Unloading Module with FreeLibrary

Created the Sunday 19 March 2023. Updated 6 months, 1 week ago.

Malware authors often use various techniques to evade detection by AV/EDR solutions. One such technique involves checking for the presence of AV/EDR DLLs that may be loaded in the malware's address space and attempting to unload them before executing their malicious code.

To do this, the malware first uses the GetModuleHandleA function to retrieve a handle to the DLL, if it's already loaded. Once it has a valid handle, the malware then calls the FreeLibrary function to free and unload the DLL from memory.

By unloading the AV/EDR DLL, the malware can avoid detection and carry out its malicious activities without being monitored or blocked by the security software. However, it is important to note that this technique only affects user-mode components of an EDR and not kernel components. Additionally, modern EDR systems have evolved to detect such evasive techniques and prevent malware from disabling them.

Technique Identifier

U0519

Technique Tags

Malware AV/EDR detection DLL address space unload GetModuleHandleA FreeLibrary

Featured Windows API's

Below, you will find a list of the most commonly used Windows API's that are currently utilized by malware authors for current evasion technique. This list is meant to provide an overview of the API's that are commonly used for this purpose. If there are any API's that you feel should be included on this list, please do not hesitate to contact us. We will be happy to update the list and provide any additional information or documentation that may be helpful.

Code Snippets

Description

This code demonstrates how the GetModuleHandleA and FreeLibrary functions can be used to unload a DLL from a process's memory.

GetModuleHandleA retrieves a handle to a module (such as a DLL) that is already loaded.
FreeLibrary frees a loaded DLL from the process's memory.

This technique can be used maliciously to unload security-related DLLs used by antivirus or EDR solutions. As such, it is important to use such code ethically and responsibly, within the bounds of the law and ethical considerations.

#include <windows.h>

int main()
{
  HMODULE hLibModule = GetModuleHandleA("av_edr_dllName.dll");
  FreeLibrary(hLibModule);
  return 0;
}

Detection Rules

                                    rule:
  meta:
    name: API pattern detection for removing EDR/AV hooks
    namespace: anti-analysis/anti-av
    authors: 
      - github.com/west-wind
    scope: basic block
    mbc:
      - Defense Evasion::Disable or Evade Security Tools [F0004]
    examples:
      - 7cacd0b11497bcdd2db0ee3ae9580bdd:0x403ED2
      - 7cacd0b11497bcdd2db0ee3ae9580bdd:0x403ED9
  features:
    - and:
      - api: GetModuleHandleA
      - api: FreeLibrary
      - or:
        - string: /(\w|\d)+\.dll/i
          description: Regex match on AV/EDR dll name

Additional Resources

External Links

The resources provided below are associated links that will give you even more detailed information and research on current evasion technique. It is important to note that, while these resources may be helpful, it is important to exercise caution when following external links. As always, be careful when clicking on links from unknown sources, as they may lead to malicious content.