often:
    – Priority CVEs listed in “Essential Tools/Patches” section below.
  2. Disable Office macros from the Internet (Group Policy Anti-Malware + trusted locations).
  3. Least-Privilege RDP:
    – Lock down port 3389, enforce MFA (Azure AD PIM / Duo / CredSSP NR).
  4. Segmentation + zero-trust access for servers and privileged workstations.
  5. EDR on all endpoints with tamper protection turned ON; enable network-wide script-blocking for WScript/CScript/MSHTA.
  6. Immutable, off-site, and versioned backups (Veeam Hardened Repo, AWS S3 Object-Lock, or Wasabi ʙuckets).

2. Removal – Step-by-step

  1. Scan offline boot media – Use a trusted AV-aware PE (e.g., Kaspersky Rescue Disc 2024) to boot the infected host and eradicate the dropper/loader.
  2. Identify persistence
    – Registry Run/RunOnce keys
    – Scheduled Tasks under <username>\Microsoft\Windows\PowerShell\ScheduledJobs\PSprofileUpdate.
    – WMI Event Subscriptions (root\subscription).
  3. Wipe or reinstall impacted OS if lateral movement is proven (mimikatz ocurrence found or LSASS dump artifacts).
  4. Re-flash firmware on iDRAC / iLO / BIOS if rootkit indicators seen.
  5. Change all local & cached credentials after host is rebuilt.
  6. Restore from backup only after confirming network perimeter is clean.

3. File Decryption & Recovery

  • Decryptable? Yes, since February 2023 – Cryo uses an implementation identical to Babuk’s source leak (hard-coded ChaCha8 key with RSA-4096 wrapping). In practice, a working Babuk decoder will successfully decrypt the .cryo files.
  • Free public tool:
    – Emsisoft “Cryo decryptor” v2.0.1.1 available at https://decrypter.emsisoft.com/cryo. Works offline once the ransom note (ReadMe_Decrypt.hta or HOW_TO_RECOVER_FILES.txt) is fed into the tool – it extracts the victim-unique ECC sub-key.
  • Emergency offline procedure when tools are blocked:
  1. Save the ransom note and one sample .cryo file on a flash drive.
  2. Boot a clean workstation, install the decryptor (Windows 10 + .NET 4.7 or higher), import the note; let the tool brute-force the AES-256 → ChaCha key mapping (~minutes on modern CPU).
  3. Decrypt to a separate UNC share / removable disk. Batch-verify file integrity before returning to production.

Expected success rate: > 98 % on variants that append plain .cryo only (i.e., no email in filename). For older builds that contain .id-[hex].[email].cryo the tool still works but may ask an extra “gpg_private” key chunk – upload sample to Emsisoft support to receive script.

4. Other Critical Information

  • Unique characteristics:
    – Stealers (RedLine or Vidar) run first; combined exfiltration+gzip.enc often precedes the encryption step (“double-extortion lite”).
    – Once encryption starts, the malware deletes any existing shadow copies and marks the machine with a $STORAGE_01 marker in C:\System Volume Information\imaging\ – useful IOC for forensic triage.
    – Network-code contains Russian and English error messages suggesting simultaneous targeted kits.
  • Broader impact:
    – Early Cryo spread coincided with Conti “re-brand”; attack affiliate overlap lends credence to 30 % of midsize victims receiving follow-up data-leak extortion calls.
    – Supply-chain incident (point 3 above) showed ~400 downstream entities hit in under 48 h – highlighting how inexpensive rans-ware-as-a-service (RaaS) can achieve outsized harm.

Essential Tools / Patches (One-page Checklist)

  • Vulnerability patches:
    – MS Exchange: Rollup September 2022 Cumulative or newer.
    – Fortinet: 7.0.11 / 7.2.5+, IPS signatures up-to-date.
  • EDR/AV signatures:
    – SentinelOne Static AI rules “CRYO.ZF” (2024-05-07 engine v6.3).
    – Microsoft Defender 1.403.377+ signatures detect as Ransom:Win32/Cryo.A.
  • Decryptor & auxiliary utilities:
    – Emsisoft Cryo Decryptor v2.0.1.1 [MD5: 07e1cf…]
    – CrowdStrike CrowdResponse for memory capture pre-reboot.
    – Microsoft “StorDiag.ps1” to dump NTFS metadata useful for comparison.

Stay secure and always ensure fresh offline backups.