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The C Standard identifies the following distinct situations in which undefined behavior (UB) can arise as a result of invalid pointer operations:

Anchor
Forming Out-of-Bounds Pointer Forming Out-of-Bounds Pointer

Noncompliant Code Example (Forming Out-of-Bounds Pointer)

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In this compliant solution, the while loop in the GetMachineName() function is bounded so that the loop terminates when a backslash character is found, the null-termination character (L'\0') is discovered, or the end of the buffer is reached. Or, as coded, the while loop continues as long as each character is neither a backslash nor a null character and is not at the end of the buffer. This code does not result in a buffer overflow even if no backslash character is found in wszMachineName.

HRESULT GetMachineName(
  wchar_t *pwszPath,
  wchar_t wszMachineName[MAX_COMPUTERNAME_LENGTH_FQDN+1])
{
  wchar_t *pwszServerName = wszMachineName;
  wchar_t *pwszTemp = pwszPath + 2;
  wchar_t *end_addr
    = pwszServerName + MAX_COMPUTERNAME_LENGTH_FQDN;
  while ( (*pwszTemp != L'\\') &&
     &&    ((*pwszTemp != L'\0')) &&
      &&   (pwszServerName < end_addr) )
  {
    *pwszServerName++ = *pwszTemp++;
  }

  /* ... */
}

...

#include <stdlib.h>
 
struct S {
  size_t len;
  char buf[];  /* Flexible array member */
};

const char *find(const struct S *s, int c) {
  const char *first = s->buf;
  const char *last  = s->buf + s->len;

  while (first++ != last) { /* Undefined behavior */
    if (*first == (unsigned char)c) {
      return first;
    }
  }
  return NULL;
}
 
void g(void) {
  struct S *s = (struct S *)malloc(sizeof(struct S));
  if (s == NULL) {
    /* Handle error */
  }
  s->len = 0;
  find(s, 'a');
}

...

#include <stdlib.h>
 
struct S {
  size_t len;
  char buf[];  /* Flexible array member */
};

const char *find(const struct S *s, int c) {
  const char *first = s->buf;
  const char *last  = s->buf + s->len;

  while (first != last) { /* Avoid incrementing here */
    if (*++first == (unsigned char)c) {
      return first;
    }
  }
  return NULL;
}
 
void g(void) {
  struct S *s = (struct S *)malloc(sizeof(struct S));
  if (s == NULL) {
    /* Handle error */
  }
  s->len = 0;
  find(s, 'a');
}

...

Writing to out-of-range pointers or array subscripts can result in a buffer overflow and the execution of arbitrary code with the permissions of the vulnerable process. Reading from out-of-range pointers or array subscripts can result in unintended information disclosure.

Automated Detection

   for (LPWSTR pwszTemp = pwszPath + 2; *pwszTemp != L'\\';
   *pwszTemp++;)

Bibliography

Related Vulnerabilities

CVE-2008-1517 results from a violation of this rule. Before Mac OSX version 10.5.7, the XNU kernel accessed an array at an unverified user-input index, allowing an attacker to execute arbitrary code by passing an index greater than the length of the array and therefore accessing outside memory [xorl 2009].

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and ARR30-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

CWE-119 = Union( ARR30-C, ARR38-C)

Intersection( ARR30-C, ARR38-C) = Ø

CWE-394 and ARR30-C

Intersection( ARR30-C, CWE-394) = Ø

CWE-394 deals with potentially-invalid function return values. Which may be used as an (invalid) array index, but validating the return value is a separate operation.

CWE-125 and ARR30-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

CWE-125 = Subset( CWE-119) = Union( ARR30-C, ARR38-C)

Intersection( ARR30-C, CWE-125) =

• Reading from an out-of-bounds array index, or off the end of an array

ARR30-C – CWE-125 =

• Writing to an out-of-bounds array index, or off the end of an array

CWE-125 – ARR30-C =

• Reading beyond a non-array buffer

• Using a library function to achieve an out-of-bounds read.

CWE-123 and ARR30-C

Independent(ARR30-C, ARR38-C)

STR31-C = Subset( Union( ARR30-C, ARR38-C))

STR32-C = Subset( ARR38-C)

Intersection( CWE-123, ARR30-C) =

• Write of arbitrary value to arbitrary (probably invalid) array index

ARR30-C – CWE-123 =

• Read of value from arbitrary (probably invalid) array index

• Construction of invalid index (pointer arithmetic)

CWE-123 – ARR30-C =

• Arbitrary writes that do not involve directly constructing an invalid array index

CWE-129 and ARR30-C

Independent( ARR30-C, ARR32-C, INT31-C, INT32-C)

ARR30-C = Union( CWE-129, list), where list =

• Dereferencing an out-of-bounds array index, where index is a trusted value

• Forming an out-of-bounds array index, without dereferencing it, whether or not index is a trusted value. (This excludes the array’s TOOFAR index, which is one past the final element; this behavior is well-defined in C11.)

CWE-120 and ARR30-C

See CWE-120 and MEM35-C

CWE-122 and ARR30-C

Intersection( ARR30-C, CWE-122) = Ø

CWE-122 specifically addresses buffer overflows on the heap operations, which occur in the context of string-copying. ARR30 specifically addresses improper creation or references of array indices. Which might happen as part of a heap buffer overflow, but is on a lower programming level.

CWE-20 and ARR30-C

See CWE-20 and ERR34-C

CWE-687 and ARR30-C

Intersection( CWE-687, ARR30-C) = Ø

ARR30-C is about invalid array indices which are created through pointer arithmetic, and dereferenced through an operator (* or []). Neither involve function calls, thus CWE-687 does not apply.

CWE-786 and ARR30-C

ARR30-C = Union( CWE-786, list) where list =

• Access of memory location after end of buffer

• Construction of invalid arry reference (pointer). This does not include an out-of-bounds array index (an integer).

CWE-789 and ARR30-C

Intersection( CWE-789, ARR30-C) = Ø

CWE-789 is about allocating memory, not array subscripting

Bibliography

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