FLOSS: FireEye Labs Obfuscated String Solver — Automatically extract obfuscated strings from…

Malware authors pack their software to resist reverse engineering and enable their operations to survive longer.

However, many features of packing are easy to automatically identify during static or dynamic analysis.

Therefore, some authors obfuscate only the most sensitive resources used by malware in an attempt to blend in.

The FireEye Labs Obfuscated String Solver (FLOSS) uses advanced static analysis techniques to automatically deobfuscate strings from malware binaries.


How it works?

FLOSS combines and automates the best manual reverse engineering techniques for string decoding. First, it uses heuristics to identify decoding routines in a sample. Then FLOSS extracts cross references and arguments to decoders using control flow analysis. Next FLOSS emulates decoder functions using extracted arguments. Finally, FLOSS diffs the emulator memory states from before and after decoder emulation and extracts human readable strings.

Algorithm

  1. Analyze control flow of malware to identify functions, basic blocks, etc.
  2. Use heuristics to find potential decoding routines
  3. Brute force emulate all code paths among basic blocks and functions
  4. Snapshot emulator state (registers, memory) at appropriate points
  5. Extract arguments to decoder functions from emulator snapshots
  6. Emulate decoder functions using extracted arguments and emulator state
  7. Diff memory state from before and after decoder emulation
  8. Extract human-readable strings from memory state difference

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