Mono is an open-source managed runtime for .NET-style applications. It provides the libraries and execution environment needed to run managed code, and it can be bundled with an application so the program carries part of its own runtime dependencies. That makes deployment easier across systems where the expected .NET components are missing or different.
In cyber security, Mono matters because threat actors can use it to package loaders, droppers, or crypters in a way that looks like a normal managed application but is harder to inspect at a glance. Bundling the runtime can reduce dependency checks, complicate static analysis, and help malware run consistently across environments. Defenders should look for unusual managed assemblies, embedded runtime files, and suspicious combinations of packaging, obfuscation, and process behavior.