In information security and programming, a buffer overflow, or buffer overrun, is an anomaly where a program, while writing data to a buffer, overruns the buffer’s boundary and overwrites adjacent memory locations.
Buffers are areas of memory set aside to hold data, often while moving it from one section of a program to another, or between programs. Buffer overflows can often be triggered by malformed inputs; if one assumes all inputs will be smaller than a certain size and the buffer is created to be that size, then an anomalous transaction that produces more data could cause it to write past the end of the buffer. If this overwrites adjacent data or executable code, this may result in erratic program behavior, including memory access errors, incorrect results, and crashes.
Exploiting the behavior of a buffer overflow is a well-known security exploit. On many systems, the memory layout of a program, or the system as a whole, is well defined. By sending in data designed to cause a buffer overflow, it is possible to write into areas known to hold executable code and replace it with malicious code, or to selectively overwrite data pertaining to the program’s state, therefore causing behavior that was not intended by the original programmer. Buffers are widespread in operating system (OS) code, so it is possible to make attacks that perform privilege escalation and gain unlimited access to the computer’s resources. The famed Morris worm in 1988 used this as one of its attack techniques.
Programming languages commonly associated with buffer overflows include C and C++, which provide no built-in protection against accessing or overwriting data in any part of memory and do not automatically check that data written to an array (the built-in buffer type) is within the boundaries of that array. Bounds checking can prevent buffer overflows, but requires additional code and processing time. Modern operating systems use a variety of techniques to combat malicious buffer overflows, notably by randomizing the layout of memory, or deliberately leaving space between buffers and looking for actions that write into those areas (“canaries”).
Various techniques have been used to detect or prevent buffer overflows, with various tradeoffs. The most reliable way to avoid or prevent buffer overflows is to use automatic protection at the language level. This sort of protection, however, cannot be applied to legacy code, and often technical, business, or cultural constraints call for a vulnerable language. The following sections describe the choices and implementations available.
Choice of programming language
Assembly and C/C++ are popular programming languages that are vulnerable to buffer overflow, in part because they allow direct access to memory and are not strongly typed. C provides no built-in protection against accessing or overwriting data in any part of memory; more specifically, it does not check that data written to a buffer is within the boundaries of that buffer. The standard C++ libraries provide many ways of safely buffering data, and C++’s Standard Template Library (STL) provides containers that can optionally perform bounds checking if the programmer explicitly calls for checks while accessing data. For example, a
vector‘s member function
at() performs a bounds check and throws an
out_of_range exception if the bounds check fails. However, C++ behaves just like C if the bounds check is not explicitly called. Techniques to avoid buffer overflows also exist for C.
Languages that are strongly typed and do not allow direct memory access, such as COBOL, Java, Python, and others, prevent buffer overflow from occurring in most cases. Many programming languages other than C/C++ provide runtime checking and in some cases even compile-time checking which might send a warning or raise an exception when C or C++ would overwrite data and continue to execute further instructions until erroneous results are obtained which might or might not cause the program to crash. Examples of such languages include Ada, Eiffel, Lisp, Modula-2, Smalltalk, OCaml and such C-derivatives as Cyclone, Rust and D. The Java and .NET Framework bytecode environments also require bounds checking on all arrays. Nearly every interpreted language will protect against buffer overflows, signaling a well-defined error condition. Often where a language provides enough type information to do bounds checking an option is provided to enable or disable it. Static code analysis can remove many dynamic bound and type checks, but poor implementations and awkward cases can significantly decrease performance. Software engineers must carefully consider the tradeoffs of safety versus performance costs when deciding which language and compiler setting to use.
Use of safe libraries
The problem of buffer overflows is common in the C and C++ languages because they expose low level representational details of buffers as containers for data types. Buffer overflows must thus be avoided by maintaining a high degree of correctness in code which performs buffer management. It has also long been recommended to avoid standard library functions which are not bounds checked, such as
strcpy. The Morris worm exploited a
gets call in fingerd.
Well-written and tested abstract data type libraries which centralize and automatically perform buffer management, including bounds checking, can reduce the occurrence and impact of buffer overflows. The two main building-block data types in these languages in which buffer overflows commonly occur are strings and arrays; thus, libraries preventing buffer overflows in these data types can provide the vast majority of the necessary coverage. Still, failure to use these safe libraries correctly can result in buffer overflows and other vulnerabilities; and naturally, any bug in the library itself is a potential vulnerability. “Safe” library implementations include “The Better String Library”, Vstr and Erwin. The OpenBSD operating system’s C library provides the strlcpy and strlcat functions, but these are more limited than full safe library implementations.
In September 2007, Technical Report 24731, prepared by the C standards committee, was published; it specifies a set of functions which are based on the standard C library’s string and I/O functions, with additional buffer-size parameters. However, the efficacy of these functions for the purpose of reducing buffer overflows is disputable; it requires programmer intervention on a per function call basis that is equivalent to intervention that could make the analogous older standard library functions buffer overflow safe.
Buffer overflow protection
Main article: Buffer overflow protection
Buffer overflow protection is used to detect the most common buffer overflows by checking that the stack has not been altered when a function returns. If it has been altered, the program exits with a segmentation fault. Three such systems are Libsafe, and the StackGuard and ProPolice gcc patches.
Microsoft’s implementation of Data Execution Prevention (DEP) mode explicitly protects the pointer to the Structured Exception Handler (SEH) from being overwritten.
Stronger stack protection is possible by splitting the stack in two: one for data and one for function returns. This split is present in the Forth language, though it was not a security-based design decision. Regardless, this is not a complete solution to buffer overflows, as sensitive data other than the return address may still be overwritten.
Buffer overflows work by manipulating pointers, including stored addresses. PointGuard was proposed as a compiler-extension to prevent attackers from being able to reliably manipulate pointers and addresses. The approach works by having the compiler add code to automatically XOR-encode pointers before and after they are used. Theoretically, because the attacker does not know what value will be used to encode/decode the pointer, he cannot predict what it will point to if he overwrites it with a new value. PointGuard was never released, but Microsoft implemented a similar approach beginning in Windows XP SP2 and Windows Server 2003 SP1. Rather than implement pointer protection as an automatic feature, Microsoft added an API routine that can be called. This allows for better performance (because it is not used all of the time), but places the burden on the programmer to know when it is necessary.
Because XOR is linear, an attacker may be able to manipulate an encoded pointer by overwriting only the lower bytes of an address. This can allow an attack to succeed if the attacker is able to attempt the exploit multiple times or is able to complete an attack by causing a pointer to point to one of several locations (such as any location within a NOP sled). Microsoft added a random rotation to their encoding scheme to address this weakness to partial overwrites.
Executable space protection
Executable space protection is an approach to buffer overflow protection which prevents execution of code on the stack or the heap. An attacker may use buffer overflows to insert arbitrary code into the memory of a program, but with executable space protection, any attempt to execute that code will cause an exception.
Some CPUs support a feature called NX (“No eXecute”) or XD (“eXecute Disabled”) bit, which in conjunction with software, can be used to mark pages of data (such as those containing the stack and the heap) as readable and writable but not executable.
Some Unix operating systems (e.g. OpenBSD, macOS) ship with executable space protection (e.g. W^X). Some optional packages include:
- Exec Shield
Newer variants of Microsoft Windows also support executable space protection, called Data Execution Prevention. Proprietary add-ons include:
Executable space protection does not generally protect against return-to-libc attacks, or any other attack which does not rely on the execution of the attackers code. However, on 64-bit systems using ASLR, as described below, executable space protection makes it far more difficult to execute such attacks.
Address space layout randomization
Main article: Address space layout randomization
Address space layout randomization (ASLR) is a computer security feature which involves arranging the positions of key data areas, usually including the base of the executable and position of libraries, heap, and stack, randomly in a process’ address space.
Randomization of the virtual memory addresses at which functions and variables can be found can make exploitation of a buffer overflow more difficult, but not impossible. It also forces the attacker to tailor the exploitation attempt to the individual system, which foils the attempts of internet worms. A similar but less effective method is to rebase processes and libraries in the virtual address space.
Deep packet inspection
The use of deep packet inspection (DPI) can detect, at the network perimeter, very basic remote attempts to exploit buffer overflows by use of attack signatures and heuristics. These are able to block packets which have the signature of a known attack, or if a long series of No-Operation instructions (known as a NOP-sled) is detected, these were once used when the location of the exploit’s payload is slightly variable.
Packet scanning is not an effective method since it can only prevent known attacks and there are many ways that a NOP-sled can be encoded. Shellcode used by attackers can be made alphanumeric, metamorphic, or self-modifying to evade detection by heuristic packet scanners and intrusion detection systems.
Checking for buffer overflows and patching the bugs that cause them naturally helps prevent buffer overflows. One common automated technique for discovering them is fuzzing. Edge case testing can also uncover buffer overflows, as can static analysis. Once a potential buffer overflow is detected, it must be patched; this makes the testing approach useful for software that is in development, but less useful for legacy software that is no longer maintained or supported.