Mimikatz Memssp Log File Detected

## Triage and analysis.

### Investigating Mimikatz Memssp Log File Detected

[Mimikatz](https://github.com/gentilkiwi/mimikatz) is an open-source tool used to collect, decrypt, and/or use cached
credentials. This tool is commonly abused by adversaries during the post-compromise stage where adversaries have gained
an initial foothold on an endpoint and are looking to elevate privileges and seek out additional authentication objects
such as tokens/hashes/credentials that can then be used to laterally move and pivot across a network.

This rule looks for the creation of a file named `mimilsa.log`, which is generated when using the Mimikatz misc::memssp
module, which injects a malicious Windows SSP to collect locally authenticated credentials, which includes the computer
account password, running service credentials, and any accounts that logon.

#### Possible investigation steps

- Investigate script execution chain (parent process tree).
- Investigate other alerts related to the user/host in the last 48 hours.
- Scope potentially compromised accounts. Analysts can do this by searching for login events (e.g., 4624) to the target
host.
- Retrieve and inspect the log file contents.
- By default, the log file is created in the same location as the DLL file.
- Search for DLL files created in the location, and retrieve any DLLs that are not signed:
  - Use the PowerShell Get-FileHash cmdlet to get the SHA-256 hash value of these files.
    - Search for the existence of these files in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.

### False positive analysis

- This file name `mimilsa.log` should not legitimately be created.

### Related rules

- Mimikatz Powershell Module Activity - ac96ceb8-4399-4191-af1d-4feeac1f1f46

### Response and remediation

- Initiate the incident response process based on the outcome of the triage.
- Isolate the involved hosts to prevent further post-compromise behavior.
- If the host is a Domain Controller (DC):
  - Activate your incident response plan for total Active Directory compromise.
  - Review the permissions of users that can access the DCs.
- Reset passwords for all compromised accounts.
- Disable remote login for compromised user accounts.
- Reboot the host to remove the injected SSP from memory.
- Reimage the host operating system or restore compromised files to clean versions.

## Config

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define `event.ingested` and default fallback for EQL rules was not added until 8.2, so you will need to add a custom pipeline to populate `event.ingested` to @timestamp for this rule to work.