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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 pos and 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:

enum { ARRAY_SIZE = 12 };
int array[ARRAY_SIZE];

These statements allocate storage for an array of 12 integers referenced by array. Arrays are indexed from 0..n-1 (where n represents an array bound). Arrays can also be declared as follows:

int array[];

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.

int array[] = { 1, 2 };

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:

int *dis = (int *)malloc(ARRAY_SIZE * sizeof(int));

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:

int * const dat = (int * const)malloc(
  ARRAY_SIZE * sizeof(int)
);
/* ... */
free(dat);

Below we consider some techniques for array initialization.  Both dis and dat arrays can be initialized as follows:

for (i = 0; i < ARRAY_SIZE; i++) {
   dis[i] = 42; /* Assigns 42 to each element of dis */ 
   dat[i] = 42; /* Assigns 42 to each element of dat */
}

The dis array can also be initialized as follows:

for (i = 0; i < ARRAY_SIZE; i++) {
   *dis = 42;
   dis++;
}
dis -= ARRAY_SIZE;

This technique, however, will not work for dat.  The 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:

int *p = dat;
for (i = 0; i < ARRAY_SIZE; i++)  {
  *p = 42; /* Assigns 42 to each element */
  p++;
}

The variable 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[0], or equivalently as *(dis+0) or simply *dis.

Risk Assessment

Arrays are a common source of vulnerabilities in C language programs because they are frequently used but not always fully understood.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

ARR00-C

High

Probable

High

P6

L2

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT CCTR00-CPP. Understand when to prefer vectors over arraysPrior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-119, Improper Restriction of Operations within the Bounds of a Memory BufferPrior to 2018-01-12: CERT:
CWE 2.11CWE-123, Write-what-where ConditionPrior to 2018-01-12: CERT:
CWE 2.11CWE-125, Out-of-bounds ReadPrior to 2018-01-12: CERT:
CWE 2.11CWE-129, Unchecked array indexingPrior to 2018-01-12: CERT:



6 Comments

  1. I think there may be a few issues with some examples on this page:

    1. Storing the result of the malloc in a constant pointer makes clearing the pointer to NULL impossible.
      Does MEM01-C then imply that constants should not be used to hold pointers from malloc() because they will remain dangling (exception is if the free is the last thing before the auto constant goes out of scope)?
      Perhaps suggest storing the pointer elsewhere for deallocation would be better.
    2. Should the ones using malloc should at least note MEM32-C?
    3. The constant version of dat cannot be initialized by the loop using dat++; The name used in the constant example should be different.
    1. Storing the result of the malloc in a constant pointer makes clearing the pointer to NULL impossible.

      Fixed

      Does MEM01-C then imply that constants should not be used to hold pointers from malloc() because they will remain dangling (exception is if the free is the last thing before the auto constant goes out of scope)?

      Obviously it can't. Technically, we could add an exception to MEM01-C, but it strikes me as so obvious as to not be worth mentioning (smile)

      Perhaps suggest storing the pointer elsewhere for deallocation would be better.
      Should the ones using malloc should at least note MEM32-C?

      Yes, I've added a note.

      The constant version of dat cannot be initialized by the loop using dat++; The name used in the constant example should be different.

      Yes it can. dat is a const pointer to a (non-const) array. You can change the integers it points to; you just can't change dat to point elsewhere.

      1. In the second last example dat is being changed to point elsewhere by dat++;

        I was saying if you use the loop in the second last example to initialize the second example defining dat, then your changing the constant pointer.

        It would have been written to go with the first example defining dat, where it is not a constant pointer.

        The second example defining dat should use a different name, datc or something, so it can't be confused with that initialization.

        1. Whoops, you're right. The concepts are sound, but the varnames used were confusing. Overhauled the varnames; dis is non-const, and dat is const.

  2. the last sentence looks broken.

    The dat identifier points to the start of the array, so adding zero is inconsequential because *(dat+i) is equivalent to *(dat+0), which is equivalent to *(dat).

    should it be the following?

    • The dat identifier points to the start of the array, so adding zero is inconsequential; *(dat+0) is equivalent to *(dat).
    1. The whole sentence seems redundant because the preceding paragraph already says:

      ...if dis is an array object [...] and i is an integer [then] dis[i] designates the ith element of dis (counting from zero).

      Let me tweak it.