The 3rd generation Subaru Impreza was released in 2007. It addressed most of the security vulnerabilities of the previous generation car.
All models now come with a factory immobiliser and the pillar-less windows are gone. Some models even have a basic alarm.
It’s worth noting that Subaru did not offer an immobiliser on any model until March 2003. Even then this only applied to models sold in New Zealand. More details of which can be found here…
Subaru Impreza Security

Generation 3 Subaru Impreza 2007-2011 (WRX >2014)
Upgrade Alarms
The CAN-BUS system on the Impreza is interesting as locking and unlocking it with the switch on the driver’s door generates the same data as a remote lock/unlock.
This is a problem for most aftermarket CAN-BUS alarms as they can be tricked into arming/disarming. It is also an issue with most Nissan’s and some Mazda’s as explained here…
The good news is that the Cobra AK4615 has it covered. It has a switch inhibit built into the alarm to prevent the vulnerability. Nearly every other brand of alarm I’ve looked at is compromised by this so be warned!
The Cobra AK4615 in action on the Impreza:
Some photos of the details:

Ultrasonic Sensors painted black to match the STi trim

Push to start Subaru Security with Cobra LED in the traditional key location
I recommend an ADR card for models with the smart key and push to start to protect against key cloning.
How to change the Remote Battery on the Smart Key
First, remove the key blade section of the remote, then use a small flat blade screwdriver and prize open as shown in the photo below.

How to open your Subaru Smart Key
Inside you’ll find a CR1632 battery.

Smart key takes a CR1632 Battery
Gen 2 Impreza Security

Late Model Generation 2 Impreza

As of March 2003 all NZ new Subaru models come with Datadot Security and an Immobiliser
Own a Gen 2 Impreza made after March 2003? If so the Cobra AK4615 can be made to work in PLIP configuration from the factory remote.
Should you have a Jap import or an older model including the Gen 1, then I’d suggest an insurance standard immobiliser as a minimum level of protection.
This blog post explains if your Subaru has an immobiliser or not…
Ultrasonic Sensors are highly recommended for cars with pillarless windows. If it’s a WRX then I highly recommend the Cobra AK4138.

March 2003-2007 NZ New Impreza came with a remote and immobiliser

Gen 2 Impreza with ultrasonic sensors installed