The incorrect use of arrays has traditionally been a source of exploitable vulnerabilities. Elements referenced within an array using the subscript operator
 are not checked unless the programmer provides adequate bounds checking. As a result, the expression
array [pos] = value can be used by an attacker to transfer control to arbitrary code.
An attacker who can control the values of both
value in the expression
array [pos] = value can perform an arbitrary write (which is when the attacker overwrites other storage locations with different content). The consequences range from changing a variable used to determine what permissions the program grants to executing arbitrary code with the permissions of the vulnerable process. Arrays are also a common source of buffer overflows when iterators exceed the bounds of the array.
An array is a series of objects, all of which are the same size and type. Each object in an array is called an array element. The entire array is stored contiguously in memory (that is, there are no gaps between elements). Arrays are commonly used to represent a sequence of elements where random access is important but there is little or no need to insert new elements into the sequence (which can be an expensive operation with arrays).
Arrays containing a constant number of elements can be declared as follows:
These statements allocate storage for an array of 12 integers referenced by
array. Arrays are indexed from
n represents an array bound). Arrays can also be declared as follows:
This array is called an incomplete type because the size is unknown. If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. At the end of its initializer list, the array no longer has incomplete type.
Although these declarations work fine when the size of the array is known at compile time, it is not possible to declare an array in this fashion when the size can be determined only at runtime. The C Standard adds support for variable length arrays or arrays whose size is determined at runtime. Before the introduction of variable length arrays in C99, however, these "arrays" were typically implemented as pointers to their respective element types allocated using
malloc(), as shown in this example:
Always check that
malloc() returns a non-null pointer, as per ERR33-C. Detect and handle standard library errors.
It is important to retain any pointer value returned by
malloc() so that the referenced memory may eventually be deallocated. One possible way to preserve such a value is to use a constant pointer:
Below we consider some techniques for array initialization. Both
dat arrays can be initialized as follows:
dis array can also be initialized as follows:
This technique, however, will not work for
dat identifier cannot be incremented (produces a fatal compilation error), as it was declared with type
int * const. This problem can be circumvented by copying
dat into a separate pointer:
p is declared as a pointer to an integer, initialized with the value stored in
dat, and then incremented in the loop. This technique can be used to initialize both arrays, and is a better style of programming than incrementing the original pointer to the array (e.g.,
dis++, in the above example), as it avoids having to reset the pointer back to the start of the array after the loop completes.
Obviously, there is a relationship between array subscripts
 and pointers. The expression
dis[i] is equivalent to
*(dis+i) for all integral values of
i. In other words, if
dis is an array object (equivalently, a pointer to the initial element of an array object) and
i is an integer,
dis[i] designates the
ith element of
dis. In fact, because
*(dis+i) can be expressed as
*(i+dis), the expression
dis[i] can be represented as
i[dis], although doing so is not encouraged. Because array indices are zero-based, the first element is designated as
dis, or equivalently as
*(dis+0) or simply
Arrays are a common source of vulnerabilities in C language programs because they are frequently used but not always fully understood.
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Key here (explains table format and definitions)
|CERT C||CTR00-CPP. Understand when to prefer vectors over arrays||Prior to 2018-01-12: CERT: Unspecified Relationship|
|CWE 2.11||CWE-119, Improper Restriction of Operations within the Bounds of a Memory Buffer||Prior to 2018-01-12: CERT:|
|CWE 2.11||CWE-123, Write-what-where Condition||Prior to 2018-01-12: CERT:|
|CWE 2.11||CWE-125, Out-of-bounds Read||Prior to 2018-01-12: CERT:|
|CWE 2.11||CWE-129, Unchecked array indexing||Prior to 2018-01-12: CERT:|