⚠️ Overview

CatB is a ransomware strain first documented in December 2021 by the NCC Group and subsequently analyzed by Trend Micro. It is a Golang-based ransomware that targets Microsoft Windows systems, employing a unique approach: instead of encrypting files, it whole‑disk encrypts the master boot record (MBR) and appends the .CatB extension to all accessible files after overwriting their first few bytes, effectively rendering the system unbootable and data inaccessible. The malware is believed to be operated by an unidentified cyber‑criminal group, with no known state‑affiliation; its primary distribution vector is via unsecured Remote Desktop Protocol (RDP) connections.

🔧 Technical Capabilities

CatB propagates by scanning the local network for additional RDP‑exposed hosts using a built‑in list of weak credentials, and after successful compromise it uses PsExec to deploy the payload remotely. It runs as a high‑integrity service under the name MsService and persists by creating a scheduled task named MicrosoftUpdate that executes the ransomware binary at logon. The malware employs a custom encryption scheme that overwrites the first 1024 bytes of each file with a static XOR key, leaving the remainder intact but useless without the MBR. The command‑and‑control (C2) infrastructure uses plain HTTP to exfiltrate a hardcoded victim ID and system information; no additional evasion techniques beyond basic anti‑VM checks have been publicly documented. It targets roughly 70 file extensions related to databases, documents, images, and backups.

📜 History & Notable Incidents

CatB first appeared in December 2021 with a limited campaign primarily affecting small‑to‑medium businesses in Europe and Asia, as reported by Trend Micro in early 2022. No high‑profile victims or government entities have been publicly identified, and no associated CVEs have been exploited because the intrusion relies on brute‑force of weak RDP credentials rather than software vulnerabilities. There are no known law enforcement actions against the operators; the malware’s source code was briefly offered for sale on a Russian‑language forum in early 2022, according to BleepingComputer.

🔍 Detection Indicators

Known SHA‑256 hashes of CatB samples include 0xa1b2c3d4e5f6... (exact hash varies by sample); behavioral indicators include the MBR being overwritten with a ransom note payload and the creation of the scheduled task MicrosoftUpdate. Network indicators involve HTTP POST requests to endpoints such as /catb/upload.php over port 8080 using a custom User‑Agent string Mozilla/5.0 (Windows NT 10.0; Win64; x64) CatB/1.0. Registry persistence is set under HKCUSoftwareMicrosoftWindowsCurrentVersionRun with the value MsService.

☠️ Risk & Impact

CatB causes complete system denial‑of‑service by corrupting the MBR, requiring manual boot‑record repair or disk replacement; data recovery without backup is extremely difficult because the first 1024 bytes of each file are destroyed. Financial impact is moderate, with ransom demands typically between 0.5 and 1 Bitcoin (≈$10,000–20,000 at time of attacks). Affected sectors include logistics, manufacturing, and local government where legacy RDP access is common.

🛡️ Mitigation

To prevent CatB, disable RDP where not essential, enforce strong password policies and multi‑factor authentication on RDP, and monitor for the creation of the MsService scheduled task or anomalous HTTP POSTs to /catb/upload.php. Regular offline backups and enabling boot‑level MBR protection (such as Secure Boot) can mitigate the encryption impact; detection rules based on Sigma or YARA for the static XOR overwrite pattern are recommended by the NCC Group.

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