Methods, systems, and apparatus, including computer programs encoded on computer storage media, for a checkout system executable code monitoring, and user account compromise determination system. The system monitors executable code initiating and executing on checkout systems, including determining hashes of the executable code. The system determines whether the executable code is malicious based on the hash, and associated information of the executable code. Additionally, the system monitors user access to checkout systems, and determines user accounts associated with being compromised. User interfaces are generated describing checkout systems associated with a risk of being compromised, and are configured for user interaction, which cause generation of updated user interfaces and access to electronic data stores to determine information relevant to the user interaction.