The invention relates to a method for computer systems based on the ARM processor, for example mobile devices, wherein the ARM processor provides fully hardware isolated runtime environments for an operating system (OS) and Trusted Execution Environment (TEE) including an embedded trusted network security perimeter. The isolation is performed by hardware ARM Security Extensions added to ARMv6 processors and greater and controlled by TrustWall software. The invention therefore comprises an embedded network security perimeter running in TEE on one or more processor cores with dedicated memory and storage and used to secure all external network communications of the host device. The invention addresses network communications control and protection for Rich OS Execution Environments and describes minimal necessary and sufficient actions to prevent unauthorized access to or from external networks. The present invention uses hardware platform security capabilities which significantly increase protection of the embedded network security perimeter itself from targeted attacks, in contrast to existing, and representing an improvement of, end-point software firewalls. In addition, embodiments of the invention do not require any modification to the OS system code or network application software.