Cephalus Rises: The Ransomware Group Hijacking Trust with DLL Sleight of Hand

A Trojan Horse in the Download Folder

Imagine a trusted guard at your front gate - only to discover he’s been replaced by an impostor. That’s the core trick behind Cephalus, a new ransomware group that has burst onto the scene with a blend of old-school entry points and cunning technical subterfuge. Discovered in August 2025, Cephalus has already targeted at least two organizations, leaving a trail of encrypted data and anxious security teams in its wake.

How Cephalus Infiltrates: Familiar Doors, Unfamiliar Tactics

Cephalus doesn’t reinvent the wheel when it comes to gaining access. The attackers used compromised RDP credentials - a method as old as remote work itself - taking advantage of organizations that neglected to enforce multi-factor authentication. But what sets Cephalus apart is what comes next: rather than deploying obviously malicious files, they hijack SentinelOne’s SentinelBrowserNativeHost.exe, a legitimate security tool, and plant it in the user’s Downloads folder. This tool then loads a booby-trapped library file (DLL), which in turn executes the ransomware code.

It’s like swapping a chef’s favorite knife for a poisoned replica: the tool looks the same, but its purpose has been fatally altered. This method, known as DLL side-loading, allows attackers to bypass many traditional security checks, as the executable itself is trusted by the system.

Double Trouble: Encryption and Extortion

Cephalus follows the playbook of modern ransomware crews by combining data encryption with data theft. Before locking files, the malware disables Windows Defender, wipes out backup copies (shadow volumes), and tweaks system settings to blind any residual defenses. The group then exfiltrates sensitive files to MEGA, a popular cloud storage platform, using automated tools like MEGAcmdUpdater.exe - sometimes even scheduling the theft via Windows’ built-in Task Scheduler.

Victims receive ransom notes that are more than just demands. Cephalus tries to up the psychological ante by referencing previous attacks, offering “proof” of stolen data, and threatening public exposure. The attackers even tailor the notes to the specific victim, sometimes providing links to sample files as evidence.

Context and Consequences

DLL side-loading isn’t new - it’s been used in targeted attacks for years, from espionage to financially motivated cybercrime. What’s notable here is the bold use of a well-known security product as the delivery vehicle, echoing past incidents where trusted software was weaponized (think of NotPetya’s abuse of accounting software or SolarWinds in supply chain attacks). Cephalus’s approach highlights a growing trend: attackers exploiting the trust we place in legitimate tools, making detection harder for defenders and multiplying the potential for damage.

For defenders, the lesson is clear: don’t just watch for strange files - watch for normal files in strange places. Monitoring for SentinelBrowserNativeHost.exe running from user directories, tightening RDP access with MFA, and restricting cloud storage use are now critical controls. As ransomware groups like Cephalus continue to evolve, so must the vigilance of those on the digital front lines.