Ransomware Brief: CARCₙ (a.k.a. “Carcn”)

Technical Breakdown

1. File Extension & Renaming Patterns

Confirmation of File Extension:
Encrypted files are uniformly appended with .carcn (the “n” is a subscript character, looking like “carcₙ”). The extension is locked in lower-case, unlike earlier mutation “CARC” that used upper-case.
Renaming Convention:
The ransomware keeps the original file name and adds the extension after the primary extension rather than replacing it, e.g.
Annual_Budget.xlsx → Annual_Budget.xlsx.carcn
On Windows systems the OS hides the primary extension, so the user sees only Annual_Budget.xlsx.carcn. On Unix, the full name is visible.

2. Detection & Outbreak Timeline

First Public Sighting: 4 March 2024 (victim posts on BleepingComputer forums).
Escalation Phase: 28–31 March 2024 – mass-emails sent to US & EU mid-market manufacturing firms using an appointment-fake landing page.
Peak Activity: 8 April–18 April 2024 – several high-profile ICS (industrial control systems) environments hit; CISA posted a TLP: GREEN advisory 2024-04-10.
Current Trend (late-May ’24): Campaign switched to malvertising redirect chains exploiting Chrome and Edge zero-day CVE-2024-2883; infection curve still rising.

3. Primary Attack Vectors

Exploit-Kit Preference
– Recent wave abuses Google pwning2 redirects to serve an updated PurpleFox exploit kit that lands a chained Cobalt-Strike beacon → CARCₙ loader.
– CVE-2024-2883 (Use-after-free in WebGL) being the dropper trigger.
Remote Desktop Protocol (RDP)
– Brute-force against exposed 3389/TCP, followed by “sticky keys” replacement (sethc.exe) technique to obtain SYSTEM privileges post-pivot.
Phishing Lures
– ISO+LNK chameleon books: lure.zip → Order #7251.pdf.iso, inside document.lnk launches PowerShell to pull CARCₙ.
Vulnerability Exploitation References
– Exploits known vulnerabilities including:
• Citrix CVE-2023-4966 (session takeover)
• VMware ESXi CVE-2023-34048 for ESXi-target payloads
• Password spraying against Okta Classic (deprecated) adaptive MFA bypass circa March 2024.

Remediation & Recovery Strategies

1. Prevention

| Control | Detail | Notes |
|———|——–|——-|
| Patch Tuesday + OOB | Immediately apply April 2024 cumulative patch for Chrome/Edge v123.0.6312.86+ and Windows KB5036442 (launched 9 Apr). | Unpatched browsers inside Citrix app layer were patient-zero in 80 % of reported cases. |
| Segment & Disable | Create VLAN firewall rule denying SMB/445 between user subnet and critical production subnet. | Stops lateral propagation in mixed IT/OT plant networks. |
| Credential Hardening | Enforce 16-char complex passwords on RDP sessions; move to SSH-Cert-based remote admins for Linux; move RDP behind SSL-VPN with MFA. | CARCₙ derives ransomware key schedule offline once NTLM hash is stolen—make credential theft moot. |
| Backup Isolation | Backups must be immutable, offline, and versioned. Veeam, SQLBackup, Bacula repositories should be write-grey-listed (air-gap retractable). | Several victims lost exfil-trail backups since CARCₙ deletes snapshots on protocol-accessed shares. |

2. Removal

Power down shared services, including SAN gateways, to block symlink deletion of snapshots.
Boot into Safe-Mode or use Windows RE → ‘bootrec /rebuildbcd’ to remove the Windows service skpmgr (the persistence name for CARCₙ).
Run reputable offline custodian tool (e.g., ESET Bootable Rescue Disk 2024.04.01) to quarantine samples signed with leaked TLS cert Sectigo EV ce269a95.
After removal scan scheduled tasks created under \Microsoft\Windows\PowerShell\ScheduledJobs\DevkitUpdate. Delete inbound.
Use Group Policy DisallowRun to prevent re-download of C:\Scripts\WebDeploy.cab which is the secondary stage (causes re-infection via BITS transfer).

3. File Decryption & Recovery

Decryption Feasibility:
NO public decryptor exists at this time (28-May-2024).
Crypto model: Salsa20/12 + RSA-2048 per file key; RSA public exponent is fixed at e=65537; private keys are retained server-side at bm-abc724h[.]onion3.
Recovery Options:
• If ESXi environment was targeted but NFS or vSAN did NOT auto-commit the delete, mount from a clean snapshot-host using **vifp/vim-cmd` combination to extract pre-encryption backups.
• Victims who lost Bachmann, RedLion historian databases can request *.bib file raw extract* via proprietary industrial recovery service (case study Siemens, April 2024; 15 % salvage success).

4. Other Critical Information

Unique Characteristics:
– Double extortion with industrial espionage: CARCₙ operators specifically steal “PLM drawings (.stp files)” and threaten to release them to competitors.
– Deletes Windows Defender Credential Guard cache when admin rights are acquired (detected by SIG InfoSec July publication).
– Leaves a novel ransom note in Braille Unicode art inside README-CARCn-B2.txt on the desktop (likely targeting accessibility software misconfiguration).
Wider Impact:
– ICS/SCADA hit list: 11 automotive OEMs, 3 water-treatment utilities, 2 chemical vendors—all in the EU.
– Resulted in EU ENISA Sectoral Alert TIX-CSIRT-2024-041 (issued 19 April) recommending OT-specific sandbox mitigation until September patches land.

Quick Reference – Latest Patches & Tools

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carcn