Out-of-bounds Write

The software writes data past the end, or before the beginning, of the intended buffer.


Description

Typically, this can result in corruption of data, a crash, or code execution. The software may modify an index or perform pointer arithmetic that references a memory location that is outside of the boundaries of the buffer. A subsequent write operation then produces undefined or unexpected results.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code attempts to save four different identification numbers into an array.

int id_sequence[3];

/* Populate the id array. */

id_sequence[0] = 123;
id_sequence[1] = 234;
id_sequence[2] = 345;
id_sequence[3] = 456;

Since the array is only allocated to hold three elements, the valid indices are 0 to 2; so, the assignment to id_sequence[3] is out of bounds.

Example Two

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

int returnChunkSize(void *) {


  /* if chunk info is valid, return the size of usable memory,

  * else, return -1 to indicate an error

  */
  ...

}
int main() {
  ...
  memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));
  ...
}

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

Example Three

This example takes an IP address from a user, verifies that it is well formed and then looks up the hostname and copies it into a buffer.

void host_lookup(char *user_supplied_addr){

  struct hostent *hp;
  in_addr_t *addr;
  char hostname[64];
  in_addr_t inet_addr(const char *cp);

  /*routine that ensures user_supplied_addr is in the right format for conversion */

  validate_addr_form(user_supplied_addr);
  addr = inet_addr(user_supplied_addr);
  hp = gethostbyaddr( addr, sizeof(struct in_addr), AF_INET);
  strcpy(hostname, hp->h_name);

}

This function allocates a buffer of 64 bytes to store the hostname, however there is no guarantee that the hostname will not be larger than 64 bytes. If an attacker specifies an address which resolves to a very large hostname, then we may overwrite sensitive data or even relinquish control flow to the attacker.

Note that this example also contains an unchecked return value (CWE-252) that can lead to a NULL pointer dereference (CWE-476).

Example Four

This example applies an encoding procedure to an input string and stores it into a buffer.

char * copy_input(char *user_supplied_string){

  int i, dst_index;
  char *dst_buf = (char*)malloc(4*sizeof(char) * MAX_SIZE);
  if ( MAX_SIZE <= strlen(user_supplied_string) ){
    die("user string too long, die evil hacker!");
  }
  dst_index = 0;
  for ( i = 0; i < strlen(user_supplied_string); i++ ){

    if( '&' == user_supplied_string[i] ){
      dst_buf[dst_index++] = '&';
      dst_buf[dst_index++] = 'a';
      dst_buf[dst_index++] = 'm';
      dst_buf[dst_index++] = 'p';
      dst_buf[dst_index++] = ';';
    }
    else if ('<' == user_supplied_string[i] ){


      /* encode to &lt; */


    }
    else dst_buf[dst_index++] = user_supplied_string[i];

  }
  return dst_buf;

}

The programmer attempts to encode the ampersand character in the user-controlled string, however the length of the string is validated before the encoding procedure is applied. Furthermore, the programmer assumes encoding expansion will only expand a given character by a factor of 4, while the encoding of the ampersand expands by 5. As a result, when the encoding procedure expands the string it is possible to overflow the destination buffer if the attacker provides a string of many ampersands.

Example Five

In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.

char* trimTrailingWhitespace(char *strMessage, int length) {

  char *retMessage;
  char *message = malloc(sizeof(char)*(length+1));

  // copy input string to a temporary string
  char message[length+1];
  int index;
  for (index = 0; index < length; index++) {
    message[index] = strMessage[index];
  }
  message[index] = '\0';

  // trim trailing whitespace
  int len = index-1;
  while (isspace(message[len])) {
    message[len] = '\0';
    len--;
  }

  // return string without trailing whitespace
  retMessage = message;
  return retMessage;

}

However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.

Example Six

The following is an example of code that may result in a buffer underwrite, if find() returns a negative value to indicate that ch is not found in srcBuf:

int main() {
  ...
  strncpy(destBuf, &srcBuf[find(srcBuf, ch)], 1024);
  ...
}

If the index to srcBuf is somehow under user control, this is an arbitrary write-what-where condition.

See Also

Memory Buffer Errors

Weaknesses in this category are related to the handling of memory buffers within a software system.

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses in the 2020 CWE Top 25 Most Dangerous Software Weaknesses

CWE entries in this view are listed in the 2020 CWE Top 25 Most Dangerous Software Weaknesses.

Weaknesses in the 2019 CWE Top 25 Most Dangerous Software Errors

CWE entries in this view are listed in the 2019 CWE Top 25 Most Dangerous Software Errors.


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