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CWE-193: Off-by-one Error
 DescriptionDescription Summary A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
Alternate Terms
Time of Introduction
Applicable PlatformsLanguages All Common Consequences
Demonstrative ExamplesExample 1 The following code allocates memory for a maximum number of widgets. It then gets a user-specified number of widgets, making sure that the user does not request too many. It then initializes the elements of the array using InitializeWidget(). Because the number of widgets can vary for each request, the code inserts a NULL pointer to signify the location of the last widget. (Bad Code) Example
Language: C int i; unsigned int numWidgets; Widget **WidgetList; numWidgets = GetUntrustedSizeValue(); if ((numWidgets == 0) || (numWidgets > MAX_NUM_WIDGETS))
{ ExitError("Incorrect number of widgets requested!");
} WidgetList = (Widget **)malloc(numWidgets * sizeof(Widget
*)); printf("WidgetList ptr=%p\n", WidgetList); for(i=0; i<numWidgets; i++) { WidgetList[i] = InitializeWidget();
} WidgetList[numWidgets] = NULL; showWidgets(WidgetList); However, this code contains an off-by-one calculation error. It allocates exactly enough space to contain the specified number of widgets, but it does not include the space for the NULL pointer. As a result, the allocated buffer is smaller than it is supposed to be (CWE-131). So if the user ever requests MAX_NUM_WIDGETS, there is an off-by-one buffer overflow when the NULL is assigned. Depending on the environment and compilation settings, this could cause memory corruption. Example 2 The following C/C++ example demonstrates the Off-by-one error in the main method of a pattern matching utility that looks for a specific pattern within a specific file. The main method uses the string copy method, strncpy, to copy the command line user input file name and pattern to the Filename and Pattern character arrays respectively. (Bad Code) Example
Language: C int main(int argc, char **argv) { char Filename[256];
char Pattern[32];
/* Validate number of parameters and ensure valid content
*/
...
/* copy filename parameter to variable, may cause
off-by-one overflow */
strncpy(Filename, argv[1], sizeof(Filename));
/* copy pattern parameter to variable, may cause off-by-one
overflow */
strncpy(Pattern, argv[2], sizeof(Pattern));
printf("Searching file: %s for the pattern: %s\n", Filename,
Pattern);
Scan_File(Filename, Pattern);
} However, the calls to strncpy use the sizeof method call for the size parameter that does not take into account that the strncpy will add a null terminator to each character array. Therefore if a user enters a filename or pattern that are the same size as (or larger than) their respective character arrays a null terminator will be added beyond the end of the buffer for the character arrays creating an off-by-one buffer overflow. In addition to creating a buffer overflow that may cause a memory address to be overwritten, if the character arrays are output to the user through the printf method the memory addresses at the overflow location may be output to the user. To fix this problem, be sure to subtract 1 from the sizeof() call to allow room for the null byte to be added. (Good Code) Example
Language: C /* copy filename parameter to variable, no off-by-one overflow
*/ strncpy(Filename, argv[2], sizeof(Filename)-1); /* copy pattern parameter to variable, no off-by-one overflow
*/ strncpy(Pattern, argv[3], sizeof(Pattern)-1); Example 3 Similarly, this example uses the strncat and snprintf functions incorrectly. The code does not account for the null character that is added by the second strncat function call, one byte beyond the end of the name buffer. (Bad Code) Example
Language: C char lastname[20]; char firstname[20]; char name[40]; char fullname[40]; strncat(name, firstname, sizeof(name)); strncat(name, lastname, sizeof(name)); snprintf(fullname, sizeof(fullname), "%s", name); By leaving a free byte at the end of the buffers for a null character to be added, the off-by-one weakness is avoided. (Good Code) Example
Language: C char lastname[20]; char firstname[20]; char name[40]; char fullname[40]; strncat(name, firstname, sizeof(name)-1); strncat(name, lastname, sizeof(name)-1); snprintf(fullname, sizeof(fullname)-1), "%s", name); Example 4 The Off-by-one error can also be manifested when reading characters of a character array using a for loop that has the incorrect size as a continuation condition and attempts to read beyond the end of the buffer for the character array as shown in the following example. (Bad Code) Example
Language: C #define PATH_SIZE 60 char filename[PATH_SIZE]; for(i=0; i<=PATH_SIZE; i++) { char c = getc();
if (c == 'EOF') {
filename[i] = '\0';
}
filename[i] = getc();
} (Good Code) Example
Language: C for(i=0; i<PATH_SIZE; i++) { ... Example 5 As another example the Off-by-one error can occur when using the sprintf library function to copy a string variable to a formatted string variable and the original string variable comes from an untrusted source. As in the following example where a local function, setFilename is used to store the value of a filename to a database but first uses sprintf to format the filename. The setFilename function includes an input parameter with the name of the file that is used as the copy source in the sprintf function. The sprintf function will copy the file name to a char array of size 20 and specifies the format of the new variable as 16 characters followed by the file extension .dat. (Bad Code) Example
Language: C int setFilename(char *filename) { char name[20];
sprintf(name, "%16s.dat", filename);
int success = saveFormattedFilenameToDB(name);
return success;
} However this will cause an Off-by-one error if the original filename is exactly 16 characters or larger because the format of 16 characters with the file extension is exactly 20 characters and does not take into account the required null terminator that will be placed at the end of the string. Observed Examples
Potential Mitigations
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Page Last Updated:
September 12, 2011
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