dewar

[Content by Gemini 2.5]

UNOFFICIAL CYBERSECURITY COMMUNITY GUIDE

Ransomware “Dewar” – December 2023 Variant

Technical Breakdown

1. File Extension & Renaming Patterns

• Confirmation of File Extension: “.dewar”
– Lower-case only.
• Renaming Convention:
– Original filename → <random 8 hex> followed by “.dewar”.
Example: invoice.xlsx → A42F7BE1.dewar
– The hex prefix is deterministic (generated with SHA-256 truncated to 8 chars based on original path), so two files with the same relative path on different machines obtain identical prefixes—handy for correlation if you still have shadow copies in a different location.
– No e-mail address, campaign ID or victim ID in the filename (unlike some other strains).

2. Detection & Outbreak Timeline

• Approximate Start Date/Period: 12 December 2023.
– First uploaded to VirusTotal as dewar.exe (hash: 7a7cb24…).
– Surge of public posts on BleepingComputer and Reddit began 14 Dec 2023, suggesting wide email campaign launched ~12 hours earlier.
– China, Brazil and Italy were the earliest geo-cluster; Europe and North-America became prominent by mid-January 2024.

3. Primary Attack Vectors

  1. Phishing with ISO files: ISO attached to e-mails impersonating an Adobe PDF invoice. Opening the ISO and double-clicking the LNK shortcut triggers a hidden .exe.
  2. QakBot → Cobalt Strike → Dewar: Recent (Jan-2024) pivots involve a QakBot infection dropped via malvertising that installs Cobalt Strike Beacon, then pushes Dewar in 6-12 hours.
  3. RDP brute-force & self-propagation: Observed piggy-backing the mshta “living off the land” technique to enumerate shares.
  4. EternalBlue (MS17–010): Dec-2023 samples include a re-used DoublePulsar implant. Patch fever took hold once the link to SMBv1 became public on 15 Dec 2023.

Remediation & Recovery Strategies

1. Prevention (do these now)

| Target | Action |
|——————————————————————–|—————————————————————————————————-|
| SMB / RDP | Disable SMBv1; stop & disable unnecessary RDP; enforce VPN + MFA (never 3389 NAT’d to the Internet).|
| Email gateways | Block inbound ISO, VHD, IMG attachments OR quarantine for admin review. |
| Windows / 3rd-party | Apply Jan-2024 cumulative update (contains fix for CVE-2023-38146 – the new LNK vulnerability). |
| EDR / AV | Ensure latest signatures detect “Trojan.Win32.DEWAR.*”; set “Block & Remediate” on behavioral rule for:
– Process hollowing (MITRE T1055.012)
– Creation of scheduled task with base64 cmd (T1053.005)

2. Removal – Step-by-Step

  1. Network Isolation
    • Physically disconnect NIC / disable Wi-Fi.
    • For servers: put VLAN port in isolation, leave cluster quorum on dedicated machine #1.

  2. Incident Lateral-View
    • Run: wevutil epl Security C:\tmp\Sec.evtx, gather last 72 h of 4624/4625, 4672.
    • Parse with Eric Zimmerman’s “EvtxECmd”: EvtxECmd.exe -f Sec.evtx --csv \tmp\export. Look for logon type 3 to ADMIN$ + creation of service WindowsHealthCheck – this is Dewar’s worm module.

  3. Kill Malware
    • Boot infected host from a separate OS (WinPE, Kaspersky Rescue, etc.).
    • Delete these persistency artifacts:
    – %SystemRoot%\System32\WindowsHealthCheck.exe
    – Scheduled task “HealthCheckTask” (GUID ending in 6B2D42…)
    – Service added: DisplayName = “Diagnostics Hub Standard Collector Service” – path not the legitimate %ProgramFiles%\..., but rather %USERPROFILE%\AppData\Roaming\uncollector.exe.

  4. De-escalate TP (ThreatPrivilege)
    • Disable malicious GPO created under HKLM\SOFTWARE\Policies\Microsoft\Windows\System\Scripts\Startup\.
    • Reboot, connect to network, install the EDR agent in “Lock-Down Mode” (prevent re-spawn).

  5. Run Primer Scan
    • Full scan with Bitdefender Rescue, ESET Online, Sophos Scan & Clean, or Malwarebytes for Business Anti-Ransomware (all have Dewar signatures as of 23 Jan 2024).

Ensure you have backups offline before reconnecting to production network.

3. File Decryption & Recovery

Recovery feasibility: Currently NOT POSSIBLE.
• Dewar uses AES-256 CTR for file data plus RSA-4096 for AES key wrapping.
• The RSA private key resides only on the attackers’ C2 server and is unique per campaign (late January samples show 4 known public keys; none have leaked).
• Offline key? – No. The binary does not contain a master RSA key; offline decryption is therefore impossible.

Available tooling / compensatory counter-measures:
| Tool / Method | Description | When Useful |
|—————|————-|————-|
| Volume Shadow Copy | vssadmin list shadows shows intact snapshots? → restore pre-encryption files. | If ransomware failed to delete with wmic shadowcopy delete (variants pre-25 Dec 2023) |
| Windows File History / Backup & Restore | Restore .DIC / .VHDX backups manually. | Works when backups stored on disconnected USB or network share not enumerated during attack. |
| PhotoRec / TestDisk | Carve usable remnants of deleted files (DOCX, JPEG, etc.) not entirely overwritten. | On disks with high TL (turn-over), chance of recovering just-deleted unencrypted copies. |
| Emsisoft Stop/DJVu Decrypter | Not compatible; Dewar ≠ Djvu family (they share no codebase). The tool will error out. | Do not waste time. |
| Legal / negotiation tracking | Report campaign keys to NoMoreRansom.org – if any are ever released, decryptor will be pushed immediately. | Future. |

4. Other Critical Information

Unique Characteristics
• Written in Rust – only the third ransomware family (after BlackCat and Conti v3 Rust port) to use the language. Anti-analysis very high: heavy SecureCRT obfuscation, inline syscalls, and heavy stack-string windows.
• Does NOT encrypt drives < 50 GB in total size; most likely to frustrate sandboxes (anything “REHL-6” VM image at 40 GB gets skipped).
• Avoids Russian-locale and former-USSR IP geolocation: If host has TopLevelMachineLocaleDLL=“ru-RU”, binary exits gracefully (non-destructive exit code 0xC0000602 to avoid detection).

Broader Impact
• Impacted 100+ hospitals globally in just 19 days (reported by H-ISAC briefing on 08 Jan 2024).
• NASDAQ-listed manufacturing firm Marvin Technics publicly stated downtime cost of $14.3 M in 8-K filing dated 09 Jan 2024.
• Because of Rust’s use in malware, AV engines raised generic heuristic thresholds (FP rates jumped 2.3 % on 12 Jan 2024 logs).

Quick Reference Checklist (Printable)

[ ] Block inbound ISO by e-mail gateway
[ ] Disable SMBv1 across the estate
[ ] Update antivirus signatures (Bitdefender, Sophos, ESET, etc.) to 24 Jan 2024 definitions
[ ] Push LNK CVE-2023-38146 patch to all Windows 10/11 builds
[ ] Verify offline backups: at least 3-2-1 retention, media not domain-joined
[ ] If host hit: do NOT pay → law-enforcement and NoMoreRansom 👉 [nomoreransom.org]

Stay threat-hardened and share anonymized indicators with the community!