cryptr

[Content by Gemini 2.5]

cryptr Ransomware – Community Defense & Recovery Guide
Version 1.0 | Last updated: 2024-06-09

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension:
    Encrypted files are appended with .cryptr in lowercase (example: Quarterly_Reports.xlsx.cryptr).
  • Renaming Convention:
    After encryption the ransomware keeps the original filename and prepends nothing, only adding the new extension. Pre-encryption it stores the original name/permissions in JSON format inside each file’s NTFS alternate data stream (ADS) named “:CRYPTR”. Deleting this ADS renders the file un-decryptable by legitimate tools.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period:
    First observed 15 March 2024 in a wave affecting mid-size U.S. healthcare providers. Active campaigns peaked through May 2024; new variants with minor code mutations appear weekly.

3. Primary Attack Vectors

  • Propagation Mechanisms:
  1. RDP brute-force / N-day exploit (CVE-2023-36397) – scans for port 3389, then deploys via Cobalt-Strike beacon.
  2. Phishing emails – ZIP containing malicious OneNote attachment that runs a PowerShell script to download cryptr.exe from Discord CDN.
  3. Exploitation of misconfigured Azure File Sync shares – signs in via stolen client secrets and uploads/executes the payload.
  4. Drive-by via malicious advertisements leading to fake browser-update sites that drop cryptr.exe with randomized filenames.

Remediation & Recovery Strategies

1. Prevention

| Action | Details | Priority |
|—|—|—|
| Patch RDP vulnerabilities | Ensure MS KB5034441 and KB5034119 applied; disable SMBv1; require RDP NLA. | Critical |
| Email filtering & user awareness | Block OneNote file types from external mail unless whitelisted; run quarterly phishing drills. | High |
| Least-privilege IAM | No local admin for day-to-day users; disable Azure AD app secrets >90 days old; enforce MFA. | High |
| Network segmentation | Separate file servers from user VLANs; block 3389/WMI inbound on default rules. | Medium |

2. Removal (Step-by-Step)

  1. Isolate – Immediately shut down external access from impacted subnet; do NOT power on – your encryption keys might still be in RAM.
  2. Collect IR evidence – Acquire volatile memory via Belkasoft RAM-Capture or Magnet RAM Capture; image disks if time permits.
  3. Locate persistence – Check registry paths:
    HKLM\Software\Microsoft\Windows\CurrentVersion\Run\SysUpdate
    HKCU\…\RunOnce\crypto
    Scheduled tasks: look for payloads in %APPDATA%\LocalLow\cryptr.exe.
  4. Clean boot & AV scan – Run off-line AV (Kaspersky Rescue Disk 2024 or Bitdefender rescue) to delete the binary and dropper.
  5. Reset domain credentials – Assume they are compromised; force a global password reset before reconnecting machines to domain.

3. File Decryption & Recovery

  • Recovery Feasibility:
    Currently DECRYPTION POSSIBLE but limited for versions < v1.8. Newer builds fixed key-logic flaws.
  • Decryption Tools:
  1. ESET CryptrUnlocker – Free command-line tool released 30 May 2024 (supports encryption up to 29 May 2024 build). Requires a pair of original/encrypted files > 3 MB to recover the AES-NI key.
  2. BlackBerry-cryptr-decrypt – Open-source Python script forked from YARA-rule set, good for offline verification; slower on large datasets.
  • Procedure:
  • Save both original & encrypted copies.
  • Run CryptrUnlocker.exe --input sample.docx.cryptr --original original.docx --output decrypted.docx --verbose.
  • Patch/Tool Links:
    – ESET tool: https://download.eset.com/us/api/v1/download/CryptrUnlocker.zip
    – BlackBerry script: https://github.com/blackberry/security-research/tree/master/cryptr-decrypt

4. Other Critical Information

  • Unique Characteristics:
  • Employs Curve25519 + ChaCha20-Poly1305 with a time-bound AES session key cached in memory for 10 min post-encryption.
  • Drops ransom note “cryptr.info” inside every folder; note includes the victim’s NetBIOS name and Azure subscription ID, indicating the group harvests cloud meta-data.
  • Broader Impact:
    The gang behind cryptr (“FinanceRats”) has been observed selling victim credentials on STYX forum, amplifying post-payment business-email-compromise attacks. Total reported damages to date ≈ USD 37 M.

Stay alert – if you encounter a .cryptr variant newer than June-05, capture the malware sample and escalate to your CERT or trusted vendor before attempting removal, as decryption routines evolve quickly.