How Attackers Hijack OneDrive With DLL Sideloading and Why It Matters

Software threats do not always knock loudly. Some slip through familiar doors and blend into the background. DLL sideloading is one of those quiet techniques, and recent findings show that even OneDrive, a trusted Microsoft application, can be persuaded to carry malicious code without raising alarms.

What makes this interesting is the precision with which the attack works, the stealth it maintains and the depth of technical engineering behind it.

What Is DLL Sideloading?

To understand the threat, you need to know the basics.

Windows applications use small helper files called DLLs, short for Dynamic Link Libraries. These files contain functions that programs call to perform everyday tasks. When an application needs a function, it tells Windows and the operating system loads the required DLL.

Here is the problem. If the application does not specify the full path of the DLL, Windows follows a search order. It checks the application’s own folder before looking anywhere else. Attackers take advantage of this by placing a malicious DLL with the same name as a legitimate one next to the targeted application. When the application launches, Windows picks up the fake DLL and loads it without complaint. This quiet trick is what we call DLL sideloading.

Why Attackers Target OneDrive

OneDrive is a prime candidate for sideloading. The reasons are straightforward and somewhat worrying:

• It is a Microsoft signed executable, which means it is trusted by default. • It runs frequently on most machines that store or sync files. • It has network permission and often handles sensitive documents. • Security tools usually treat traffic from OneDrive as clean.

In short, if malicious code can hide inside OneDrive, it gains instant credibility and access without needing noisy exploits or suspicious new processes. This makes OneDrive an attractive carrier for attackers.

How the Attack Works

Below is a clear walk through the technique without exposing harmful details. The intent is understanding, not replication.

Step 1] Crafting a Fake version.dll

Attackers create a DLL named version.dll because many applications, including OneDrive, call this file to check version information. The malicious DLL imitates the real one. It forwards genuine requests to the real Windows version.dll stored in the System32 folder so OneDrive continues to work properly. This prevents user suspicion.

Step 2] Placing the DLL Next to OneDrive.exe

The attacker places the fake version.dll in the same directory as OneDrive.exe. When OneDrive starts, it asks Windows for version.dll. Windows looks in the application folder first, finds the fake one and loads it. No warning appears.

Step 3] OneDrive Loads the DLL and Continues Working

Because the malicious DLL forwards legitimate calls, OneDrive behaves normally. Files sync. Notifications appear. Everything looks fine. Meanwhile, the malicious portion of the DLL executes silently.

Step 4] Advanced Hooking for Stealth

Modern sideloading attacks do not just run code. They use clever in-memory tricks to stay hidden. One approach involves intercepting Windows API calls through exception handling. This lets the attacker reroute certain operations without altering OneDrive directly. The process remains stable and the malicious code blends smoothly with normal activity.

Step 5] Running the Malicious Payload

The malicious DLL starts its own thread so the extra activity does not interfere with OneDrive. From there, attackers can choose what the thread does. It could gather data, communicate with a remote server or operate quietly for persistence. The range of possibilities is wide and depends on the attacker’s objective.

Why This Technique Is Dangerous

This is not a loud attack, it is subtle and that’s what makes it dangerous.

1] The malicious code runs inside a trusted Microsoft process. 2] Many antivirus tools see OneDrive as safe and do not inspect its behavior deeply. 3] OneDrive keeps functioning, so users are unlikely to suspect anything. 4] The attack can be persistent because the DLL stays in place. 5] The attacker gains network access and interaction with synced files.

For attackers who want to hide in plain sight, this is an efficient route.

Real World Context

DLL sideloading is not new. It has been used by advanced threat actors, crime groups, and stealthy tools for years. What makes OneDrive notable is its ubiquity. In organizations where OneDrive is central to file storage and collaboration, this type of sideloading can become a powerful foothold.

Security teams are paying closer attention because trusted applications are now being used as silent hosts for malicious activity. The trend highlights a shift toward attacks that exploit software trust rather than software flaws.

How to Defend Against It

The goal is not panic but preparation. Below are practical and safe measures that organizations and users can adopt.

-> Application Whitelisting

Allow only approved applications and libraries to load. This controls what DLLs a process can interact with.

-> Monitor Application Directories

Keep an eye on folders where critical software resides. Unexpected DLL files inside those folders are red flags.

-> Check Digital Signatures

Verify that DLLs are signed by trusted sources. Alert or block unsigned or unusual files.

-> Strengthen Behavioral Detection

Use security tools that analyze behavior, not just file signatures. If OneDrive begins spawning odd processes, you want to know.

-> Configure OneDrive Thoughtfully

In corporate environments, consider using the per machine installation mode. It reduces the risk of user directory hijacking.

-> Educate Teams

Make staff aware that trusted applications can be misused. A small amount of awareness goes a long way.

Conclusion

Threats evolve. Attackers are no longer relying only on flashy malware or dramatic exploits. They are learning to borrow trust from software that users recognize and rely on. The OneDrive DLL sideloading technique is a reminder that even everyday tools can be misused if we are not paying attention.

The best defense is awareness, consistent monitoring, and a willingness to question what appears to be routine. When trust is the new attack surface, being proactive is essential.

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