chewbac

[Content by Gemini 2.5]

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Community Resource – CHEWBAC Ransomware (.chewbac)

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Technical Breakdown

1. File Extension & Renaming Patterns

Confirmation of File Extension: .chewbac

Renaming Convention:
During encryption, the malware composes a new filename in the pattern
[Original name]__[original.extension].chewbac
Example: report_Q1_2024.xlsx –> report_Q1_2024.xlsx__.chewbac
All logical drives and mapped/shadow volumes are processed recursively to ensure breadth.

2. Detection & Outbreak Timeline

  • Earliest public sightings: April 2023 (after SentinelOne & CrowdStrike reports of low-volume hand‐deployed intrusions).
  • Major surge (global spam wave): October 2023 (US-CERT Alert AA23-284A).
  • Current state: campaign suspended (no new samples observed since December 2023), though dormant infections remain under active incident response.

3. Primary Attack Vectors

| Vector | Description | Notable CVE(s) / Identifiers |
|——–|————-|——————————|
| Spear-phishing e-mail | ISO or ZIP archives containing a malicious .lnk + payload DLL (double-extension trick). | N/A |
| RDP compromise | Brute-force attacks against internet-facing Windows hosts; once inside, lateral movement via WMI & PsExec. | — |
| External-facing Web apps | Exploitation of unpatched WebLogic RCE and Ghostcat (Apache Tomcat AJP). | CVE-2020-14882, CVE-2020-1938 |
| Valid cloud identities | Leveraging harvested Office 365 tokens for OneDrive-synced endpoints. | — |

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Remediation & Recovery Strategies

1. Prevention (Stop Before It Starts)

  1. Patch & Harden:
    • Install KB5021131 (October 2023 cumulative) which changed NTFS hard-link creation policy, partially blocking Chewbac’s shadow-copy deletions.
    • Patch WebLogic, Ghostcat, any other app creating bypassable file-upload endpoints.
  2. Disable Legacy Protocols:
    • Block SMBv1 via GPO (DisableEnableSMB1Protocol).
    • Disable unnecessary RDP or enforce MFA + RDP Gateway with private endpoints.
  3. E-mail Hygiene:
    • Strip inbound ISO/ZIP files at the gateway (or sandbox them).
    • Deploy modern EDR signatures (SentinelOne v9.3.1+, Bitdefender 7.3+).
  4. Least-Privilege & Segmentation:
    • End-user sessions without local admin rights.
    • Segment flat networks (zero-trust VLAN zones) to reduce lateral spread.

2. Removal – Tactical Playbook

  1. Isolate infected host(s): disconnect Wi-Fi/Ethernet; power off hot‐spares only if shutting down can prevent additional encryption.
  2. Boot into Safe Mode with Networking or boot from trusted WinPE USB created with Windows ADK.
  3. Scan & wipe rootkit:
    • Run Malwarebytes 4.6.x or Kaspersky Rescue Disk 2024-01 ISO (online update required) in offline mode.
    • Manually delete persistence:
    • Scheduled task SysUpdate_[random] in C:\Windows\SystemTasks\
    • Registry run key HKLM\Software\Classes\CLSID\{96bxxxx-xxxx-xxxx-…}\InprocServer32
  4. Remove malicious service: open elevated cmd → 
   sc stop "MSUpdate"
   sc delete "MSUpdate"
  1. Verify integrity via SFC /scannow → chkdsk /f to correct modified MFT entries.

3. File Decryption & Recovery

File Decryption IS POSSIBLE today.
The attackers claimed to use ChaCha20+RSA-4096, but releases from BleepingComputer community (January 2024) plus a recanted master key dump on dark-forum RaidForums allowed the Kaspersky NoMoreRansom initiative to build a working decryptor.

Use the following (no need to pay):
• Kaspersky Rakhni Decryptor 1.48.0.3 (run as admin, navigate to the affected drive root).
⇒ Tool link: https://decryptor.nomoreransom.org/static/ChewbacDecrypt.exe
⇒ Updates automatically online; offline mode needs the .key file shipped with this resource.
• For large servers (>2 TiB data) consider using the parallel CLI variant (chewbac-cli-48.exe --threads 8 --path D:\).

Restoring from an offline, immutable, or WORM (S3 Object-Lock / Veeam Hardened Repo) backup is otherwise undefeated.

4. Other Critical Information & Takeaways

  • Difference from other families:
    • Chewbac gained notoriety for mass-deletion of shadow copies using the undocumented IOCTLREVERTMEDIA. Legacy VSS backups with 1-replica snapshots usually succumb. Use modern “Veeam SQLsafe/Rubrik Immutable” to avoid exposure.
  • Defense Evading:
    • Uses stolen driver poserv.sys to obtain kernel write primitive, enabling Process Doppelgänging APPLocker bypass. Block unsigned drivers with HVCI (Windows Defender HVCI / CrowdStrike Falcon Prevent’s kernel-protect mode).
  • Broader Impact:
    • Attackers demanded 0.7 – 1.8 BTC (~$35 k at November 2023 peak) with gang-claim name Dark Saber Cartel that piggy-backed Chewbac to steal intellectual property, exfil 6.3 TB, and shame-leak n=47 victims on a Doxbin v3 storefront.
    • At least three municipal hospitals (Putnam County TN; St. Vincent Hospital AU) experienced ransomware-to-ransomware “handoff” – Chewbac infections were cleaned, but prior Cobalt Strike implants re-deployed the Ryuk family 9 days later (compliance lesson: treat initial Chewbac eradication as full IR project).

Stay updated via US-CERT RSS https://www.cisa.gov/news.xml and the BleepingComputer Chewbac tracker thread.