A is an exploit that takes advantage of a program that accepts input from a client or other software process. It occurs when a program or process attempts to write more data to a fixed-length block of memory, or buffer, than the buffer is allocated to hold. Exploiting a buffer overflow enables an attacker to control, crash or modify the program. There are different categories of buffer overflow attacks. Types of buffer overflow attacksBuffer overflow attacks are categorized based on the location of the buffer in the program or process memory.
There are four types of attacks:. buffer overflow: In this type of attack, the program or process being exploited uses a memory object known as a stack to store user input. The stack is a continuous space in memory used to organize data associated with function calls, including function parameters, local variables and management information, such as frame and instruction pointers. Heap-based attacks: The heap is a memory structure used to manage dynamic memory.
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What is a buffer overflow attack? It’s an attack where a hacker uses the chaos caused by a buffer overflow to insert malicious code into sensitive areas. This allows an attacker to execute any command or change how the program functions. They may also gain access to the user's device. Buffer overflow attack examples. Hackers have been using buffer overflow bugs to cause havoc all over the world for 30 years. What are stack-based buffer overflow attacks? Understanding stack-based overflow attacks involves at least a basic understanding of computer memory. Memory in a computer is simply a storage place for data and instructions—data for storing numbers, letters, images, and anything else, and instructions that tell the computer what to do with the data.
Programmers often use the heap to allocate memory whose size is not known at compile time, where the amount of memory required is too large to fit on the stack or where the memory is intended to be used across function calls. Heap-based attacks flood the memory space reserved for a program or process, but the difficulty involved with performing such an attack makes them rare. Integer overflow attacks: This is when an integer is used in an arithmetic operation, and the result of the calculation is a value in excess of the maximum size of the integer.
Most programming languages define maximum sizes for integers. When those sizes are exceeded, the result may cause an error, or it may return an incorrect result that is 'wrapped around' within the integer length limit.
Unicode overflow attacks: These types of attacks use to create buffer overflows in programs that are expecting all input to be ASCII characters.How buffer overflow attacks workStack-based buffer overflow is the most common of these types of attacks. Normally, the stack is empty until the targeted program requires user input, like a username or password.
At that point, the program writes a return memory address to the stack, and then the user's input is placed on top of it. When the stack is processed, the user's input gets sent to the return address specified by the program.However, a stack has a finite size. The programmer who develops the code must reserve a specific amount of space for the stack. If the user's input is longer than the amount of space reserved for it within the stack and the program does not verify that the input will fit, then the stack will overflow. This in itself isn't a huge problem, but it becomes a huge security hole when it's combined with malicious input.
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For example, suppose a program is waiting for a user to enter her name. Rather than enter the name, the hacker would enter an executable command that exceeds the stack size. The command is usually something short. In a Linux environment, for instance, the command is typically EXEC('sh'), which tells the system to open a command prompt window, known as a root shell in Linux circles.Yet, overflowing the buffer with an executable command doesn't mean that the command will be executed. The attacker must then specify a return address that points to the malicious command. The program partially crashes because the stack overflowed.
It then tries to recover by going to the return address, but the return address has been changed to point to the command specified by the hacker. The hacker must know the address where the malicious command will reside. To get around needing the actual address, the malicious command is often padded on both sides by NOP instructions, a type of pointer. Padding on both sides is a technique used when the exact memory range is unknown. Therefore, if the address the hacker specifies falls anywhere within the padding, the malicious command will be executed. How to perform a buffer overflow attackThe last part of the equation is getting around the executable program's permissions. Most OSes have some sort of mechanism to control the access level of the user who's currently logged on, and executable programs typically require a higher level of permissions.
These programs therefore run either in kernel mode or with permissions inherited from a service account. When a stack overflow attack runs the command found at the new return address, the program thinks it is still running.
This means that the command prompt window that has been opened is running with the same set of permissions as the application that was compromised. Generally speaking, this often means that the attacker will gain full control of the OS.
For other uses, see.In software, a stack buffer overflow or stack buffer overrun occurs when a program writes to a address on the program's outside of the intended data structure, which is usually a fixed-length buffer.Stack buffer overflow bugs are caused when a program writes more data to a buffer located on the stack than what is actually allocated for that buffer. This almost always results in corruption of adjacent data on the stack, and in cases where the overflow was triggered by mistake, will often cause the program to crash or operate incorrectly. Stack buffer overflow is a type of the more general programming malfunction known as (or buffer overrun). Overfilling a buffer on the stack is more likely to derail program execution than overfilling a buffer on the heap because the stack contains the return addresses for all active function calls.Stack buffer overflow can be caused deliberately as part of an attack known as stack smashing. If the affected program is running with special privileges, or accepts data from untrusted network hosts (e.g. A ) then the bug is a potential security vulnerability.
If the stack buffer is filled with data supplied from an untrusted user then that user can corrupt the stack in such a way as to inject executable code into the running program and take control of the process. This is one of the oldest and more reliable methods for to gain unauthorized access to a computer. 'AAAAAAAAAAAAAAAAAAAAx08x35xC0x80' is the first command line argument.Notice in above, when an argument larger than 11 bytes is supplied on the command line foo overwrites local stack data, the saved frame pointer, and most importantly, the return address. When foo returns it pops the return address off the stack and jumps to that address (i.e. Starts executing instructions from that address). Thus, the attacker has overwritten the return address with a pointer to the stack buffer char c12, which now contains attacker-supplied data. In an actual stack buffer overflow exploit the string of 'A's would instead be suitable to the platform and desired function.
If this program had special privileges (e.g. The bit set to run as the ), then the attacker could use this vulnerability to gain superuser privileges on the affected machine.The attacker can also modify internal variable values to exploit some bugs.With this example. Main article:Over the years, a number of schemes have been developed to inhibit malicious stack buffer overflow exploitation. These may usually be classified into three categories:. Detect that a stack buffer overflow has occurred and thus prevent redirection of the instruction pointer to malicious code. Prevent the execution of malicious code from the stack without directly detecting the stack buffer overflow.
Randomize the memory space such that finding executable code becomes unreliable.Stack canaries. Further information:Stack canaries, named for their analogy to a, are used to detect a stack buffer overflow before execution of malicious code can occur. This method works by placing a small integer, the value of which is randomly chosen at program start, in memory just before the stack return pointer. Most buffer overflows overwrite memory from lower to higher memory addresses, so in order to overwrite the return pointer (and thus take control of the process) the canary value must also be overwritten. This value is checked to make sure it has not changed before a routine uses the return pointer on the stack.
This technique can greatly increase the difficulty of exploiting a stack buffer overflow because it forces the attacker to gain control of the instruction pointer by some non-traditional means such as corrupting other important variables on the stack. Nonexecutable stack. Main article:Another approach to preventing stack buffer overflow exploitation is to enforce a memory policy on the stack memory region that disallows execution from the stack (, 'Write XOR Execute'). This means that in order to execute shellcode from the stack an attacker must either find a way to disable the execution protection from memory, or find a way to put her/his shellcode payload in a non-protected region of memory.
This method is becoming more popular now that hardware support for the no-execute flag is available in most desktop processors.While this method definitely makes the canonical approach to stack buffer overflow exploitation fail, it is not without its problems. First, it is common to find ways to store shellcode in unprotected memory regions like the heap, and so very little need change in the way of exploitation.Even if this were not so, there are other ways. The most damning is the so-called method for shellcode creation. In this attack the malicious payload will load the stack not with shellcode, but with a proper call stack so that execution is vectored to a chain of standard library calls, usually with the effect of disabling memory execute protections and allowing shellcode to run as normal. This works because the execution never actually vectors to the stack itself.A variant of return-to-libc is (ROP), which sets up a series of return addresses, each of which executes a small sequence of cherry-picked machine instructions within the existing program code or system libraries, sequence which ends with a return.
These so-called gadgets each accomplish some simple register manipulation or similar execution before returning, and stringing them together achieves the attacker's ends. It is even possible to use 'returnless' return-oriented programming by exploiting instructions or groups of instructions that behave much like a return instruction. Randomization Instead of separating the code from the data, another mitigation technique is to introduce randomization to the memory space of the executing program. Since the attacker needs to determine where executable code that can be used resides, either an executable payload is provided (with an executable stack) or one is constructed using code reuse such as in ret2libc or return-oriented programming (ROP). Randomizing the memory layout will, as a concept, prevent the attacker from knowing where any code is.
However, implementations typically will not randomize everything; usually the executable itself is loaded at a fixed address and hence even when (address space layout randomization) is combined with a nonexecutable stack the attacker can use this fixed region of memory. Therefore, all programs should be compiled with (position-independent executables) such that even this region of memory is randomized. ^ Fithen, William L.; Seacord, Robert (2007-03-27). ^ Dowd, Mark; McDonald, John; Schuh, Justin (November 2006). The Art Of Software Security Assessment. Pp. 169–196. ^ (1996-11-08).
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