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 FireEye Labs Obfuscated String Solver (FLOSS) 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.Please review the theory behind FLOSS here.
To try FLOSS right away, download a standalone executable file from the releases page: https://github.com/fireeye/flare-floss/releases
For a detailed description of installing FLOSS, review the documention here.
Standalone nightly builds:
Usage
Extract obfuscated strings from a malware binary:
$ floss /path/to/malware/binary$ ./floss -hFor a detailed description of testing FLOSS, review the documention here.
Sample Output
$ floss malware.bin
FLOSS static ASCII strings
!This program cannot be run in DOS mode.
_YY
RichYY
MdfQ
.text
`.rdata
@.data
.idata
.didat
.reloc
U F
?;}
A@;E
_^[
HttHt-H
'9U
WS2_32.dll
FreeLibrary
GetProcAddress
LoadLibraryA
GetModuleHandleA
GetVersionExA
MultiByteToWideChar
WideCharToMultiByte
Sleep
GetLastError
DeleteFileA
WriteFile
[..snip...]
FLOSS static UTF-16 strings
,%d
FLOSS decoded 4 strings
WinSta0\Default
Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings
ProxyEnable
ProxyServer
FLOSS extracted 81 stack strings
WinSta0\Default
'%s' executed.
ERR '%s' error[%d].
Software\\Microsoft\\Windows\\CurrentVersion\\Internet Settings
ProxyEnable
ProxyServer
wininet.dll
InternetOpenA
0\A4
InternetSetOptionA
InternetConnectA
InternetQueryOptionA
Mozilla/4.0 (compatible; MSIE 7.0; Win32)
-ERR
FILE(%s) wrote(%d).
Invalid ojbect.
SetFilepoint error[%d].
b64_ntop error[%d].
GetFileSize error[%d].
Creates file error[%d].
KCeID5Y/96QTJc1pzi0ZhEBqVG83OnXaL+oxsRdymHS4bFgl7UrWfP2v=wtjNukM
[..snip...]