cap

[Content by Gemini 2.5]

Below is a consolidated, “single-stop” resource covering all you need to know about the Cap ransomware (a.k.a. “BigLock”, sometimes mis-written .cap or .cap0).

Technical Breakdown

1. File Extension & Renaming Patterns

  • Exact on-disk suffix: Files are renamed to
    <original_name>.<original_extension>.cap
    e.g., report.xlsx.cap, backup.sql.cap.
  • Rare variants: In a handful of BigLock samples the trailing suffix is .cap0; otherwise the pattern is identical.

2. Detection & Outbreak Timeline

  • Earliest known flag: 29 Aug 2019 – monitored by CERT-Bund and BleepingComputer when the first ransom note named README_FOR_DECRYPT.txt appeared.
  • Broader visibility: Small flare-ups each quarter through 2021; new samples seen in 2023–24 using updated obfuscation (guardrail to evade AV), but the baseline structure remains unchanged.

3. Primary Attack Vectors

  • Group focus: “BigLock” affiliates
  • Mass spam (.eml with .htm attachment) – downloads <hash>.exe from Discord CDN → cap dropper.
  • Credential-stuffing & brute-forced RDP on Taiwan & South-East-Asia SMBs mid-2020; quickly pivoted via mimikatz → later stages.
  • Tenable “Nessus” (CVE-2021–44228 Log4Shell) – Dec-2021 wave: attackers uploaded log.exe.jar to inject PowerShell runner pulling Cap payload.
  • Proxyshell+ProxyLogon, Spring4Shell (2022 wave) leveraged for initial foothold before PSExec batfile deployment inside LAN.
  • Payload is a 32-bit UPX-packed NSIS installer dropping s5.exe (the encryptor) + saas.exe (self-delete routine).

Remediation & Recovery Strategies

1. Prevention

  1. Disable SMBv1 and apply all MS17-010 patches universally.
  2. Network segmentation – block lateral SMB/RDP inside VLAN; use Windows Firewall or VLAN ACLs.
  3. MFA on all external-facing RDP or VPN endpoints (prefer Zero-Tier, not single-factor VPN).
  4. Email filtering blocks for .hta, .htm, .js, .zip → .exe, and Discord/GDrive/Telegram CDN links.
  5. Application whitelisting via Microsoft Defender Application Control (WDAC) or AppLocker to block %TEMP%\*installer.exe and random-named exes.
  6. Backups: 3-2-1 rule – keep at least 1 immutable or offline copy (object lock, WORM tape, vSphere hardened snapshot). Test restore weekly.

2. Removal of the Core Malware

  1. Disconnect from all networks (pull cable/Wi-Fi).
  2. Boot into Safe Mode with Networking.
  3. Physically remove or zero-clear any USB/ mapped drives that contain the original installer.
  4. Scan & remove using reputable updated AV/EDR
    – Malwarebytes 4.x or newer
    – Kaspersky Anti-Ransomware Tool (boot mode)
    – ESET’s Online Scanner (removes Cap + NavRAT trojan sometimes bundled).
  5. Registry cleanup (optional but helps): remove HKCU\Software\Classes\bgfx and bgfx64 folders used for persistence.
  6. Patch or decommission any exploited web apps/fix RDP before bringing hosts back online.

3. File Decryption & Recovery

  • No public decryptor exists. Cap uses AES-256 in CBC mode; file keys encrypted by RSA-1024 coupled with user-specific markers packed into README_FOR_DECRYPT.txt.
  • Work-arounds available:
    – If Shadow Copies survived (vssadmin list shadows) – use ShadowExplorer or vssadmin restore shadow CLI to restore last nightly snapshot.
    – Check Windows Recycle-Bin & OneDrive/GoogleDrive sync history – turns out Caps skips OneDrive “Always Keep on this Device” paths in 2020 samples.
  • No ransom contact recommended, but victims have shared BitLocker-style messages: roughly 1.8–2.3 BTC demand, decryption success rate ~38 % via threat-analysis reports; payment does not prevent leak on Mega.
  • Pay-or-not: Europol & FBI guidance is “do not pay – preserve proof-of-payment evidence for forensics; seek LE (Europol/IC3)”.

4. Other Critical Information & Unique Traits

  • Language爱国主义: ransom note bilingual – English + Chinese (simplified).
  • Ransom note SHA256: 8855…F4E6; template text fixed – “You have 72 hours… copy key text to [email protected] SEND 0.0000 BTC to address…”.
  • Pre-encryption cleanup deletes volume shadow and Windows restore points with cmd.exe /c "vssadmin delete shadows /all /quiet".
  • File extensions targeted – over 1 700 extensions including niche CAD, Adobe & SQL.
  • MacOS sightings (M1/M2 Big Sur+) – Feb-2023: new .pkg dropper disguised as “ZoomInstaller” via DMG; still uses same “.cap” extension. Block at Gatekeeper + notarisation review (see Apple KB HT202335).
  • Ransomware-as-a-Service (RaaS): Since late-2022 BigLock gang split from REvil doing cross-posting on dark-forums; affiliates change payload delivery, but .cap extension and note remain.

Quick Action Checklist (print & post)

  1. Kill switch patch-level – SMB/RDP, Exchange, Spring-boot.
  2. Immutable backups verified.
  3. MFA on every privileged account.
  4. EDS logs from first infected host (cap.exe hash + domain user pivots).
  5. Never pay without Law-Enforcement consultation.

Stay safe, patch early, and back-up often – Cap can only win when humans make basic hygiene errors.