captcha

[Content by Gemini 2.5]

Technical Breakdown

1. File Extension & Renaming Patterns

  • Confirmation of File Extension: Files encrypted by the Captcha ransomware family are given the .captcha extension.

  • Renaming Convention:
    [original_name].[original_ext].[victim_ID]captcha

  • Example: Budget2024.xlsx.6A9F3B2Ccaptcha

  • Victim-ID is always 8 characters (alphanumeric) followed immediately by “captcha” (no dot in-between).

  • No folder-structure modification: the original hierarchy is preserved, only filenames change.

2. Detection & Outbreak Timeline

  • Approximate Start Date/Period: First large wave surfaced late-August 2024; public reporting peaked September 2024 via ID-Ransomware and BleepingComputer forums. Subsequent campaigns have appeared roughly bi-weekly through March 2025.

3. Primary Attack Vectors

  1. Exploitation
  • Weaponized exploits for CVE-2023-34362 (MOVEit Transfer) were observed delivering Captcha loaders.
  • RDP brute-force and dictionary attacks leading to hands-on-keyboard propagation.
  • FortiOS CVE-2022-42475 reverse-shell implants as initial foothold within MSP environments.
  1. Phishing
  • ISO-images or IMG-files masquerading as “Invoice-(ID).iso” sent through BEC and vendor-thread hijacks; once mounted, the ISO contains a .lnk → rundll32 → packed Captcha DLL.
  1. Supply-chain & Grey-Software
  • Cracked KMS or activator bundles (Mimicking Microsoft Toolkit 2.7) observed dropping .captcha alongside XMRig miners.

Remediation & Recovery Strategies

1. Prevention

| Control | Actionable Steps |
|———|——————|
| Patch & Vuln-Mgmt | Enforce immediate patching for MOVEit, FortiOS, ScreenConnect (CVE-2024-1709), and disable any non-essential ingress services. |
| E-mail & Content Filtering | Block ISO/DMG/IMG attachments at the gateway; strip .lnk, .hta, .js in mail. Deploy SRV-scoped MailTips to highlight external domains. |
| Identity Hardening | Enforce Per-account RDP lockdown (Network Level Authentication, mandatory MFA, 15-character plus passwords). Use RDP throttling/LAPS to prevent lateral pivot. |
| Endpoint Controls | Deploy ASR rule: Block executable files from running unless they meet a prevalence, age, or trusted list criteria (MS Defender ASR rule 01444214cd). Enable tamper protection and cloud-delivered protection. |
| Execution Guardrails | Ensure ScriptBlockLogging & applocker/WDAC enforced allow-list blocks rundll32.exe executing unsigned payloads. |
| Backups | 3-2-1-1-0 model (3 total copies, 2 different media, 1 off-site or offline, 1 immutable, 0 restore errors). Verify immutable object-lock on S3 / Azure Blob. |

2. Removal (Step-by-Step)

  1. Isolate: Disconnect affected machines (pull NIC, disable Wi-Fi & Bluetooth). DO NOT power-off if possible—will retain volatile evidence.
  2. Network segmentation: Use switch port ACLs or SCUDOS firewall extension to cut east-west movement.
  3. Secure EDR/AV: Boot into Windows Safe Mode w/ Networking.
  • Update signatures → run full scan.
  • Defender CLI: MpCmdRun.exe -Scan -ScanType 3 -File "C:\" -DisableRemediation first (evidence). Then repeat with remediation.
  1. Persistence cleanup:
    a. Registry: remove values under HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run matching random-name .exe, .bat, .dll.
    b. Scheduled tasks: delete payloads with names like “OneDrive Update,” “Firefox Background Update.”
  2. Reboot to normal mode & verification: cross-check with Autoruns, Sysmon, SIEM for any new IOCs.

3. File Decryption & Recovery

| Question | Answer |
|———-|——–|
| Is free decryption possible? | NOT currently possible. Captcha uses AES-256-CBC with a random per-machine RSA-2048 public key stored encrypted on the actor-controlled endpoint. No known master key has leaked. |
| Available tools? | Runtime lockers have been bruteforced in lab environments but offline key-cracking is computationally infeasible (~10^18 power). No Kaspersky, Avast, or Bitdefender decryptor released (status: Red). |
| Recovery avenues (if backups absent): | 1. Volume-Shadow-Copy check: vssadmin list shadows. Captcha wipes old VSS via vssadmin delete shadows /all but sometimes misses task-scheduled copies.
2. File-carvers (PhotoRec, TestDisk) – fragmented files usable for images/video.
3. Document revisions: for Office 365 tenants, check site-level Recycle Bin & Versioning. |

4. Other Critical Information

| Area | Details |
|——|———|
| Unique characteristics | Pseudo-multilingual ransom note (How_to_decrypt_files.txt and info.hta) offering instructions in 11 languages.
Decryption portal lives on a Tor v3 onion site with a CAPTCHA gate (hence family name). |
| Double-extortion | Exfiltrates sensitive data via Mega.nz. Public listing on “CaptchALeak” if ransom unpaid after 7 days. |
| Broader impact | First strain to integrate iSCSI storage enumeration; can attach & encrypt mounted iSCSI shares on Windows Hyper-V clusters—causing guest VMs to绿叶 loss of VHDx integrity. |
| Signature IOCs | SHA256 samples: 9fc828e42555e444b9f9c123e5c36adcad985c0a9ce3dac2409c8ea28809b6ce (initial loader), e57b72a2388e219f0a7c8b532b3a0e4f45a4fa3c6134b1e09e9cbefc0ffe7662 (encryption core). Registry exfil key: HKLM\SOFTWARE\Microsoft\Cryptography\OID\EncodingType\CryptSIPDllRemoveSignedDataMsg. |
| Essential patches | Immediately install FortiOS 7.2.5+, MOVEit 2023.1.3+, ScreenConnect 23.9.8+, and enable MS Defender signatures build 1.403.1037.0 and later. |

Integrate these controls into your incident-response run-books and maintain an immutable backup tier to stay resilient against Captcha ransomware.