What Is Buffer Overflow? Attacks and Risks Explained

Hackers can take over a computer system with something as simple as a line of code. They achieve this by exploiting buffer overflows, which occur when a fixed-length block of memory tries to store more data than its allocated space can hold. This causes the data to spill over into adjacent memory locations, disrupting their operations. In this article, learn what a buffer overflow attack is, how they work, and how to protect against them.

What is a buffer overflow?

Buffer overflow refers to a common cybersecurity vulnerability that happens when a program or process writes more data into a buffer than it can handle. In other words, too much data passes into memory that doesn't have enough space. Then, the extra data ends up overflowing into adjacent memory locations. The excess data therefore overwrites any data previously stored there.

A buffer is a temporary area for data storage. Buffers have a fixed size and only store a defined amount of data. Any excess data will overwrite the memory locations adjacent to the destination buffer.

Buffer overflows are one of the most common software weaknesses present today. They can happen inadvertently or when a cyber attacker causes it. Either way, it opens opportunities for attack.

One real-life example of a buffer overflow attack is the 1988 Morris Worm. The Morris Worm was a computer worm that exploited a buffer overflow vulnerability in Unix programs. The worm infected over 60,000 computers worldwide, causing an estimated $10 billion in damages.

Buffer overflows are likely to affect all types of software. They result from malformed inputs, code errors, or failure to allocate enough space for the buffer and checking for overflow issues.

When these overflows happen, the program behaves unpredictably, leading to incorrect and data corruption. These overflows can be avoided if the program has built-in instructions to discard data when too much is sent to a memory buffer.

What is a buffer overflow attack?

A buffer overflow attack is a common cyberattack that exploits buffer overflow vulnerabilities to gain control of a computer system. It takes place when a program writes data outside the bounds of a buffer, overwriting portions of memory connected to the buffer space.

Cybercriminals exploit buffer overflow problems by overwriting the memory of an application. They craft malicious input that causes a program to overflow a buffer and then inject their own code into the program. Once your program runs this code, the attacker gains unauthorized access to a system.

What's the purpose of a buffer overflow attack?

Attackers often use buffer overflow attacks to achieve the following:

• Disruption of operations. Attacks can disrupt the operations of a computer system, causing the system to become unresponsive or glitchy.
• Denial of Service (DoS) attacks. Within an application's memory space, there are pointers, code, and other reliable data that are critical to the program's ability to execute. Overwriting this piece of data could cause the program to crash, leading to a DoS attack.
• System manipulation. These types of attacks alter the flow of an application by overwriting parts of its memory. In doing so, the attacker causes the system to behave in ways it wasn't intended to, such as bypassing security measures.
• Execution of arbitrary code. Unauthorized users can overflow the buffer with their own malicious code and cause the application to execute the code in order to gain control over the system. This allows them to do almost anything, from creating a backdoor for future access to launching other attacks against other systems.
• Unauthorized access to systems. In some cases, attackers use buffer overflow to escalate privileges within a system. Malicious actors could have the same permissions as a system administrator, allowing them to perform activities that they shouldn't be able to.

Buffer overflow attacks are a serious threat to software systems. Therefore, understanding how they work is crucial for developing secure applications.

How a buffer overflow attack works

A buffer overflow attack can lead to memory corruption. This potentially allows a threat actor to send arbitrary code to a program. The attack typically involves exploiting vulnerabilities in the way that a program handles input, such as insufficient input validation or poor buffer size management.

By carefully crafting malicious input, attackers manipulate the memory layout of a program and overwrite critical data, including return addresses and function pointers. That way, they are able to redirect the program's execution flow and run their own malicious code instead.

Steps of a buffer overflow attack

For better understanding, the stages involved in a buffer overflow attack are as follows.

1. Identifying the vulnerable program. First, an attacker must discover a program with a buffer overflow vulnerability. This often involves testing various inputs to see if they can cause the program to behave unexpectedly.
2. Crafting malicious input. The attacker creates arbitrary code. It's often designed to exploit the vulnerability found in the program. This malicious input is larger than the buffer can accommodate.
3. Sending malicious input. The input, which is more than a buffer can handle, goes to the program over a network connection or by tricking the user into entering malicious input into the program.
4. Memory overflow. If the program does not properly validate inputs, it allows the input sent to overflow a buffer. This overflow causes data to spill into adjacent memory locations.
5. Executing arbitrary code. With control of the program's execution flow, the attacker potentially sets new values for the return pointer, which is the address where the program should go next. This all happens when the attacker manages to change the return pointer to point to a location of their choosing.
6. Gaining access or control. Most buffer overflow attacks aim to gain unauthorized control over a system. After executing the arbitrary command, the attacker gains administrative rights or other control.

Suppose a web application is waiting for users to upload images to their profiles. The application stores these images in a designated server folder. Rather than upload the images, an attacker can manipulate the image upload process by entering an executable command that exceeds the stack size.

They might upload an image file with an excessively long file name designed to exceed the allocated file name buffer size. This then successfully causes a buffer attack.

Types of buffer overflow attacks

The consequences of buffer overflow vulnerabilities can be severe. Hacker attack techniques vary between operating systems. They involve not only overriding the bounds of the buffers, but also violating programming languages to trigger additional actions.

If an attacker controls what data is written beyond the buffer's boundaries, then they can manipulate the program's behavior.

Buffer overflows are mostly categorized into stack-based overflows or heap-based overflows. Both are found in the device's RAM.

Here are the various types of buffer overflow attacks.

Stack-based attacks

A stack-based attack leverages a vulnerability in certain software programs that handle data input. The stack adheres to a last-in, first-out memory architecture. It's a fixed block of memory that stores data associated with function calls, such as function parameters, local function variables, and management information, including instruction pointers and frames.

It's worth noting that the size of a stack is typically defined at the start of a program. Therefore, a specific amount of space must be reserved for the stack. If the user's input exceeds the amount of space in the memory or stack, the stack overflows. This then causes the entire program to crash.

Stack attacks are the most common way to exploit buffer overflow issues.

Heap-based attacks

Unlike stack-based overflow, which targets the stack, heap-based attacks target dynamically allocated memory on the heap. This happens when a buffer in the heap - which is a region of a computer's memory space used for dynamic memory allocation - is overflowed. The attacker can overwrite data in the heap, potentially leading to arbitrary code execution.

Heap-based vulnerabilities, such as the zero-day vulnerability reported by Google Chrome, can lead to unauthorized code execution if exploited. However, these vulnerabilities are difficult to exploit, so bad actors often use stack attacks rather than heap-based attacks.

Integer overflow attacks

This type of attack arises when an integer value is incremented to a value too large to store in the associated representation. When this type of overflow occurs, the value can wrap to become a very small or negative number, leading to unexpected behavior.

Format string attacks

Format string attacks can occur when a program uses user-supplied format strings as part of a formatted output function. A format string attack involves misusing format functions like printf and sprintf in C programming language. If the format string is not properly sanitized, an attacker will exploit it to read arbitrary memory locations.

Unicode overflow attacks

Unicode overflow attacks happen when software inadequately processes unicode characters, leading to buffer overflows. It's worth noting that Unicode uses more bytes than the American Standard Code for Information Interchange (ASCII) for character representation. Any software that doesn't handle this appropriately becomes vulnerable to an overflow attack.

Who is vulnerable to buffer overflow attacks?

Coding languages are particularly vulnerable to buffer overflow attacks. Languages such as C and C++ don't have protections against overwriting or accessing data in their memory.

They also don't perform automatic bounds checking on buffers. It's up to the programmer to ensure that the data written to a buffer does not exceed the buffer size.

Windows, MacOS, and Linux all use code written in C and C++. Malicious actors use common programming constructs to manipulate memory directly.

To avoid buffer overflow vulnerabilities, use modern programming languages like C#, Java and Perl, Python, and COBOL. These languages reduce the chance of coding errors. They use built-in safety mechanisms to mitigate a buffer overflow, but can't prevent it altogether. Ultimately, buffer overflow can happen in any programming environment as long as there's direct memory access.

How to prevent buffer overflow attacks

Preventing a buffer overflow attack is easier than recovering from one. Thankfully, there are many ways to keep your computer safe. They include:

• Always validating and sanitizing user inputs. Input validation ensures that the input data meets the expected format and constraints, while sanitation removes any potentially malicious characters from the input. Implementing the two effectively reduces the risk of buffer overflow vulnerabilities.
• Managing buffer size. This involves carefully allocating the memory enough space for buffers. Developers should always ensure that the size of the buffer is sufficient to hold the expected data and any input exceeding the buffer's capacity.
• Using OS runtime protections like ASLR. Address space layout randomization (ASLR) is a security technique used to prevent buffer overflow attacks. It works by randomizing the memory addresses where system libraries, executable code, and data are loaded. By enabling ASLR, the attackers won't predict the location of vulnerable code.
• Using memory-safe programming languages. Some high-level programming languages, like C++, are more vulnerable to buffer overflows than others. if you're developing a security-sensitive application, opt for languages with strong memory protection.
• Avoiding vulnerable functions. Some functions aren't safe, as spammers can use them to carry out buffer overflow attacks. Choose safer alternatives to protect yourself.
• Checking bounds. When writing code that accesses buffers, check the bounds of the buffer before you access it. This helps ensure you don't accidentally write outside of the bounds of the buffer.
• Using safe coding practices. Check the return value of functions that allocate memory often and avoid using pointers unless absolutely necessary.

Additionally, make sure to keep your software up to date. Regardless of what you're trying to protect your device or network from, ensuring that your software has the latest updates and patches is a crucial aspect of computer security.

Frequently asked questions

What is a buffer?

A buffer is an area of memory used to temporarily store data while it's being transferred from one place to another.

Why is buffer overflow a vulnerability?

Buffer overflow is a vulnerability because it allows attackers to overwrite a program's memory, potentially leading to unauthorized access or code execution.

What are well-known buffer overflow attacks?

The Morris Worm, released in 1988, and the SQL Slammer, released in 2003, are two famous buffer overflow attacks.