Ask and you shall receive (Part 1)


It is really annoying not being able to learn basic information about penetration testing without struggling to locate the proper information.  This post is about delivering the payload the proper way, the bible is says ask and you shall receive (again this is basic hacking methodology that most penetration testers don't use). So the question I am going to answer in this post is how can someone deliver his or her exploit payload in order to:

A. Bypass:
  1. Network Based Intrusion Prevention (IPS).
  2. Network Based Intrusion Detection  (IDS).
  3. Host Based Intrusion Prevention (IPS).
  4. Host Based Intrusion Detection (IDS).
  5. Network Firewall Device.
  6. Web Application Firewalls.
  7. Deep Content Inspection Devices. 
B. Deliver in short amount of time to: 
  1. Large scale networks
  2. Low bandwidth networks (happening not so often).      
So imagine that your client says to you that you have to test 100 IP's in lets say three days (how can you test for conficker vulnerability) or that you have an internal penetration test and all hosts have host based Intrusion Prevention software, how do you try to bypass the network filtering? Well it is very simple you treat the delivery in a different way. You have to take into consideration the network stack as a separate entity from the vulnerable process. For the sake of this post I am going to use an exploit developed in a previous post. But first a little about the stack and IPS/IDS, on how it works.

A little about data Encapsulation and the TCP/IP Protocol Stack

The packet is the basic unit of information that is transferred across a network. The packet consists, at a minimum, of a header with the sending and receiving hosts' addresses, and a body with the data to be transferred. As the packet travels through the TCP/IP protocol stack, the protocols at each layer either add or remove fields from the basic header. When a protocol on the sending host adds data to the packet header, the process is called data encapsulation. Moreover, each layer has a different term for the altered packet, in our example we are going to use rlogin program and as shown in the following figure that is how data are treated.

Note: As you can see the data (in our case the exploit payload) are broken in packets, segments, datagram, frames and then reassembled again in frames, datagrams, segment and packets and finally into data in it's pure original form.

A little about how IPS/IDS works

Intrusion prevention systems (IPS), also known as intrusion detection and prevention systems (IDPS), are network security appliances that monitor network and/or system activities for malicious activity. The main functions of intrusion prevention systems are to identify malicious activity, log information about said activity, attempt to block/stop activity, and report activity.

Intrusion prevention systems are considered extensions of intrusion detection systems because they both monitor network traffic and/or system activities for malicious activity. The main differences are, unlike intrusion detection systems, intrusion prevention systems are placed in-line and are able to actively prevent/block intrusions that are detected.

More specifically, IPS can take such actions as sending an alarm, dropping the malicious packets, resetting the connection and/or blocking the traffic from the offending IP address. An IPS can also correct Cyclic Redundancy Check (CRC) errors, unfragment packet streams, prevent TCP sequencing issues, and clean up unwanted transport and network layer options.

Intrusion prevention systems classification  types

Intrusion prevention systems are classified in 4 major categories: 
  1. Network-based intrusion prevention system (NIPS): monitors the entire network for suspicious traffic by analyzing protocol activity.
  2. Wireless intrusion prevention systems (WIPS): monitors a wireless network for suspicious traffic by analyzing wireless networking protocols.
  3. Network behavior analysis (NBA): examines network traffic to identify threats that generate unusual traffic flows, such as distributed denial of service (DDoS) attacks, certain forms of malware, and policy violations.
  4. Host-based intrusion prevention system (HIPS): an installed software package which monitors a single host for suspicious activity by analyzing events occurring within that host.
IPS/IDS detection methods

The majority of intrusion prevention systems utilize one of three detection methods: signature-based, statistical anomaly-based, and stateful protocol analysis.
  1. Signature-Based Detection: This method of detection utilizes signatures, which are attack patterns that are preconfigured and predetermined. A signature-based intrusion prevention system monitors the network traffic for matches to these signatures. Once a match is found the intrusion prevention system takes the appropriate action. Signatures can be exploit-based or vulnerability-based. Exploit-based signatures analyze patterns appearing in exploits being protected against, while vulnerability-based signatures analyze vulnerabilities in a program, its execution, and conditions needed to exploit said vulnerability. 
  2. Statistical anomaly-based detection: This method of detection baselines performance of average network traffic conditions. After a baseline is created, the system intermittently samples network traffic, using statistical analysis to compare the sample to the set baseline. If the activity is outside the baseline parameters, the intrusion prevention system takes the appropriate action.
  3. Stateful Protocol Analysis Detection: This method identifies deviations of protocol states by comparing observed events with “predetermined profiles of generally accepted definitions of benign activity.”

Yes but how how?

All this information is pretty cute but how am I going to use this knowledge to exploit my target. Well all this information is not so useless if you have the proper tool kit and right mindset. You have to think and it will all become clear to you it is like matrix. But first lets analyze our exploit scenario, we have an attacker machine, a victim machine with lets say a network filtering entity. The following figure explains conceptually the attack scenario:

Note: See how the Network filter is placed in the victim machine and reassembles all the packets before getting injected to the vulnerable process, which in our example is Free Float FTP Server v1.0. Check out that the Network filter in our example is set in the Transport and Internet Layer. I am placing the bind shell box to the attackers TCP/IP stack because conceptually the payload is going to be sent through the attacker stack, meaning that what you see as a port is behind your stack. 

Redefining buffer overflow  concept (it is all in your mind)

Well if someone asks you what is a buffer overflow you tell him the process of injecting a string, just a string nothing else, when you do an SQL injection you sent a string from your browser to a vulnerable Web Application database, buffer overflows is the same thing, just a set of characters and you can own the world. Now the string that you sent gains (the buffer overflow) a different meaning when injected to the vulnerable process. So lets revisit the exploit developed in previous post called Over The Flow the simple way:

Note: What we do in the exploit shown above is we open a raw socket using Python to sent our exploit to the vulnerable process then connect to the target and sent the exploit.

Converting the buffer to a simple file

Instead of opening a socket to sent the file we will use another delivery method such as the Netcat tool, but first we export the buffer overflow to a file:

Note: See how easy it was to convert the buffer overflow to a file. You should also take into consideration the fact that the file is a single line starting with the ftp USER variable and ending with a CRLLF sequence which designates the end of the file.

So the final file format is:

USER + buffer overflow + CRLF sequence

Note: Simplistically speaking this is a generic form of how all buffer overflows look like when reassembled and before injected to the vulnerable process. This can get you a feeling now how IPS signatures are created.

Netcat as a payload launcher

The rest of the post is easy to describe. Now we can sent our payload using the proper command syntax of Netcat to transfer the payload to the target machine. We just have to issue the following command:

nc 21 > mybuffer.txt

Note: And boom you got your shell back, that easy (the command syntax is based in unix-like systems such as Linux). This exploit attempt was reproduced in numerous penetration test so you better be sure that is works because it is a real threat. Image what are the possibilities in real world hacks.

Rapid payload delivery

Now that we know how to deliver a payload with Netcat and we can build a Python server component that launches multiple thread connections waiting for reverse shells to initiate a connection to your attacking machine. A conceptual representation is shown below in this figure.

Note: See how the vulnerable process lances the reverse shell and the multi threaded Python listener accepts the remote connections amazing is not? Again imagine the possibilities of exploitation when using a stable exploit that does not crush the service and remains undetected or how can someone write a costume IPS signature to identify the reverse shell connection. By the way this is a good way to test your IPS heuristics behavior. This type of IPS counter measure can be easily defeated using an IP list randomizer (within your shellcode) and a bot net from compromised machines with a seemingly random IP list as reverse shell receivers (you should understand by now that this is a very realistic scenario already used from conflikor). 

Delivering the payload the right way

A better approach would be to use unicornscan instead of netcat, hping2, sbd, nmap or hping3. With unicornscan you cam easily fool host based signature network filters.Unicornscan is an attempt at a User-land Distributed TCP/IP stack for information gathering and correlation. It is intended to provide a researcher a interface for introducing a stimulus into and measuring a response from a TCP/IP enabled device or network.

Some of its features include asynchronous stateless TCP scanning with all variations of TCP flags, asynchronous stateless TCP banner grabbing, and active/passive remote OS, application, and component identification by analyzing responses. It allows you to specify more information, such as source port, packets per second sent, and randomization of source IP information, if needed. For this reason, it may not be the best choice for initial port scans; rather, it is more suited for later “fuzzing” or experimental packet generation and detection. A much more interesting tool for this job would be fragroute.

Fragroute was created by Dug Song (@dugsong on twitter.) It has the ability to take traffic destined for a particular host and do all sorts of things with it. It can delay, duplicate, drop, fragment, overlap, reorder, etc. It was created primarily to test network based intrusion detection systems, firewalls, and IP stack behavior.

More specifically fragroute intercepts, modifies, and rewrites egress traffic destined for a specified host, implementing most of the attacks described in the Secure Networks "Insertion, Evasion, and Denial of Service: Eluding Network Intrusion Detection" paper of January 1998.

It features a simple rule set language to delay, duplicate, drop, fragment, overlap, print, reorder, segment, source-route, or otherwise monkey with all outbound packets destined for a target host, with minimal support for randomized or probabilistic behavior.This tool was written in good faith to aid in the testing of network intrusion detection systems, firewalls, and basic TCP/IP stack behavior.

Examples of using other tools

Below you can see multiple examples and get multiple ideas on how to by pass network and host based IPS/IDS.

hping3 –file mybuffer.txt –data 21 

Note: You can use hping3  to transfer the buffer over flow over TCP/IP stack and play wth the.

fragroute -f frag-3 

Note: You can use fragroute to transfer the buffer over flow over and test your IPS signatures.

sbd 21 < mybuffer.txt 

Note: You can use sbd to transfer the buffer over flow over an encrypted connection and bypass reverse SSL proxies.

nc 21 > mybuffer.txt

Note: You can use nc to transfer the buffer over flow over TCP/IP stack in a simple penetration test without any filtering. 

netcat6 21 > mybuffer.txt

Note: You can use netcat6 to transfer the buffer over flow over IPv6 forgotten services that system administrators don't think that can be done anything.  

cryptcat 21 < mybuffer.txt 

Note: You can use sbd to transfer the buffer over flow over an encrypted connection and bypass reverse SSL proxies.


In this post we redefined what a buffer overflow is and we showed alternative ways to deliver the payload.

  1. http://docs.oracle.com/cd/E19http://docs.oracle.com/cd/E19683-01/806-4075/ipov-32/index.html
  2. http://www.nask.pl/run/n/IDS_IPS_work
  3. http://en.wikipedia.org/wiki/Intrusion_prevention_system
  4. http://oreilly.com/pub/h/1058 
  5. http://www.downloadnetcat.com/ 
  6. http://www.compsec.org/security/index.php/port-scanners/93-port-scanners-unicorn-scan.html
  7. http://pentestmonkey.net/cheat-sheet/shells/reverse-shell-cheat-sheet 
  8. http://linux.softpedia.com/get/System/Networking/sbd-14900.shtml 
  9. http://www.radarhack.com/tutorial/shadowinteger_backdoor.pdf 
  10. http://www.dest-unreach.org/socat/doc/socat.html#FILES 
  11. http://www.dest-unreach.org/socat/ 
  12. http://www.hellboundhackers.org/articles/634-cryptcat:-advanced-usage.html