Malware tricks for Pentesters Bsides Lisbon 2017

1. FileLess Malware
Infections
Malware tricks for Pentesters
Ramon Pinuaga
Bsides Lisbon 2017

2. 2
1) Presentation
2) Real world examples
3) Process
• Infection
• Persistence
4) Conclusions
Index

3. PRESENTATION

4. 4
• Pentester for many, many years.
• Current position: Cybersecurity
Audit Manager at PROSEGUR
Spain.
• I prefer the offensive side of
security.
Who are you?

5. 5
• Malicious code that doesn't need to create or drop
regular files on the system.
• Move away from the traditional monolithic malware
or pentesting framework.
• For persistence we usually need to leave at least
something on the system but we can hide it and make
it very small.
What is FileLess Malware?

7. 7
• We have 2 main ways to achieve an infection without
files:
• Not using malware (or code) at all. E.G. planting a
configuration only backdoor on a system. That way we
don’t control the system all the time but we can access it
later.
• Using code that never touches disk. In his clear form. For
persistence we always need a way to keep at least the first
stage of the code on the system.
FileLess or MalwareLess?

8. 8
• Evading Antivirus detection: No file, No scan, No
VirusTotal upload. 
• Leaving a smaller forensic trail: Less artifacts.
• Difficult environments (hard to upload things).
• Helps in bypassing Application Whitelisting (SRP,
Applocker, etc.)
Why FileLess in pentesting?

10. 10
• Long-term persistence.
• Kernel-level access.
• High level hiding.
• Quickness and simplicity: Only userland, No rootkits,
No NSA like implants.
Pentest vs APT

11. DEMO: RDP + Sethc
• Enable remote desktop
• Hijack sethc.exe
• Change RDP port
• Open windows firewall

13. 13
• Keeping all in memory. (Problem: no persistence)
• Storing the code in non-file or non-regular storage
(classics):
• Outside the filesystem: UEFI, HDD Firmware, Hidden disk
areas, $EA, etc. (We are not going that far, remember; only
userland)
• Network / External systems.
• Alternate Data Streams (ADS).
• Registry.
How to keep code without files?

14. 14
• WMI (subscriptions).
• Windows events (.evt).
• Inside Documents (.doc, .xls, .pdf).
• File/Directory names (0-day).
• Environment variables (more 0-day).
Novel non-regular storage

15. DEMO: Code in filenames
• Stage 1: Run key
• Stage 2: Environment
• Stage 3: Dir names

17. 17
• Remote injection in memory -> Remote call or exploit.
• Load of remote binaries (EXEs, DLLs) -> Via SMB,
WebDAV, etc.
• Scripting languages -> Loaded remotely or from the
command line.
• PowerShell (Powershell.exe)
• Javascript/Vbscript (Cscript.exe, Wscript.exe, Mshta.exe,
Rundll32.exe, Regsvr32.exe).
• .Net assemblies (InstallUtil.exe, IEExec.exe, RegAsm.exe).
How to execute code without files?

18. 18
• Tools already installed on the system (no new files).
• Tools that allow receiving external input (via
command line or via the network).
• Bonus: Tools signed by Microsoft.
Our FileLess pentest framework

19. REAL WORLD EXAMPLES

20. 20
• Worms (memory only):
• Slammer.
• Poweliks.
• WMIGhost.
• Empire.
• Duqu 2.0 (Kaspersky).
Real world examples

21. 21
• Worm that infected thousands of computers and
impacted general Internet traffic in some areas.
• The worm exploited a buffer overflow vulnerability in
Microsoft SQL Server resolution service (1434/UDP).
• Only 376 bytes and fitted into a single UDP packet.
Slammer (2003)

22. 22
• Infection via Word macro (No FileLess).
• Persistence via Autostart registry key
(HKLMSoftwareMicrosoftWindowsCurrentVersion
Run).
• Minimal first stage: Uses clever rundll32 trick to run
Javascript code.
• Next stages also stored in the registry (encoded). Runs
PowerShell code.
• PowerShell injects a DLL in another process memory,
without touching disk.
Poweliks (2014)

23. 23
Poweliks – Rundll32 trick

24. 24
• Infection via Word macro.
• Dropper and UAC bypass binaries touch disk (not fully
FileLess).
• Then it register the permanent and necessary WMI
classes: event definition, event filter and event
consumer.
• It uses Javascript for payload code in the event
consumer active script.
WMIghost (2014)

25. 25
• PowerShell based RAT.
• It tries to be as FileLess as possible.
• Mostly working from memory only.
• Various options for persistent storage: Registry, ADS,
Eventlog and of course WMI subscriptions.
Empire (2015)

26. 26
• Unknown infection vector.
• Only a few selected hosts were used for on-disk
persistence.
• These hosts injected the malware remotely into other
systems memory.
• For this task the malware gained domain
administrator privileges and then it deployed MSI
packages (via a new service or a scheduled task).
Duqu 2.0 (2015)

28. 28
• First stage: Minimal. Usually a small vbs or js (not
directly PowerShell).
• Second Stage: Main script based on PowerShell. More
complex and powerful logic that injects a binary into
another process.
• Third stage: Binary. Usually a PE DLL payload. More
similar to traditional malware, but never touches disk.
Common FileLess behavior

29. PROCESS

30. 30
• An ideal FileLess pentest operation should cover the following
phases:
1. FileLess Infection.
2. Installation of FileLess Backdoors.
3. Gain FileLess Persistence.
Operation Process

31. 31
• Infection without sending any files.
• Not common. Even known FileLess APT operations
use some kind of files in this stage.
• Preferably, we need to deliver the exploit before the
application layer.
• Inside a stream.
• At the lower network layers (e.g. SMB or SSL exploits).
• Open network services (e.g. Eternalblue).
FileLess Infection

32. 32
• Configuration only backdoors (no code).
• Some popular:
• Create user + Remote exec (Psexec/Sc, WMI, SchTasks,
WinRM, PSRemoting).
• Binary Image Hijack + Remote Desktop.
• Silver/Golden tickets.
• Proxy + Decrease security.
FileLess Backdoors

34. 34
• First stage: Registry Autostart entries.
• Run entries.
• Scheduled tasks.
• Image hijacks.
• WMI.
• Services (not very elegant).
• Usually too noisy! For a human analyst but harder to
detect with automated tools because we are not
using any files.
FileLess persistence

35. CONCLUSIONS

36. 36
• Full pentest operations are possible without using any
files (or almost).
• We need some “resident” artifacts on the system, but
these can be very small and can be easily hidden.
Conclusions

37. 37
• Questions? Comments?
• https://twitter.com/rpinuaga
Thanks

38. 38
• A lot of ideas taken from:
• Casey Smith: https://twitter.com/subtee
• Didier Stevens: https://twitter.com/DidierStevens
• Alex Abramov: https://twitter.com/codereversing
• Rob Fuller: https://twitter.com/mubix
• Cneelis: https://twitter.com/Cneelis
• Matt Nelson: https://twitter.com/enigma0x3
• Matt Graeber: https://twitter.com/mattifestation
• James Foreshaw - https://twitter.com/tiraniddo
Previous research

39. www.prosegur.com

40. DEMO: Proxy + Authenticode
• Convince the user to execute a .REG file
• Configure Proxy
• Disable Authenticode validation
• Wait for EXE download