🎯 Goal

Today I explored how DLL files work in Windows and how security teams verify whether a file is legitimate using digital signatures.

The goal was to understand:

  • how Windows loads DLL files
  • how attackers abuse DLL execution
  • how defenders verify file authenticity.

🛠 What I Did

I studied the role of DLL (Dynamic Link Library) files in Windows.

DLLs are shared libraries that programs use to load functionality dynamically.

Instead of embedding all code inside a program, applications call functions from DLL files.

Example flow:

Application starts
↓
Program loads required DLL
↓
Functions inside the DLL execute

This modular design improves performance and reduces duplication.

🔗 Key Cybersecurity Connections

Attackers often abuse DLL behavior through techniques such as:

  • DLL sideloading
  • malicious DLL injection
  • loading attacker-controlled libraries.

Example attack pattern:

Legitimate application starts
↓
Application loads DLL from writable directory
↓
Malicious DLL executes attacker code

Because the application itself is trusted, this technique can bypass some security controls.

🔍 Investigation Questions

If suspicious DLL activity appeared in telemetry, a SOC analyst might investigate questions such as:

  • Which process loaded the DLL?
  • From which directory was the DLL loaded?
  • Is the DLL located in a user-writable directory rather than a system directory?
  • Does the DLL have a valid digital signature?
  • What is the parent process of the application loading the DLL?
  • Did the process establish network connections after loading the DLL?

These questions help determine whether the DLL loading behavior is legitimate or part of a malicious technique such as DLL sideloading.

🚨 Detection Opportunities

Possible detection strategies for DLL abuse include:

  • alerting when applications load DLLs from non-standard directories
  • detecting DLL loads from user-controlled paths such as AppData or Temp
  • monitoring unsigned DLL execution by trusted applications
  • detecting abnormal parent → child process relationships
  • correlating DLL execution with network activity or persistence mechanisms

Endpoint telemetry such as Sysmon process and image load events can provide valuable visibility into these behaviors.

🧭 MITRE ATT&CK Techniques

The techniques discussed in this topic relate to several MITRE ATT&CK entries:

  • T1574.002 – DLL Side-Loading
  • T1055 – Process Injection
  • T1218 – Signed Binary Proxy Execution

Mapping activity to ATT&CK helps analysts categorize malicious behaviors and build more effective detection strategies.

📚 Digital Signatures

To verify the authenticity of files, Windows uses code signing.

A digital signature proves:

  • who published the software
  • that the file has not been modified.

Signed files contain cryptographic metadata verifying the publisher.

Security analysts often check:

  • whether the file is signed
  • whether the signature is valid
  • which organization issued the certificate.

⚠ Challenges

One important observation is that malware is often unsigned.

However, attackers sometimes obtain or steal legitimate certificates to sign malicious code.

This means that a signed file is not automatically trustworthy.

Analysts must still evaluate:

  • file origin
  • execution context
  • behavior.

📚 What I Learned

When investigating suspicious DLL execution, analysts often examine:

  • file location
  • digital signature status
  • parent process
  • command-line parameters

Combining these signals helps determine whether a DLL is malicious.

➡ Next Steps

Next areas to explore include:

  • real malware families
  • how malware spreads across networks
  • how security teams detect these threats.

🧠 Reflection

Understanding DLL loading behavior helps explain how attackers exploit trusted applications.

Many attacks rely on misconfigurations or weak trust assumptions rather than complex exploits.

🧩 Lessons Learned

What worked

Studying how Windows verifies file signatures clarified how trust is established.

What broke

At first it seemed that signed files should always be safe.

Why it broke

Attackers can sometimes steal or misuse legitimate certificates.

Fix / takeaway

Trust should be evaluated using multiple indicators, not only signatures.

📈 Skill Progression Context

Learning about DLL behavior prepares for deeper topics such as:

  • malware analysis
  • persistence mechanisms
  • Windows process investigation.