ProcEnvInjection - Remote code injection by abusing process environment strings

Created the Monday 13 June 2022. Updated 9 months, 2 weeks ago.

This method allows to inject custom code into a remote process without using WriteProcessMemory - It will use the lpEnvironment parameter in CreateProcess to copy the code into the target process. This technique can be used to load a DLL into a remote process, or simply execute a block of code.

The lpEnvironment parameter in CreateProcess allows us to specify a custom environment string for the target process. The environment string contains a set of environment variable entries, such as PATH=C:\Windows\system32;C:\Windows. Each environment variable in the list is separated by a null terminator character, and the final entry in the list is a blank string (two null terminator characters). When a new process is created, the environment string will be copied to the virtual memory of the process and it can then be accessed via the PEB.

In summary, the injector process takes the following steps:

  1. Create a generic "code loader" block which doesn't contain any 0x00 characters - values will be encoded with XOR if necessary.
  2. Use GetEnvironmentStringsW to retrieve the existing environment string and copy this to a temporary buffer. Our "generic code loader" entry will be appended to the end of the existing entries. Some programs make use of the environment variables, so it is not a good idea to overwrite the existing entries.
  3. Create a suspended instance of the target EXE process using CreateProcess with our custom environment string lpEnvironment. We will also use the CREATE_UNICODE_ENVIRONMENT flag to specify a wide-char environment value, otherwise the string will be converted from ANSI to wide-char which will break our loader code.
  4. Use NtQueryInformationProcess to retrieve the PEB address for the target process.
  5. Call NtCreateThreadEx to call Sleep(0) in the target process and wait for the thread to exit. This will force the necessary PEB fields to become initialised in the target process.
  6. Calculate the address of the environment string in the target process (PEB-> RtlUserProcessParameters -> Environment)
  7. Locate the address of our loader code within the environment string. Call VirtualProtectEx to make this data executable.
  8. Call NtCreateThreadEx to execute the loader code within the target process. This code will read the final payload back from the injector process and execute it.
  9. Restore the original memory protection after the payload has finished executing.
  10. Call ResumeThread to continue normal execution of the target process.

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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.

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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.

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