FLOSS

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

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 the output of the strings.exe utility that we commonly use during basic static analysis.

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: a 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.

Not all compilers use string formats that the classic strings.exe algorithm supports. For example, if strings are UTF-8 encoded or stored without a NULL-terminator. FLOSS can identify and extract strings from programs compiled from the following languages:

1. Go
2. Rust

The strings FLOSS extracts specific to a compiler are much easier to inspect by humans.

Please consult the documentation to learn more about the language-specific string extraction.

FLOSS enriches static strings by default with file structure context and tags (global prevalence, open-source libraries, expert rules, and more). Stack, tight, and decoded strings still appear after the layout-aware static listing when deobfuscation is enabled.

$ floss sample.exe
$ floss sample.exe -j

Features:

• extract ASCII and UTF-16LE strings
• show strings next to right-aligned, colored context, including tags and file offset
• render strings within PE section range delimiters
• annotate strings from known PE structures, like the import table
• don't show junk strings that overlap with instructions
• mute strings known to be globally prevalent, via an embedded database
• mute strings from popular open source libraries, via embedded databases
• highlight strings that match expert rules, via embedded databases

Tag databases and FLIRT signature files are tracked with Git LFS; contributors cloning the repo may need Git LFS installed to fetch those files. Maintenance of tag databases is documented in scripts/tags/README.md and the per-database notes under floss/tags/data/.

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

See the installation documentation for a detailed description of all methods to install FLOSS.

Extract obfuscated strings from a malware binary:

$ floss malware.exe

Only extract stack and tight strings:

$ floss --string-type stack tight -- suspicious.exe

Do not extract static strings:

$ floss --no-string-type static -- backdoor.exe

Display the help/usage screens:

$ floss -h  # show all supported arguments

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

FLOSS also contains additional Python scripts in the scripts directory which can be used to load its output into other tools such as Binary Ninja or IDA Pro. For detailed description of these scripts review the documentation here.

Subscribe to the FLARE mailing list for community announcements by sending an email with the subject "subscribe" to flare-external@google.com.