DLL Side-Loading Attack - A Deep Dive

What is DLL Side-Loading?​

DLL (Dynamic Link Library) side-loading is a stealthy attack technique where adversaries trick a legitimate, trusted application into loading a malicious DLL instead of the legitimate one.
Since the malicious DLL runs inside the context of a trusted signed application, it helps attackers:
Bypass antivirus / security tools
Blend in with normal processes
Gain persistence

How Does DLL Side-Loading Work?​

Trusted Application Execution​

A legitimate signed application is executed by the user (e.g., Microsoft Office, Adobe Reader, or even a security tool).

DLL Search Order Hijacking​

When the application tries to load a DLL, Windows follows a predefined search order (Current Directory → System32 → PATH variables).
If a malicious DLL with the same name exists in a location searched before the real DLL, Windows will load the attacker’s DLL.

Malicious Code Execution​

The fake DLL runs inside the trusted application’s process.
This allows the attacker to execute arbitrary code with the same privileges as the application.

Real-World Examples​

PlugX Malware (APT10 / APT27)
PlugX used DLL side-loading with signed applications (like McAfee or Microsoft tools).
Attackers bundled a legitimate EXE + malicious DLL + encrypted payload.
When the EXE ran, the malicious DLL was loaded → payload executed stealthily.
APT41 (Chinese Cyber Espionage Group)
Used DLL side-loading in gaming applications and enterprise software.
Targeted both espionage and financial gain.
ShadowPad Backdoor
Delivered through DLL side-loading with legitimate software installers.
Later used in critical infrastructure attacks.

Why Attackers Love DLL Side-Loading?​

Trusted Process Execution → Malware hides inside signed applications.
AV / EDR Evasion → Security products trust the parent process.
Privilege Escalation → If the trusted app runs with admin rights, attacker DLL inherits them.
Stealth & Persistence → Hard to detect because malicious code looks like it belongs to a legit process.

Defense Against DLL Side-Loading​

1. Application Whitelisting​

Use AppLocker or Windows Defender Application Control (WDAC).
Allow only signed and verified DLLs to load.

2. Enable “Safe DLL Search Mode”​

Prevents applications from loading DLLs from unsafe locations (like current working directory).

3. Digital Signature Enforcement​

Ensure that DLLs must be digitally signed before being loaded.

4. Logging & Monitoring​

Use Sysmon Event IDs:
Event ID 7 → DLL loaded
Event ID 1 → Process creation
Event ID 10 → Process access
Monitor for unusual DLL loads from non-standard directories.
Use EDR to detect DLL injection / side-loading activity.

5. Patch & Update​

Keep OS and third-party applications updated, as vendors regularly patch DLL hijacking issues.

6. Threat Hunting & YARA Rules​

Create rules to detect known malicious DLLs and suspicious EXE + DLL combinations.

Key Takeaway
DLL side-loading is not a vulnerability in itself, but a feature abuse of Windows DLL search order.
Attackers weaponize trusted applications as a shield to run malware silently.
Organizations must harden DLL loading policies, enforce code signing, and monitor anomalies to stay protected.
DLL Side-Loading = Using trusted apps as a weapon against you.