The two volume set LNCS 12972 + 12973 constitutes the proceedings of the 26th European Symposium on Research in Computer Security, ESORICS 2021, which took place during October 4-8, 2021. The conference was originally planned to take place in Darmstadt, Germany, but changed to an online event due to the COVID-19 pandemic. The 71 full papers presented in this book were carefully reviewed and selected from 351 submissions. They were organized in topical sections as follows: Part I: network security; attacks; fuzzing; malware; user behavior and underground economy; blockchain; machine learning; automotive; anomaly detection; Part II: encryption; cryptography; privacy; differential privacy; zero knowledge; key exchange; multi-party computation.