A Next-Generation Platform for Analyzing Executables
Thomas W. Reps and Gogul Balakrishnan and Junghee Lim and Tim Teitelbaum
In recent years, there has been a growing need for tools that an analyst can use to understand the workings of COTS components, plugins, mobile code, and DLLs, as well as memory snapshots of worms and virus-infected code. Static analysis provides techniques that can help with such problems; however, there are several obstacles that must be overcome:
- For many kinds of potentially malicious programs, symbol-table and debugging information is entirely absent. Even if it is present, it cannot be relied upon.
- To understand memory-access operations, it is necessary to determine the set of addresses accessed by each operation. This is difficult because
- While some memory operations use explicit memory addresses in the instruction (easy), others use indirect addressing via address expressions (difficult).
- Arithmetic on addresses is pervasive. For instance, even when the value of a local variable is loaded from its slot in an activation record, address arithmetic is performed.
- There is no notion of type at the hardware level, so address values cannot be distinguished from integer values.
- Memory accesses do not have to be aligned, so word-sized address values could potentially be cobbled together from misaligned reads and writes.
We have developed static-analysis algorithms to recover information about the contents of memory locations and how they are manipulated by an executable. By combining these analyses with facilities provided by the IDAPro and CodeSurfer toolkits, we have created CodeSurfer/x86, a prototype tool for browsing, inspecting, and analyzing x86 executables. From an x86 executable, CodeSurfer/x86 recovers intermediate representations that are similar to what would be created by a compiler for a program written in a high-level language. CodeSurfer/x86 also supports a scripting language, as well as several kinds of sophisticated pattern-matching capabilities. These facilities provide a platform for the development of additional tools for analyzing the security properties of executables.
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