Buffer Access Using Size of Source Buffer

Description

When the size of the destination is smaller than the size of the source, a buffer overflow could occur.

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

In the following example, the source character string is copied to the dest character string using the method strncpy.

...
char source[21] = "the character string";
char dest[12];
strncpy(dest, source, sizeof(source)-1);
...

However, in the call to strncpy the source character string is used within the sizeof call to determine the number of characters to copy. This will create a buffer overflow as the size of the source character string is greater than the dest character string. The dest character string should be used within the sizeof call to ensure that the correct number of characters are copied, as shown below.

...
char source[21] = "the character string";
char dest[12];
strncpy(dest, source, sizeof(dest)-1);
...

Example Two

In this example, the method outputFilenameToLog outputs a filename to a log file. The method arguments include a pointer to a character string containing the file name and an integer for the number of characters in the string. The filename is copied to a buffer where the buffer size is set to a maximum size for inputs to the log file. The method then calls another method to save the contents of the buffer to the log file.

#define LOG_INPUT_SIZE 40

// saves the file name to a log file
int outputFilenameToLog(char *filename, int length) {

  int success;

  // buffer with size set to maximum size for input to log file
  char buf[LOG_INPUT_SIZE];

  // copy filename to buffer
  strncpy(buf, filename, length);

  // save to log file
  success = saveToLogFile(buf);

  return success;

}

However, in this case the string copy method, strncpy, mistakenly uses the length method argument to determine the number of characters to copy rather than using the size of the local character string, buf. This can lead to a buffer overflow if the number of characters contained in character string pointed to by filename is larger then the number of characters allowed for the local character string. The string copy method should use the buf character string within a sizeof call to ensure that only characters up to the size of the buf array are copied to avoid a buffer overflow, as shown below.

...
// copy filename to buffer
strncpy(buf, filename, sizeof(buf)-1);
...

See Also