Refinements for Open Automata -- The Cubicle Fuzzy Loop : A Fuzzing-Based Extension for the Cubicle Model Checker -- Guiding Symbolic Execution with A-star -- Robustness Testing of Software Verifiers -- Decoupled Fitness Criteria for Reactive Systems -- Capturing Smart Contract Design with DCR Graphs -- An Active Learning Approach to Synthesizing Program Contracts -- Ranged Program Analysis via Instrumentation -- Attack time analysis in dynamic attack trees via integer linear programming -- SSCalc A Calculus for Solidity Smart Contracts -- ATM: a Logic for Quantitative Security Properties on Attack Trees -- Refactoring of Multi-Instance BPMN Processes with Time and Resources -- Verified Scalable Parallel Computing with Why3 -- Exact and Efficient Bayesian Inference for Privacy Risk Quantification -- A Formalization of Heisenbugs and Their Causes -- Verifying Read-Copy Update under RC11 -- QNNRepair: Quantized Neural Network Repair -- Timeout Prediction for Software Analyses -- PART Tool Papers -- PMC-VIS: An Interactive Visualization Tool for Probabilistic Model Checking.

This book constitutes the refereed proceedings of the 21st International Conference on Software Engineering and Formal Methods, SEFM 2023, held in Eindhoven, The Netherlands, during November 6-10, 2023. The 18 regular papers presented in this book, together with 1 invited paper and 1 tool paper, were carefully reviewed and selected from 41 submissions. The SEFM conference series aims to bring together researchers and practitioners from academia, industry and government, to advance the state of the art in formal methods, to facilitate their uptake in the software industry, and to encourage their integration within practical software engineering methods and tools.