With the advent of network connectivity and complex software applications, life-critical systems like medical devices are subject to a plethora of security risks and vulnerabilities. Security threats and attacks exploiting these vulnerabilities have been shown to compromise patient safety by hampering essential functionality. This necessitates incorporating security from the very design of software. Isolation of software functionality into different modes and switching between them based on risk assessment, while maintaining a fail-safe mode ensuring device’s essential functionality is a compelling design. Formal modeling is an essential ingredient for verification of such a design. Hence, in this paper, we formally model a trustworthy multi-modal framework for life-critical systems security and in turn safety. We formalize a multiple mode based software design approach of operation with a fail-safe mode that maintains critical functionality. We ensure trustworthyness by formalizing a composite risk model incorporated into the design for run-time risk assessment and management.