The amount of memory that needs to be reserved is decided at runtime and it is managed by the program, not the operating system. Access to the heap is slower but the space on the heap is only limited by the size of virtual memory. In a simple program, you may want the user to enter an email address. Therefore, you create a string variable. You allocate 64 bytes to the variable because you do not expect an email string to be longer than 64 characters.
However, you trust the user input too much and do not check if the length of the entered string exceeds the size of the buffer. As a result, the user enters characters and the remaining 36 characters are stored in memory allocated to another variable. This causes the value of that variable to change and the behavior of the program to change as well.
In most cases, this leads to a simple memory segmentation fault but it may have more serious consequences. To understand, how this may influence program execution, we shall assume that the vulnerability is a stack overflow and it appears in a C program. A C program uses the stack to store a set of data for every function.
The set of data is called a stack frame and it includes the function identifier, values of local variables, and the return address. Here is a simple source code example to explain how the stack works:. When you run the program, it starts with the main function. The program stores the values of the main function variables on the top of the stack mv1 and mv2. Then the main function calls the func function and it stores the values of its variables on the top of the stack fv1 and fv2.
When the func function finishes running, the top of the stack is forgotten, the current function returns to main , and the program has access to mv1 and mv2 again. Technical Content Writer at Invicti. Drawing on his experience as an IT journalist and technical translator, he does his best to bring web security to a wider audience on the Netsparker blog and website. Get a demo Get a demo. Product Overview Features. Why Netsparker?
Web Security Blog. How buffer overflow attacks work Zbigniew Banach - Thu, 08 Aug -. What causes a buffer overflow? The following is the source code of a C program that has a buffer overflow vulnerability: char greeting[5]; memcpy greeting, "Hello, world! The arguments and the return value of the readIpAddress function During this function call, three different pieces of information are stored side-by-side in computer memory.
Contents of the stack frame when the readIPAddress function is called If the problem was caused by random malformed user input data, the new return address most likely will not point to a memory location where any other program is stored, so the original program will simply crash.
The content of ip. Can you prevent buffer overflows? Since the discovery of the stack buffer overflow attack technique, authors of operating systems Linux, Microsoft Windows, macOS, and others have been trying to find prevention techniques: The stack can be made non-executable, so even if malicious code is placed in the buffer, it cannot be executed.
The operating system may randomize the memory layout of the address space memory space. When malicious code is then placed in a buffer, the attacker cannot predict its address. Other protection techniques for example, StackGuard modify a compiler in such a way that each function calls a piece of code that makes sure the return address has not changed.
Buffer overflows and the Web However, even programmers who use high-level languages should know and care about buffer overflow attacks. How to avoid buffer overflow vulnerabilities Programmers can mitigate the risk of buffer overflow attacks by always validating user input length.
Article written by: Piotr Sobolewski Download. About the Author Zbigniew Banach. Stay up to date on web security trends. Understanding session fixation attacks.
Application Security Bruce Lynch. Application Delivery Data Security. Data Security Application Delivery Application Security. Nik Hewitt. Terry Ray. Latest Articles. App Security Edge Security DDoS Essentials. Essentials Protocols. The computer is brilliant, and if you can change the value of the return address, you can send it wherever you like. Exploits will often write the instructions in the same buffer they overflow and then point execution back to the buffer itself, which allows an attacker to hand a program code and then force it to execute the code.
One caveat is that none of these examples will work on remotely modern operating systems anymore. Operating system developers, application developers, hardware engineers, and even compilers have all reacted and made performing stack overflow attacks much harder. It has been nearly 20 years since the heyday of stack overflow attacks, and there are a lot of protections in place that prevent them from working as well now as they did back then.
Some of these protections include stack canaries, Address Space Layout Randomization ASLR , compiler warnings, and hardware changes to prevent execution of code on the stack. First and foremost, the best defense against stack-based overflow attacks is the use of secure coding practices—mostly through stopping the use of functions that allow for unbounded memory access and carefully calculating memory access to prevent attackers from modifying adjacent values in memory.
Quite simply, if attackers can only access the memory of the variable they intend to change, they cannot affect code execution beyond the expectations of the developer and architect.
Unfortunately, there are thousands of programs that implemented the unsafe, unbounded functions to access memory, and recoding all of them to meet secure coding practices is simply not feasible. For those legacy programs, operating system manufacturers implemented several mitigations to prevent poor coding practices that result in arbitrary code execution.
We can see this in action somewhat in our example by toggling the protections and pushing further in our overflow. One quick change that compilers made in the immediate aftermath of the stack-based attacks was starting to include protections on important pieces of data, such as return addresses.
Since most stack overflow attacks involved overflowing one data location and writing to another, the compiler placed a sacrificial known value between buffers and important data, then the program would check to see whether the sacrificial value had been changed before using the important data.
If that value had been changed, it was likely that the important data was also altered, so execution would stop immediately. One method is by finding the canary value through an unbounded read of memory or guessing. In some cases, canary values are static and predictable.
Once attackers know the canary value, they can replace it in the overwrite. This resulted in a program crash, which is expected when memory structures are corrupted with bad data. This is likely the result of overwriting the return value, and then the processor crashing when trying to access the new memory. Changes to hardware and operating systems took longer, but they did happen. One of the first mitigations introduced by hardware and operating system vendors was the NX, or no-execute bit.
It allowed operating systems to define certain areas of memory as non-executable, and when flagged as such, the CPU would simply not execute that memory. In theory, there should never be executable code on the stack, as it is designed for storing data values only. Based on that understanding, operating systems classified the stack as non-executable, preventing arbitrary code from being placed on the stack and executed.
In addition to bypasses for this mitigation, it quickly became apparent that despite being a poor practice, multiple legitimate programs placed instructions on the stack and executed them, and NX broke them all. That forced operating systems to allow some programs to opt out of the protection, and those programs were well-known to hackers and continued to be targeted.
Aside from those programs that opted out, the most common bypass for NX was through the use of return-oriented programming ROP , which leverages pre-existing code in instructional memory to perform desired tasks.
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