The present volume contains the proceedings of the 6th International workshop on Formal Aspects of Security and Trust (fast 2009), held in Eindhoven, The Netherlands, 5-6 November 2009, as part of Formal Methods Week 2009. fast is sponsored by IFIP WG 1.7 on Foundations of Security Analysis and Design. The previous ?ve fast workshop editions have fostered cooperation among researchers in the areas of security and trust, and we aimed to continue this tradition. As computing and networkinfrastructures become increasingly per- sive, and as they carry increasing economic activity, society needs well-matched security and trust mechanisms. These interactions increasingly span several - terprises and involve loosely structured communities of individuals. Participants in these activities must control interactions with their partners based on trust policies and business logic. Trust-based decisions e?ectively determine the se- ritygoalsforsharedinformationandfor accessto sensitiveorvaluableresources. fast sought original papers focusing of formal aspects of: security and trust policy models; security protocol design and analysis; formal models of trust and reputation; logics for security and trust; distributed trust management s- tems; trust-based reasoning; digital assets protection; data protection; privacy and id issues; information ?ow 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. The fast proceedings contain-in addition to an abstract of the invited talk by Anindya Banerjee-revisionsoffull papersacceptedforpresentationat fast.