CWE-193: Off-by-one Error
Presentation Filter:
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. ![]()
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The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance. Languages Class: Language-Independent (Undetermined Prevalence) The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Example 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 In this example, the code does not account for the terminating null character, and it writes one byte beyond the end of the buffer. The first call to strncat() appends up to 20 characters plus a terminating null character to fullname[]. There is plenty of allocated space for this, and there is no weakness associated with this first call. However, the second call to strncat() potentially appends another 20 characters. The code does not account for the terminating null character that is automatically added by strncat(). This terminating null character would be written one byte beyond the end of the fullname[] buffer. Therefore an off-by-one error exists with the second strncat() call, as the third argument should be 19. (bad code) Example Language: C char firstname[20];
char lastname[20]; char fullname[40]; fullname[0] = '\0'; strncat(fullname, firstname, 20); strncat(fullname, lastname, 20); When using a function like strncat() one must leave a free byte at the end of the buffer for a terminating null character, thus avoiding the off-by-one weakness. Additionally, the last argument to strncat() is the number of characters to append, which must be less than the remaining space in the buffer. Be careful not to just use the total size of the buffer. (good code) Example Language: C char firstname[20];
char lastname[20]; char fullname[40]; fullname[0] = '\0'; strncat(fullname, firstname, sizeof(fullname)-strlen(fullname)-1); strncat(fullname, lastname, sizeof(fullname)-strlen(fullname)-1); Example 3 The Off-by-one error can also be manifested when reading characters from a character array within a for loop that has an incorrect continuation condition. (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(); In this case, the correct continuation condition is shown below. (good code) Example Language: C for(i=0; i<PATH_SIZE; i++) {
... Example 4 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.
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Relationship This is not always a buffer overflow. For example, an off-by-one error could be a factor in a partial comparison, a read from the wrong memory location, an incorrect conditional, etc. Research Gap Under-studied. It requires careful code analysis or black box testing, where inputs of excessive length might not cause an error. Off-by-ones are likely triggered by extensive fuzzing, with the attendant diagnostic problems.
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