DLL Hijacking: The Silent Execution Vector

By XploitHunter

TL;DR

DLL Hijacking is a technique where an attacker tricks an application into loading a malicious Dynamic Link Library (DLL) instead of a legitimate one. This happens due to improper DLL search order or missing dependencies. Successful exploitation can lead to code execution, privilege escalation, and persistence within the target system.

Uncovering the Roots of DLL Hijacking

DLL Hijacking occurs because Windows applications follow a predefined DLL search order when loading dependencies. If a required DLL is not found in a secure location, the system continues searching in other directories.

Attackers exploit this behavior by placing a malicious DLL in a location that is searched before the legitimate one. As a result, the application unknowingly executes attacker-controlled code.

Common root causes include:

• Applications using relative paths
• Missing or misconfigured DLL dependencies
• Lack of integrity verification
• Weak file permission controls.

Types of DLL Hijacking

• Classic DLL Hijacking — Replacing a legitimate DLL with a malicious one
• DLL Search Order Hijacking — Placing a DLL in a higher priority directory
• Phantom DLL Hijacking — Exploiting missing DLLs
• Side-loading — Using trusted signed applications to load malicious DLLs

Real-World Impact

DLL hijacking can lead to:

• Remote Code Execution (RCE)
• Privilege Escalation
• Persistence on compromised systems
• Bypassing application whitelisting

Preparing the environment

• Kali Linux VM (Attacker)
• Victim VM (Target)
• Procmon Tool
• Vulnerable Thick Client Application

How the Attack Works

Step 1: Monitor DLL Loading with ProcMon

Apply filters in ProcMon:

• Process Name → DVTA
• Path → .dll
• Result → NAME NOT FOUND

This helps identify missing DLLs that the application is trying to load.

Fig 1. Searching for missing .dll files when application opens

Step 2: Create a Malicious DLL

Create a DLL that executes arbitrary code when loaded with the help of metasploit:

Fig 2. Creating a malicious .dll file

Step 3: Place the Malicious DLL

Place the DLL in a directory that appears earlier in the search order (e.g., application directory).

Fig 3. Place the malicious .dll file in the application directory.

Step 4: Execute the Application

Run the vulnerable application. If successful, the application loads the malicious DLL and executes the payload.

Fig 4. Upon running the application reverse shell is observed.

Mitigation Strategies

To prevent DLL Hijacking attacks:

• Use absolute paths when loading DLLs
• Enable Safe DLL Search Mode
• Restrict write permissions on application directories
• Implement code signing and validation
• Use application whitelisting (AppLocker / WDAC)
• Monitor unusual DLL loads using EDR solutions

Conclusion

DLL Hijacking is a simple yet highly effective attack vector that exploits insecure application design. While easy to overlook, it can lead to severe consequences including system compromise and persistence. Proper coding practices, strict access controls, and continuous monitoring are essential to mitigate this risk.