Buffer Overflow Attacks

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Many attacks target server applications such as those hosted on web servers. Web servers are highly susceptible to several types of attacks. If you’re planning on taking the Security+ exam, you should have a basic understanding of buffer overflow attacks as a common server attack.

For example, can you answer this question?

Q. Checking the logs of a web server, you see the following entry:

198.252.69.129 –[1/Sep/2015:05:20]”GET /index.php?username=ZZZZZZZZZZZZZZZZZZZZBBBBBBBBCCCCCCCHTTP/1.1″ “http://gcgapremium.com/security/” “Chrome31”

Which of the following is the BEST choice to explain this entry?

A. A SQL injection attack

B. A pharming attack

C. A phishing attack

D. A buffer overflow attack

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

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Buffer Overflows and Buffer Overflow Attacks

A buffer overflow occurs when an application receives more input, or different input, than it expects. The result is an error that exposes system memory that would otherwise be protected and inaccessible. Normally, an application will have access only to a specific area of memory, called a buffer. The buffer overflow allows access to memory locations beyond the application’s buffer, enabling an attacker to write malicious code into this area of memory.

Buffer Overflow Attacks

As an example, an application may be expecting to receive a string of 15 characters for a username. If input validation is not used and it receives more than 15 characters, it can cause a buffer overflow and expose system memory. The following HTTP GET command shows an example of sending a long string to the system to create a buffer overflow:

GET /index.php?username=ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ

The buffer overflow exposes a vulnerability, but it doesn’t necessarily cause damage by itself. However, once attackers discover the vulnerability, they exploit it and overwrite memory locations with their own code. If the attacker uses the buffer overflow to crash the system or disrupt its services, it is a DoS attack.

More often, the attacker’s goal is to insert malicious code in a memory location that the system will execute. It’s not easy for an attacker to know the exact memory location where the malicious code is stored, making it difficult to get the computer to execute it. However, an attacker can make educated guesses to get close.

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A popular method that makes guessing easier is with no operation (NOP, pronounced as “no-op”) commands, written as a NOP slide or NOP sled. Many Intel processors use hexadecimal 90 (often written as x90) as a NOP command, so a string of x90 characters is a NOP sled. The attacker writes a long string of x90 instructions into memory, followed by malicious code. When a computer is executing code from memory and it comes to a NOP, it just goes to the next memory location. With a long string of NOPs, the computer simply slides through all of them until it gets to the last one and then executes the code in the next instruction. If the attacker can get the computer to execute code from a memory location anywhere in the NOP slide, the system will execute the attacker’s malicious code.

The malicious code varies. In some instances, the attackers write code to spread a worm through the web server’s network. In other cases, the code modifies the web application so that the web application tries to infect every user who visits the website with other malware. The attack possibilities are almost endless.

Remember this

Buffer overflows occur when an application receives more data than it can handle, or receives unexpected data that exposes system memory. Buffer overflow attacks often include NOP instructions (such as x90) followed by malicious code. When successful, the attack causes the system to execute the malicious code. Input validation helps prevent buffer overflow attacks.

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A buffer overflow attack includes several different elements, but they happen all at once. The attacker sends a single string of data to the application. The first part of the string causes the buffer overflow. The next part of the string is a long string of NOPs followed by the attacker’s malicious code, stored in the attacked system’s memory. Last, the malicious code goes to work.

In some cases, an attacker is able to write a malicious script to discover buffer overflow vulnerabilities. For example, the attacker could use JavaScript to send random data to another service on the same system.

Although error-handling routines and input validation go a long way to prevent buffer overflows, they don’t prevent them all. Attackers occasionally discover a bug allowing them to send a specific string of data to an application causing a buffer overflow. When vendors discover buffer overflow vulnerabilities, they are usually quick to release a patch or hotfix. From an administrator’s perspective, the solution is easy: Keep the systems up to date with current patches.


 

Q. Checking the logs of a web server, you see the following entry:

198.252.69.129 –[1/Sep/2015:05:20]”GET /index.php?username=ZZZZZZZZZZZZZZZZZZZZBBBBBBBBCCCCCCCHTTP/1.1″ “http://gcgapremium.com/security/” “Chrome31”

Which of the following is the BEST choice to explain this entry?

A. A SQL injection attack

B. A pharming attack

C. A phishing attack

D. A buffer overflow attack

Answer is D. A buffer overflow attack sends more data or unexpected data to a system in the hopes of overloading it and causing a problem. In this case, it is sending a series of letters as the username (?username=ZZZZ….), which is likely longer than any expected username. Input validation can prevent this from succeeding.

A SQL injection attack uses specific SQL code, not random letters or characters.

A pharming attack attempts to redirect users from one website to another website.

A phishing attack sends unwanted email to users.

See Chapter 7 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on identifying application attacks.

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