Please join us on our exciting adventure to China where our precious son is waiting for us to bring him home! Psalm 68:6 God sets the lonely in families.
"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 ruleset 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 behaviour. This tool was written in good faith to aid in the testing of network intrusion detection systems, firewalls, and basic TCP/IP stack behaviour." read more...
Ethical hacking is identifying weakness in computer system and/or computer networks and coming with countermeasures that protect the weakness. Ethical hackers must abide by the following rules- 1-Get written permission from the owner of the computer system and/or computer network before hacking. 2-Protect the privacy of the organisation been hacked etc. Ethical Hacking and Ethical Hacker are terms used to describe hacking performed by a company or individual to help identity potential threats on a computer or network.
An Ethical Hacker attempts to byepass system security and search for any weak point that could be exploited by Malicious Hackers.
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:
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: 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:
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.
In Hacking the main focus is over gathering the information about victim or victim's machine. Which will help to find out which type of exploit will works according to the given circumstances. Gathering the network and host information means to find out by which network, the which victim's machine is connected and communicating over the network. Moreover, scanning is also performed for gathering information about open and closed ports. After that they'll able to find the vulnerabilities in the target system and try to get access to the system.
Types Of Scan
As a CEH you should know the scan types and uses:
SYN
SYN scan doesn't complete the TCP three way handshake that is why it is known as a half-open scan. An attacker send a SYN packet to the victim machine if SYN/ACK packet is received back to attacker, then it clarify that the port is listening due to the acknowledgment by the victim that it has completed the connection. While if the attacker is received the RST/ACK packet then it assumed that the port is closed or open.
XMAS
XMAS scan works only on target system that has the RFC 793 development of TCP/IP and it doesn't works against any version of windows. XMAS scan send a packet with by setting up the FIN, URG and PSH flags of the TCP header. The function of this scan is if the port is active there will be no response but if the port is closed the target responds with a RST/ACK packet.
FIN
A FIN scan send a packet by setting up only the FIN flag of the TCP. This scan is similar to XMAS scan. FIN scan receives no response if the port is active while if the port is closed it receives the RST/ACK packet.
NULL
NULL scan is also similar to the XMAS scan. But the only difference is that it sends a packet without setting up the any flag of TCP header. NULL scan receives no response if the port is open but if the port is closed it receives the RST/ACK packet.
IDLE
It is just like spoofing an IP address by sending a SYN packet to the victim's machine to find out which services are available over the system. This scan is completed with the help of another system called as "Zombie" (that is not receiving or transmitting any information).
What is the process of hacking or phases of hacking? Hacking is broken up into six phases:The more you get close to all phases,the more stealth will be your attack.
1-Reconnaissance-This is the primary phase of hacking where hacker tries to collect as much as information as possible about the target.It includes identifying the target,domain name registration records of the target, mail server records,DNS records.The tools that are widely used in the process is NMAP,Hping,Maltego, and Google Dorks.
2-Scanning-This makes up the base of hacking! This is where planning for attack actually begins! The tools used in this process are Nessus,Nexpose,and NMAP. After reconnaissance the attacker scans the target for services running,open ports,firewall detection,finding out vulnerabilities,operating system detection.
3-Gaining Access-In this process the attacker executes the attack based on vulnerabilities which were identified during scanning! After the successful, he get access to the target network or enter in to the system.The primary tools that is used in this process is Metasploit.
4-Maintaining Access-It is the process where the hacker has already gained access in to a system. After gaining access the hacker, the hacker installs some backdoors in order to enter in to the system when he needs access in this owned system in future. Metasploit is the preffered toll in this process.
5-Clearning track or Covering track-To avoid getting traced and caught,hacker clears all the tracks by clearing all kinds of logs and deleted the uploaded backdoor and anything in this process related stuff which may later reflect his presence!
6-Reporting-Reporting is the last step of finishing the ethical hacking process.Here the Ethical Hacker compiles a report with his findings and the job that was done such as the tools used,the success rate,vulnerabilities found,and the exploit process.
We're a busy famiy of six on a great adventure to China to bring home our long awaited son, Liam. He is currently in the Yangquan City Orphanage in Shanxi,China. We hope to travel this February to bring him home.
