A burgeoning cybersecurity threat, the Fog ransomware, has begun aggressively targeting educational and recreational organizations across the United States. Since early May 2024, security analysts have been tracking its proliferation through multiple incident response cases. The overwhelming majority, 80 percent, of impacted organizations fall within the education sector, with the remaining 20 percent operating in recreation. All observed attacks have been localized within the U.S., suggesting a geographically focused cybercriminal campaign.
The Fog ransomware operates as a distinct software variant, separate from the groups actively deploying it. This distinction is crucial, as ransomware operations often involve independent affiliate teams utilizing pre-existing malware. While the exact organizational structure behind Fog remains under investigation, evidence points towards coordinated malicious activity. The most recent documented attack within these investigated cases occurred on May 23, 2024, providing a critical timeframe for implementing defensive measures.
Fog Ransomware’s Attack Methodology and Compromised VPN Credentials
Security researchers at Arctic Wolf Labs, who began investigating these incidents in early May, identified the malware’s presence. Forensic analysis across all investigated cases revealed a consistent entry vector: threat actors gained network access by exploiting compromised VPN credentials. These credentials were used to breach networks through two different VPN gateway vendors, highlighting a significant vulnerability in remote access security protocols.
Once inside victim networks, the attackers employed a multi-stage approach. This strategy blended common penetration testing tactics with the eventual deployment of the ransomware. Administrators’ accounts were targeted through pass-the-hash techniques, which were then leveraged to establish Remote Desktop Protocol (RDP) connections to Windows servers running virtualization and backup systems like Hyper-V and Veeam. In some instances, credential stuffing was utilized to facilitate lateral movement across the compromised environment. Tools like PsExec were deployed across multiple hosts, with RDP and SMB protocols serving as pathways to access targeted systems.
Before initiating encryption, a critical defensive layer was systematically removed. Windows Defender was disabled on affected servers, leaving them exposed. The ransomware payload itself exhibits characteristics common to other well-known variants, with identical code blocks found in samples from different attack cases. Upon execution, the malware creates a log file named DbgLog.sys in the %AppData% directory to record its operational status. Its initialization routines interact with NTDLL.DLL, specifically using the NtQuerySystemInformation function to gather system details for thread allocation.
The ransomware offers several command-line options, including NOMUTEX for concurrent execution, TARGET for specifying discovery locations, and CONSOLE for displaying output. A JSON configuration block dictates the encryption process, specifying the RSA public key, file extensions (typically .FOG or .FLOCKED), ransom note filenames, and procedures for shutting down services. File discovery leverages standard Windows APIs such as FindFirstVolume and FindFirstFile, utilizing Unicode variants throughout the process. The encryption itself is managed by a thread pool scaled to the system’s processors, ranging from two to sixteen threads. It employs CryptImportKey and CryptEncrypt functions before renaming encrypted files with the designated extensions and dropping ransom notes. Finally, the tool vssadmin.exe is executed with the command delete shadows /all /quiet to eliminate volume shadow copies, effectively removing automated backup recovery options for victims.
The threat actors’ motivations appear purely financial, evidenced by the rapid pace of encryption and a lack of observed data exfiltration. This suggests an emphasis on quick payouts rather than elaborate extortion schemes involving the public leak of stolen data. To mitigate these risks, organizations are strongly advised to prioritize securing their VPN infrastructure by implementing robust multi-factor authentication. Maintaining secure, off-site backup systems is paramount, alongside the deployment of comprehensive defense-in-depth strategies. The continued emergence of ransomware like Fog underscores the ongoing need for vigilance and proactive cybersecurity measures.