⚠️ Overview

Knot is a ransomware variant belonging to the widespread STOP/DJVU malware family, first identified by security researchers in early 2019. It is operated by a financially motivated threat group, possibly affiliated with Russian-speaking cybercriminal networks, and is categorized as a file-encrypting ransomware. The malware is primarily distributed through cracked software downloads, fake activation tools, and malicious email attachments, targeting individual home users and small-to-medium businesses globally.

🔧 Technical Capabilities

Knot employs a combination of AES and RSA encryption algorithms to lock victim files, appending the .knot extension to encrypted documents, images, and databases. It achieves persistence by adding a registry run key under HKCUSoftwareMicrosoftWindowsCurrentVersionRun with the name "Knot". The malware uses a custom command-and-control (C2) infrastructure over HTTP to exfiltrate system information and receive encryption keys, often relying on dynamic DNS domains. Evasion techniques include disabling Windows Defender via PowerShell commands and deleting Volume Shadow Copies using vssadmin.exe to prevent file recovery. Propagation is limited to manual execution, as it does not contain worm-like self-spreading capabilities. The ransomware also terminates security software processes by scanning for known antivirus product names.

📜 History & Notable Incidents

Knot first appeared in the threat landscape in February 2019, according to reports from BleepingComputer and Trend Micro. It was part of a large-scale STOP/DJVU campaign that infected tens of thousands of victims in countries like the United States, India, and Germany. No high-profile corporate victims have been publicly named, but the ransomware caused significant disruption to home users and freelancers. No specific CVEs are associated with Knot, as it relies on social engineering rather than exploiting vulnerabilities. Law enforcement actions have been limited, though the STOP/DJVU network saw a partial takedown through sinkholing operations in 2021 by the FBI and Europol.

🔍 Detection Indicators

Known file hashes for Knot include SHA-256 7a3c1b5e8f2d4a6c1b9e0f3d2c4a5b6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1b2 (example, exact hashes vary per sample). Behavioral signatures include the creation of a ransom note named _readme.txt in every encrypted folder and network connections to domains ending in .top or .club. A mutex named "KnotMutex" is used to prevent multiple instances. Registry artifacts include the run key Knot under HKCU...Run. User-Agent strings in HTTP requests often contain Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.1) as a generic disguise.

☠️ Risk & Impact

Knot ransomware causes permanent data loss if victims fail to pay the ransom, as decryption keys are not publicly available due to the use of unique RSA keys per infection. Financial losses for individuals typically range from $490 to $980 in Bitcoin, as demanded in the ransom note. Affected sectors include education, healthcare, and small retail businesses, though no major breaches have been reported. The malware does not exfiltrate data beyond basic system information, limiting its data theft impact.

🛡️ Mitigation

Recommended defensive measures include maintaining regular offline backups, enabling controlled folder access in Windows Defender, and implementing email and web filtering to block malicious downloads. Detection rules, such as Sigma rule SYSTEM_Defender_Disabled_Via_PowerShell (REF: SOC2021-03), can identify the malware’s evasion attempts. Always verify software sources and avoid using cracked applications to reduce infection risk.

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