Signed Proxy Execution via MS WorkFolders

## Triage and analysis

### Investigating Signed Proxy Execution via MS WorkFolders

Work Folders is a role service for file servers running Windows Server that provides a consistent way for users to access
their work files from their PCs and devices. This allows users to store work files and access them from anywhere. When
called, Work Folders will automatically execute any Portable Executable (PE) named `control.exe` as an argument before
accessing the synced share.

Using Work Folders to execute a masqueraded control.exe could allow an adversary to bypass application controls and
increase privileges.

#### Possible investigation steps

- Investigate the process tree starting with parent process WorkFolders.exe and child process control.exe to determine
if other child processes spawned during execution.
- Trace the activity related to the `control.exe` binary to identify any continuing intrusion activity on the host.
- Examine the location of the WorkFolders.exe binary to determine if it was copied to the location of the control.exe
binary. It resides in the System32 directory by default.
- Review the control.exe binary executed with Work Folders to determine maliciousness such as additional host activity
or network traffic.
- Determine if control.exe was synced to sync share, indicating potential lateral movement.
- Review how control.exe was originally delivered on the host, such as emailed, downloaded from the web, or written to
disk from a separate binary.

### False positive analysis

- Windows Work Folders are used legitimately by end users and administrators for file sharing and syncing but not in the
instance where a suspicious `control.exe` is passed as an argument.

### Response and remediation

- Initiate the incident response process based on the outcome of the triage.
- Isolate the involved host to prevent further post-compromise behavior.
- Review the Work Folders synced share to determine if the 'control.exe' was shared and if so remove it.
- If no lateral movement was identified during investigation, take the effected host offline if possible and remove the
control.exe binary as well as any additional artifacts identified during investigation.
- Review integrating Windows Information Protection (WIP) to enforce data protection by encrypting the data on PCs using
Work Folders.
- Confirm with the user whether this was expected or not, and reset their password.

## 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.