This book constitutes the thoroughly refereed post-workshop proceedings of the 6th International Workshop on Formal Aspects in Security and Trust, FAST 2009, held under the auspices of IFIP WG 1.7 in Eindhoven, The Netherlands, in November 2009 as an event of the Formal Methods Week, FMweek 2009. The 18 revised papers presented together with an abstract of the invited lecture were carefully reviewed and selected from 50 submissions. The papers focus of formal aspects in security and trust policy models, security protocol design and analysis, formal models of trust and reputation, logics for security and trust, distributed trust management systems, trust-based reasoning, digital assets protection, data protection, privacy and id issues, information flow analysis, language-based security, security and trust aspects in ubiquitous computing, validation/analysis tools, Web service security/trust/privacy, grid security, security risk assessment, and case studies.