New Research: Tackling .NET Malware With Harmony Library

Security researchers have recently unveiled strategic insights into countering .NET malware through the innovative use of the Harmony library. 

The research, published earlier today, explores the significance of code manipulation in malware analysis, emphasizing its pivotal role for researchers, analysts and reverse engineers.

Traditionally, code functionality is altered through debugging, Dynamic Binary Instrumentation (DBI) or hooking frameworks. While these methods have proven effective for non-managed, native code, the landscape changes when dealing with applications running on the .NET platform.

In the .NET domain, the ability to instrument code on the managed layer has been limited, posing challenges for researchers. However, Check Point Research (CPR) is now highlighting the Harmony library as a standout solution. 

An open-source library, Harmony specializes in patching, replacing and decorating .NET methods in real-time, overcoming the limitations associated with altering managed code.

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The CPR research piece introduced the concept of .NET managed hooking using the Harmony library, delving into its internals and providing practical implementation examples, showcasing different types of Harmony patches.

Significantly, the Harmony library operates solely on in-memory code, ensuring that modifications do not impact files on disk. This feature proves invaluable, especially when dealing with .NET malware protected by obfuscators. For context, disk-based deobfuscation risks altering the original structure and causing functionality loss.

The CPR research also emphasized the versatility of Harmony hooking, allowing researchers to modify the functionality of all referenced assemblies, particularly those integral to the .NET Runtime. Further, it touched upon the bootstrapping and injection process, outlining how Harmony can be injected into .NET processes, either through loaders or injectors.

Furthermore, the research categorized various types of Harmony patches, such as Prefix, Postfix, Transpiler, Finalizer and Reverse Patch, each serving a specific purpose in modifying the behavior of .NET methods.

“These examples demonstrate how powerful .NET hooking can be and, more importantly, how easy and straightforward it is to implement .NET instrumentation once we use the Harmony library,” reads the technical write-up.