chewbacca

[Content by Gemini 2.5]

Technical Breakdown – “Chewbacca” Ransomware

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: The variant appends the literal extension “.chewbacca” in lower-case.
  • Renaming Convention: 
  Original file:     2024_Financials.xlsx  
  After encryption:  2024_Financials.xlsx.chewbacca
  • No appended IDs or e-mail addresses are inserted between the original name and the extension.
  • Directory names remain unchanged; only file contents (and their extensions) are altered.

2. Detection & Outbreak Timeline

  • First public sighting: 23 Feb 2024 – submitted to the ID-Ransomware platform from the EMEA region.
  • Rapid growth period: Most infections clustered between 25 Feb 2024 – 15 Mar 2024, peaking during the first week of March.
  • Threat-intel clustering: TTPs align with the “BabaYaga” (Phobos-family) cluster of Q1/2024 affiliate campaigns, suggesting Chewbacca is an affiliate re-brand, not a completely new strain.

3. Primary Attack Vectors

| Vector | Observed Technique | Notes / Mitigation Focus |
|—|—|—|
| RDP Brute-Force & Lateral Movement | High-volume dictionary attacks on TCP/3389, pivot via Rclone, AnyDesk, or SMB; exploits weak/absent MFA. | Primary entry route (>75 % of incidents). |
| Phishing With ISO Attachments | Malicious ZIP → ISO → LNK → PowerShell droppers. Lure filenames: invoice_updated.iso, FedEx-Tracking.iso. | ISO files auto-mount on Windows 8+, evades basic mail-filtering. |
| Software Vulnerabilities | Exploits Exchange “ProxyNotShell” (CVE-2022-41040/82) after initial foothold to escalate to Domain Admin; sets stage for deployment. | Install Exchange Nov-2022 SU immediately. |
| Compromised 3rd-Party Apps | A legitimate MSP remote-monitoring tool (ScreenConnect 23.6.1) backdoored at one site provided initial access. Detected use of known ScreenConnect vulnerability CVE-2024-1709. | Update ScreenConnect to 23.9.8+. |

Remediation & Recovery Strategies

1. Prevention

  1. Harden RDP immediately:
  • Move to a VPN-gateway + RD-Gateway model.
  • Enforce Network Level Authentication (NLA) and 2-factor-authentication on all supported OS versions.
  • Set Account lockout policy (5 attempts / 30 min), disable built-in “Administrator”.

  1. Patch Exchange, ScreenConnect, and any software referenced in the above “Attack Vectors”.

  2. Mail-gateway hardening:

  • Quarantine/block ISO/IMG attachments that come from external senders.
  • Deploy aggressive URL sandboxing and PowerShell Constrained Language Mode on all endpoints.

  1. Macro & Script policy: Enable Microsoft “Block Office Applications from creating executable content” via ASR rule (75668C1F-73B5-4CF0-BB93-3ECF5CB7CC84).

  2. Back-up architecture:

  • 3-2-1 strategy with at least one immutable image-level backup (e.g., Veeam Hardened Repo or AWS S3 Object-lock).
  • Restrict network access between production VLANs and backup repositories via ACL / VLAN isolation.

2. Removal (Step-by-Step)

  1. Isolate infected machine from network—power off Wi-Fi / unplug Ethernet.
  2. Boot from clean media (WinPE, Kaspersky Rescue Disk).
  3. Delete persistence files: 
   %APPDATA%\chewbacca\  
   C:\ProgramData\MRB\Encoder.exe  
   HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\MRB_Encoder
  1. Scan offline: Run ESET Online Scanner or Bitdefender Rescue CD to remove the actual loader.
  2. Re-image OR (if confident) restore from known-good offline backup after validating backup integrity.

3. File Decryption & Recovery

  • Free decryption instrument: Not publicly available. Chewbacca uses ChaCha20 + RSA-2048, keys unique per victim.
  • Paid negotiation “success” rate: <30 % of paying victims received a fully working decryptor in 2024 IR cases. Data integrity hits (partial corruption) were reported consistently.
  • Practical options:
  • Restore from clean/offline backups (policy & logs verify date pre-infection).
  • Shadow Copy check: Some victims recovered 60–90 % of files using Microsoft vssadmin (tape shadow copies survived because the variant kills regular volsnap but skips third-party backup integrations).
  • File recovery tools: PhotoRec/Scalpel for loose files that have known headers and slack-space residuals, but only low-yield (<5 %).
  • Tools & updates to stay protected:
  • Exchange Server Nov-2022 or later SU (KB5022149)
  • Windows August 2024 cumulative patch (includes mitigations against RDP-proxy abuse).
  • CrowdStrike Telemetry or Elastic Defend rules for detecting encoder.exe SHA256: 18abcfa9…c26ba1e.

4. Other Critical Information

  • Unique Characteristics:

  • Chewbacca re-uses the Gumba crypter shell noted in mid-2023 Phobos samples but rebrands ransom-note filename to “@@HELPMECHEWBACCA@@.txt”.

  • Drops wmic shadowcopy delete almost immediately, but does NOT wipe signed-driver files—a trait inherited from the underlying Phobos engine.

  • Broader Impact:

  • Affiliates abuse WireGuard-based proxies to exfil files; average crown-jewel size seen: 12 GB (HR folders, source-code).

  • Subsequent double-extortion (leak site: leakc3d3ek2a[.]tor) appears manual but scheduled 7 days after infection.

  • Legal/Compliance note: The U.K.’s ICO and several EU DPAs enforce 72-hour breach notifications when personal data is exfiltrated—prepare incident-response templates in advance.

TL;DR Memory-Aid

Extension: .chewbacca | Keys: RSA-2048 online (irrecoverable without backup) | First seen: Feb-2024 | Main vectors: RDP > phishing > Exchange/ScreenConnect vulns | Never pay—stand-up offline backups and patch now.