What is Buffer Overflow
Buffers are memory storage regions that temporarily hold data while it is being transferred from one location to another. A buffer overflow (or buffer overrun) occurs when the volume of data exceeds the storage capacity of the memory buffer. As a result, the program attempting to write the data to the buffer overwrites adjacent memory locations.
For example, a buffer for log-in credentials may be designed to expect username and password inputs of 8 bytes, so if a transaction involves an input of 10 bytes (that is, 2 bytes more than expected), the program may write the excess data past the buffer boundary.
Buffer overflows can affect all types of software. They typically result from malformed inputs or failure to allocate enough space for the buffer. If the transaction overwrites executable code, it can cause the program to behave unpredictably and generate incorrect results, memory access errors, or crashes.
A buffer is also a sequential section of memory allocated to contain anything from a character string to an array of integers. A buffer overflow, or buffer overrun, occurs when more data is put into a fixed-length buffer than the buffer can handle. The extra information, which has to go somewhere, can overflow into adjacent memory space, corrupting or overwriting the data held in that space. This overflow usually results in a system crash, but it also creates the opportunity for an attacker to run arbitrary code or manipulate the coding errors to prompt malicious actions.
What is a Buffer Overflow Attack
Attackers exploit buffer overflow issues by overwriting the memory of an application. This changes the execution path of the program, triggering a response that damages files or exposes private information. For example, an attacker may introduce extra code, sending new instructions to the application to gain access to IT systems.
If attackers know the memory layout of a program, they can intentionally feed input that the buffer cannot store, and overwrite areas that hold executable code, replacing it with their own code. For example, an attacker can overwrite a pointer (an object that points to another area in memory) and point it to an exploit payload, to gain control over the program.
CAUSES OF BUFFER OVERFLOW ATTAKS
Coding errors are typically the cause of buffer overflow. Common application development mistakes that can lead to buffer overflow include failing to allocate large enough buffers and neglecting to check for overflow problems. These mistakes are especially problematic with C/C++, which does not have built-in protection against buffer overflows. Consequently, C/C++ applications are often targets of buffer overflow attacks.
Examples Of Buffer Overflow Attack
What are the different types of buffer overflow attacks?
There are a number of different buffer overflow attacks which employ different strategies and target different pieces of code. Below are a few of the most well-known.
- Stack overflow attack – This is the most common type of buffer overflow attack and involves overflowing a buffer on the call stack*.
- Heap overflow attack – This type of attack targets data in the open memory pool known as the heap*.
- Integer overflow attack – In an integer overflow, an arithmetic operation results in an integer (whole number) that is too large for the integer type meant to store it; this can result in a buffer overflow.
- Unicode overflow – A unicode overflow creates a buffer overflow by inserting unicode characters into an input that expect ASCII characters. (ASCII and unicode are encoding standards that let computers represent text. For example the letter ‘a’ is represented by the number 97 in ASCII. While ASCII codes only cover characters from Western languages, unicode can create characters for almost every written language on earth. Because there are so many more characters available in unicode, many unicode characters are larger than the largest ASCII character.)
*Computers rely on two different memory allocation models, known as the stack and the heap; both live in the computer’s RAM. The stack is neatly organized and holds data in a Last-In, First-Out model. Whatever piece of data was most recently placed in the stack will be the first to come out, kind of like how the last bullet inserted into an ammunition magazine will be the first to be fired. The heap is a disorganized pool of extra memory, data does not enter or leave the heap in any particular order. Since accessing memory from the stack is much faster than accessing from the heap, the heap is generally reserved for larger pieces of data or data that a programmer wants to manage explicitly.
In some cases, an attacker injects malicious code into the memory that has been corrupted by the overflow. In other cases, the attacker simply takes advantage of the overflow and its corruption of the adjacent memory. For example, consider a program that requests a user password in order to grant the user access to the system. In the code below, the correct password grants the user root privileges. If the password is incorrect, the program will not grant the user privileges.
printf ("\n Correct Password \n");
pass = 1;
/* Now Give root or admin rights to user*/
printf ("\n Root privileges given to the user \n");
However, there is a possibility of buffer overflow in this program because the gets() function does not check the array bounds.
Here is an example of what an attacker could do with this coding error:
Enter the password :
Root privileges given to the user
In the above example, the program gives the user root privileges, even though the user entered an incorrect password. In this case, the attacker supplied an input with a length greater than the buffer can hold, creating buffer overflow, which overwrote the memory of integer “pass.” Therefore, despite the incorrect password, the value of “pass” became non zero, and the attacker receives root privileges.
Protection Against Buffer Overflow Attacks
Luckily, modern operating systems have runtime protections which help mitigate buffer overflow attacks. Let’s explore 2 common protections that help mitigate the risk of exploitation:
- Address space randomization – Randomly rearranges the address space locations of key data areas of a process. Buffer overflow attacks generally rely on knowing the exact location of important executable code, randomization of address spaces makes that nearly impossible.
- Data execution prevention – Marks certain areas of memory either executable or non-executable, preventing an exploit from running code found in a non-executable area.
Software developers can also take precautions against buffer overflow vulnerabilities by writing in languages that have built-in protections or using special security procedures in their code.
Despite precautions, new buffer overflow vulnerabilities continue to be discovered by developers, sometimes in the wake of a successful exploitation. When new vulnerabilities are discovered, engineers need to patch the affected software and ensure that users of the software get access to the patch.
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