Saturday, February 24, 2018

Using NMAP NSE for Identifying Vulnerabilities

When I sat for the OSCP exam, automated vulnerability scanners were banned. However, the usage of nmap nse script was allowed. Nmap has built-in NSE(Network Scripting Engine) capability for network discovery, backdoor detection, vulnerability detection and even exploitation. Among other tools such as burpsuite, nikto, dirbuster, owasp-zap, I found nmap's nse script insanely useful for vulnerability detection.

Here are some common usage:

$ nmap --script http-vuln-cve2013-0156 www.victim.com -p80

Starting Nmap 6.40 ( http://nmap.org ) at 2018-02-19 04:49 EST
Nmap scan report for www.victim.com (x.x.x.x.x)
Host is up (0.022s latency).
PORT   STATE SERVICE
80/tcp open  http
| http-vuln-cve2013-0156:
|   VULNERABLE:
|   Parameter parsing vulnerabilities in several versions of Ruby on Rails allow object injection, remote command execution and Denial Of Service attacks (CVE-2013-0156)
|     State: VULNERABLE
|     Risk factor: High
|     Description:
|       All Ruby on Rails versions before 2.3.15, 3.0.x before 3.0.19, 3.1.x before 3.1.10, and 3.2.x before 3.2.11 are vulnerable to object injection, remote command execution and denial of service attacks.
|       The attackers don't need to be authenticated to exploit these vulnerabilities.
|     
|     References:
|       https://community.rapid7.com/community/metasploit/blog/2013/01/10/exploiting-ruby-on-rails-with-metasploit-cve-2013-0156
|       https://groups.google.com/forum/?fromgroups=#!msg/rubyonrails-security/61bkgvnSGTQ/nehwjA8tQ8EJ
|_      http://cvedetails.com/cve/2013-0156/

Nmap done: 1 IP address (1 host up) scanned in 1.45 seconds

The above example illustrate the usage of nmap on port 80 to identify a web vulnerability in Ruby on Rails. The usage of metasploit was limited to once per exam. Meaning, you can only use it to exploit 1 vulnerability out of the 5 boxes that you had to root. Think of it like a lifeline if you would. Speaking from my experience, I opine that metasploit is an essential tool in penetration testing. However,  OSCP exam's goal is to teach one to fully understand an exploit's internal working, metaploit unfortunately, makes it too easy to pawn, hence, why its usage is severely limited. You can however, use metaploit auxiliary functions to scan and identify vulnerabilities without any restrictions during the exam.

NSE script can be used to scan for literately hundreds of known vulnerabilities. Caution, it is not comprehensive enough to replace a full fledged commercial vulnerability scanner.

Here's another example:


$ nmap --script http-vuln-cve2017-5638 www.victim.com -p80

Starting Nmap 7.60 ( https://nmap.org ) at 2018-02-19 18:04 +08
Nmap scan report for www.victim.com (x.x.x.x.x)
Host is up (0.29s latency).

PORT   STATE SERVICE
80/tcp open  http
| http-vuln-cve2017-5638:
|   VULNERABLE:
|   Apache Struts Remote Code Execution Vulnerability
|     State: VULNERABLE
|     IDs:  CVE:CVE-2017-5638
|       Apache Struts 2.3.5 - Struts 2.3.31 and Apache Struts 2.5 - Struts 2.5.10 are vulnerable to a Remote Code Execution
|       vulnerability via the Content-Type header.
|         
|     Disclosure date: 2017-03-07
|     References:
|       https://cwiki.apache.org/confluence/display/WW/S2-045
|       http://blog.talosintelligence.com/2017/03/apache-0-day-exploited.html
|_      https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-5638

Nmap done: 1 IP address (1 host up) scanned in 1.63 seconds

The above illustrate the detection of Apache Struts vulnerability. If you are not sure which nse script to use, you may use a wildcard such as:

$ nmap --script "http-vuln-cve*" www.victim.com -p80

Remember to include the "", the command above will scan www.victim.com on port 80 for all http vulnerabilities in nse scripts. More commands can be found  at Nmap's official website.

I personally like to make sure I have all the latest nse scripts loaded before I scan:

$ sudo nmap --script-updatedb

Starting Nmap 7.60 ( https://nmap.org ) at 2018-02-19 18:10 +08
NSE: Updating rule database.
NSE: Script Database updated successfully.
Nmap done: 0 IP addresses (0 hosts up) scanned in 0.61 seconds

Here's an example of me using it to detect shellshock:

$ nmap -p80 --script "http-shellshock*" --script-args uri=/cgi-bin/status www.victim.com

Starting Nmap 7.60 ( https://nmap.org ) at 2018-02-19 18:36 +08
Nmap scan report for www.victim.com (x.x.x.x.x)
Host is up (0.33s latency).

PORT   STATE SERVICE
80/tcp open  http
| http-shellshock:
|   VULNERABLE:
|   HTTP Shellshock vulnerability
|     State: VULNERABLE (Exploitable)
|     IDs:  CVE:CVE-2014-6271
|       This web application might be affected by the vulnerability known as Shellshock. It seems the server
|       is executing commands injected via malicious HTTP headers.
|           
|     Disclosure date: 2014-09-24
|     References:
|       https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-6271
|       http://www.openwall.com/lists/oss-security/2014/09/24/10
|       https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-7169
|_      http://seclists.org/oss-sec/2014/q3/685

Nmap done: 1 IP address (1 host up) scanned in 1.67 seconds


One important note when using the nse script to detect vulnerabilities, is to adjust the --script-args(highlighted above), always read up on the online nse usage to ensure proper argument usage, or else you will end up missing the vulnerability completely when it is staring at you right in the eyes!! Important, do not completely rely on nmap alone, you'll need to enumerate further combining different toolsl! Also, do not completely believe if a particular vuln scanner do not show you the intended results, vulnerability scanners are known for false negatives which often mislead a pentester in ignoring a vulnerability completely. My recommendation would be to combine other tools such as burpsuite, nikto, dirbuster or owasp-zap for enumeration techniques.

Another important point to note, other than the obvious false positive or false negative results,  not all vulnerabilities are detectable via scanners. While majority of CVE tagged vulnerabilities are detectable via automated scanners, there are few vulnerabilities that rely on code logic flaws that can only be discovered via customized queries or/with human interaction or code review. Common misconception that vulnerability scanners are the ultimate tool but it is far from the truth. That was the one most important lessons that I've learnt from offensive-security syllabus.

Friday, February 2, 2018

USB Rubber Ducky

Introduction


The USB Rubber Ducky is a product by Hak5. It is HID(Human Interface Device) a.k.a a keyboard, disguised as a USB thumb drive. Inside this device is a micro SD memory card containing a programmable executable for predefined keystroke commands, it works against different Operating Systems.

Product Details


This product can be purchased at hak5 website for around 45 Dollars (US). There are several components inside the package.


Figure A:Packaging containing the product



Figure B: (From Left to Right) USB housing, HID, Cradle, OTG connector




Figure C: The USB housing is used to disguise the HID to look like a regular USB thumb drive.

Practical Usage


The Rubber Ducky is designed to run a series of preset keystroke commands as directed via a keyboard when plugged into a PC's USB port. For example, it can be programmed to invoke a shutdown sequence, deactivate AntiVirus software or Windows Defender, brute force pin-codes on Android mobile devices. It can also be used to exfiltrate user credentials via SMB, steal documents, download scripts, wipe out drives or anything you can conjure from an attached keyboard. All of this is done using a simple scripting language affectionately called the Ducky script. For practical usage, pentesters can utilize it as part of a social engineering scheme into tricking unsuspecting users in an organization. For example, simply drop a few Rubber Ducks on the office floor and wait for someone to pick it up and insert it into his PC. The rubber ducky was also featured in the award winning TV series, Mr Robot. Where the hacker dropped several devices outside a police station, the next scene showed a police officer plugging it into the station's PC. You guess what happened next...

Programming the HID


Before finding practical usages for it, you will first need to write its payload. Firstly, place the Micro SD memory chip into the cradle as pictured below. 


By placing it in the cradle, the HID is now detected as a regular USB drive, thus making it safe for you to plug it into your PC's USB port.



Figure D: Snapshot of a ducky code. This payload is designed to connect to a netcat listener on a VPS, simply a reverse shell invoked when this USB drive is plugged into the victim's PC. 

Luckily, Hak5 has loads of prewritten ducky code, you may download it or write your own custom code. There are loads of material on the web for those of you interested in developing custom payloads.

Next, the ducky code will need to be compiled using the duckencoder into an 'inject.bin' format that is placed inside the memory card.

$ ./duckencoder.jar -i payloads/reverseshell.txt -o /media/usb/inject.bin
Hak5 Duck Encoder 2.6.3

Loading File ..... [ OK ]
Loading Keyboard File ..... [ OK ]
Loading Language File ..... [ OK ]
Loading DuckyScript ..... [ OK ]
DuckyScript Complete ..... [ OK ]

Figure E: illustrates the compilation of the revershell duckcode into binary format.

After compilation, the memory card can be transferred back to the HID device and disguised as a USB thumb drive. See figure below:

  

Figure F: Illustrates the memory card placed inside the HID device before the housing is completely assembled.


Once that is done, insert the disguised 'USB thumb drive' into the victim's PC and watch it in action:





Figure G: This video demonstrates automatic keystroke injection on a Virtual Guest Host when a USB Rubber Ducky is plugged-in. The window on the right displays a netcat listener on a VPS, waiting to receive its payload which happens to be a Windows cmd.exe reverse shell. 

The USB Rubber Ducky also works on Android Mobile devices, the supplied OTG connector allows it to be plugged into a micro USB port commonly available on Android phones. The Ducky code will need to be adjusted to suit Android keystrokes. It is commonly used to brute force pin-codes to unlock phones.

Caveats

For starters, most canned payloads can be easily detected by commercial AVs. You might want to write your own payload if you are serious about bypassing Windows Defender or commercial AV products. Secondly, the Ducky executes keystrokes only when a user is logged-on his PC. Just as any connected Keyboard, you will first need to be authenticated and able to type commands into the OS. Furthermore, Ducky runs with the same privilege as the logged-on user. Thirdly, on Windows OS, some keystroke commands might require UAC bypass like the 'run as' command.

Conclusion


In a nutshell, the USB Rubber Ducky is a smart programmable automated keystroke injector. Allowing pentesters to exfiltrate data or test an endpoint security policy. Conversely, it can be used for malicious purposes. It exploits the fundamental flaw in the USB design; upon connection into a PC, the USB device is allowed to declare itself as anything (mobile phone, mouse, pendrive, etc), there is no sanity check performed, after all, this device is seen as a harmless keyboard. The moral of the story, never insert an unknown USB device into your PC! You'll never know what you might unravel ;-)