Algebraic State-Transition Diagrams (ASTDs) are extensions of common automata and statecharts that can be combined with process algebra operators like sequence, choice, guard and quantified synchronization. They were previously introduced for the graphical representation, specification and proof of information systems. In an attempt to use ASTDs to specify cyber attack detection, we have identified a number of missing features in ASTDs. This paper extends the ASTD notation with state variables (attributes), actions on transitions, and a new operator called flow which corresponds to AND states in statecharts and is a compromise between interleaving and synchronization in process algebras. We provide a formal structured operational semantics of these extensions and illustrate its implementation in an OCaml-based interpreter called iASTD and the model checker ProB. Extended ASTDs are illustrated in a case study in cyber attack detection