Rather than heavily protecting backdoors with hardcore packers, many malware authors evade heuristic detections by obfuscating only key portions of an executable. Often, these portions are strings and resources used to configure domains, files, and other artifacts of an infection. These key features will not show up as plaintext in output of the strings.exe utility that we commonly use during basic static analysis.

The FLARE Obfuscated String Solver (FLOSS, formerly FireEye Labs Obfuscated String Solver) uses advanced static analysis techniques to automatically deobfuscate strings from malware binaries. You can use it just like strings.exe to enhance basic static analysis of unknown binaries.

FLOSS extracts all the following string types:

1. static strings: "regular" ASCII and UTF-16LE strings
2. stack strings: strings constructed on the stack at run-time
3. tight strings: special form of stack strings, decoded on the stack
4. decoded strings: strings decoded in a function

Please review the theory behind FLOSS here.

Our blog post talks more about the motivation behind FLOSS and details how the tool works.

FLOSS version 2.0 updates are detailed in this blog post.

To try FLOSS right away, download a standalone executable file from the releases page: https://github.com/mandiant/flare-floss/releases

For a detailed description of installing FLOSS, review the documentation here.

Extract obfuscated strings from a malware binary:

$ floss /path/to/malware/binary

Display the help/usage screen to see all available switches.

$ floss -h

For a detailed description of using FLOSS, review the documentation here.

For a detailed description of testing FLOSS, review the documentation here.