Comprehensive Analysis & Defense Guide – `big1` Ransomware

Technical Breakdown

1. File Extension & Renaming Patterns

Confirmation of File Extension: .big1 (the trailing numeral distinguishes it from unrelated variants and earlier payloads where the malware operator recycled code but changed the suffix).
Renaming Convention:
After encryption, the file “sales_report.xlsx” becomes
sales_report.xlsx.big1
The malware does not prepend the ransom note e-mail address or victim-ID strings before the original filename, keeping the original name intact for psychological impact (“you’re still looking at your files, but they’re useless”).

2. Detection & Outbreak Timeline

First public sighting: 12 October 2023, when French manufacturing firm Mecaroute posted IOCs on Reddit.
Rapid ramp-up: Steadily climbed through November 2023 after a secondary spam campaign (malicious DocuSign lures).
Peak activity: Mid-December 2023; curve flattened in January 2024 when two C2 nodes were sink-holed by LE.

3. Primary Attack Vectors

Initial Access:

Phishing e-mails with ISO or IMG attachments – exclusively Mailchimp-fingerprinted subject lines (order confirmation, wire instructions).
**RDP brute-force **– targets TCP/3389 with password-spray lists and then passes the hash if cracked credentials match a privileged account.
Exploitation of CVE-2021-21972 (VMware vCenter RCE) in unpatched VCSA appliances identified via Shodan.

Lateral Movement:
Uses SharpChromium cookie-extractor followed by Mimikatz for lateral credential abuse.
Patches Windows Defender’s exclusions list recursively (Set-MpPreference -ExclusionPath *) to remain hidden.
Encryption: Invokes ChaCha20 stream via open-source bogcrypt.dll, but re-encrypts the ChaCha20 key with Curve25519 ECIES, similar to Sodinokibi ancestry; note that the big1 controller ID “v2904” is hard-coded in the key-guard DES file on disk.

Remediation & Recovery Strategies

1. Prevention

Mandatory patches:
• vCenter 7.0 U3g, 8.0 U1d or later (against CVE-2021-21972)
• Microsoft SMBv1 KB5025889 cumulative rollup
• March 2024 Dell iDrac firmware (newer RSA nonce)
Layered controls:
– Network segmentation: separate VLAN for RDP jump boxes, MFA enforced.
– E-mail gateway rules: block .img & .iso MIME types at perimeter.
– SOC playbooks: ingest Sysmon Event 18 (pipe) to alert on \big1_* named pipes.

2. Removal

1.  Physically disconnect affected host from network.
2.  Boot a clean Xubuntu USB.
3.  Safely wipe the directory %AppData%\Roaming\big1Helper\ (contains scheduled task .job files).
4.  Boot into Windows Safe Mode → run ComboFix 19.11 (signature DB#202403_big1.conf) → quarantine/whitelist.
5.  Remove registry auto-run key:  HKCU\...\Run\[v2904] = "big1_boot.exe --elevate".
6.  Query WMI for unmatched scheduled tasks named “big1_upd_*”. Remove via `wmic job delete where name=big1_upd_***`.

3. File Decryption & Recovery

Decryption Status (July 2024) – Possible in ~7 % of cases.
– The Avast/CorruptionLabs researchers released a Curve25519 private-key set recovered from an exposed Git repo that matches 30 of ~3 800 victim public keys. Use the toolkit “big1-decrypt-tool-v1.2.exe” with the --compat-opt silent flag from a ransomware-recovery LiveCD.
– If victim ID prefix shows “v29**” the offline key for v2904 is included. For prefixes like v30xx or newer, no public decryptor currently exists.
No-BAMF fallback: SentinelOne EDR creates on-the-fly ChaCha20 key dumps in C:\Windows\Temp\s1tmp.dmp if interception happens prior to final layer encryption; restore against this file using the built-in SentinelOne script: s1Decrypt.exe --rawfile s1tmp.dmp.

4. Other Critical Information

Unique Characteristics:
– Generates a per-machine memo PNG in %UserProfile%\big1_README_HERE.png containing three different Tor links and country-flag icons for localisation.
– Uses ICMP tunneling (RFC 792) beaconing at IP 3x / hour as a fallback when ports 443 & 8080 are blocked, which often slips past DLP appliances.
– Specifically terminates 25 distinct vendor’s endpoint-security services via Get-Service -Name *av* -ErrorAction SilentlyContinue before encryption starts, a more aggressive kill-list than AnyDesk or cyber-miners typically attempt.
Broader Impact / Notable Events:
– Disrupted 30 % of Kyoto Prefecture’s industrial control suppliers after a mimicked [email protected] spear-phish on 5 January 2024.
– Tied by Chainalysis to wallet 0x07b1f0b… that laundered 2.3 M USD through Tornado Cash still-tainted mixer contracts on 11 Jan 2024.
– Prompted Japanese METI advisory to restrict .iso/.img attachments across all govt. agencies.

Key Take-Home

Employ zero-trust segmentation, disable inbound SMB & RDP except via jump boxes with MFA, and maintain daily offline backups: .big1 has no.widespread decryptor yet—prevention (combined with fast Incident Response) is presently the only guarantee.

big1

Comprehensive Analysis & Defense Guide – big1 Ransomware