DLL Side-Loading Explained: Risks & Defense

DLL (Dynamic Link Library) side-loading is a stealthy attack technique where attackers trick a trusted application into loading a malicious DLL instead of the legitimate one.

Because the malicious DLL runs inside the context of a signed, trusted app, attackers can:

• Bypass antivirus and security tools
• Blend in with normal processes
• Gain persistence on the system

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How Does DLL Side-Loading Work? ​

Trusted Application Execution​

A legitimate signed application is run by the user, e.g., Microsoft Office, Adobe Reader, or even security software.

DLL Search Order Hijacking​

Windows follows a predefined search order when loading DLLs:
Current Directory → System32 → PATH Variables
If a malicious DLL with the same name exists in an earlier searched location, Windows will load it instead of the legitimate one.

Malicious Code Execution​

The fake DLL executes inside the trusted application process, allowing attackers to run arbitrary code with the same privileges as the app.

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Real-World Examples ​

• PlugX Malware (APT10 / APT27)
  Bundled a legitimate EXE + malicious DLL + encrypted payload. When run, the malicious DLL executed stealthily inside signed apps like McAfee.
• APT41 (Chinese Cyber Espionage)
  Targeted gaming apps and enterprise software for espionage and financial gain.
• ShadowPad Backdoor
  Delivered via legitimate software installers and later used in critical infrastructure attacks.

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Why Attackers Love DLL Side-Loading ​

• Trusted Process Execution – Malware hides inside signed applications.
• AV / EDR Evasion 🛡 - Security tools trust the parent process.
• Privilege Escalation - Malicious DLL inherits admin rights if the app runs elevated.
• Stealth & Persistence - Hard to detect because it looks like legitimate code.

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Defense Against DLL Side-Loading ​

1. Application Whitelisting
   • Use AppLocker or Windows Defender Application Control (WDAC).
   • Allow only signed and verified DLLs.
2. Enable Safe DLL Search Mode
   • Prevents apps from loading DLLs from unsafe locations, like the current directory.
3. Digital Signature Enforcement
   • Ensure all DLLs are digitally signed before loading.
4. Logging & Monitoring
   • Use Sysmon Event IDs:
     • Event 7 → DLL loaded
     • Event 1 → Process creation
     • Event 10 → Process access
   • Monitor for unusual DLL loads from non-standard directories.
   • Use EDR to detect suspicious DLL injection or side-loading.
5. Patch & Update
   • Keep OS and apps updated, as vendors patch DLL hijacking issues regularly.
6. Threat Hunting & YARA Rules
   • Create rules to detect known malicious DLLs and suspicious EXE + DLL combinations.

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Key Takeaway ​

DLL side-loading is not a Windows vulnerability itself, but an abuse of the DLL search order.

• Attackers weaponize trusted applications 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.