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Base · Medium

CWE-193: Off-by-one Error

A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.

CWE-193 · Base Level ·18 CVEs ·1 Mitigations

Description

A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.

Potential Impact

Availability

DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU), DoS: Resource Consumption (Memory), DoS: Instability

Integrity

Modify Memory

Confidentiality, Availability, Access Control

Execute Unauthorized Code or Commands, Bypass Protection Mechanism

Demonstrative Examples

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
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 (CWE-193). 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 out-of-bounds write (CWE-787) when the NULL is assigned. Depending on the environment and compilation settings, this could cause memory corruption.
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
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
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);
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
#define PATH_SIZE 60
                 char filename[PATH_SIZE];
                 for(i=0; i<=PATH_SIZE; i++) {
                 
                   char c = fgetc(stdin);
		   if (c == EOF) {
		   
		     filename[i] = '\0';
		   
		   }
		   else {
		   
		     filename[i] = c;
		   
		   }
		 
		 }
If i reaches PATH_SIZE, then the loop continues. However, filename[PATH_SIZE] is actually out of bounds, since the valid index range is from 0 to PATH_SIZE-1.
In this case, the correct continuation condition is shown below.
Good
for(i=0; i<PATH_SIZE; i++) {...
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
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.

Mitigations & Prevention

Implementation

When copying character arrays or using character manipulation methods, the correct size parameter must be used to account for the null terminator that needs to be added at the end of the array. Some examples of functions susceptible to this weakness in C include strcpy(), strncpy(), strcat(), strncat(), printf(), sprintf(), scanf() and sscanf().

Detection Methods

  • Automated Static Analysis High — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea

Real-World CVE Examples

CVE IDDescription
CVE-2003-0252Off-by-one error allows remote attackers to cause a denial of service and possibly execute arbitrary code via requests that do not contain newlines.
CVE-2001-1391Off-by-one vulnerability in driver allows users to modify kernel memory.
CVE-2002-0083Off-by-one error allows local users or remote malicious servers to gain privileges.
CVE-2002-0653Off-by-one buffer overflow in function usd by server allows local users to execute arbitrary code as the server user via .htaccess files with long entries.
CVE-2002-0844Off-by-one buffer overflow in version control system allows local users to execute arbitrary code.
CVE-1999-1568Off-by-one error in FTP server allows a remote attacker to cause a denial of service (crash) via a long PORT command.
CVE-2004-0346Off-by-one buffer overflow in FTP server allows local users to gain privileges via a 1024 byte RETR command.
CVE-2004-0005Multiple buffer overflows in chat client allow remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2003-0356Multiple off-by-one vulnerabilities in product allow remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2001-1496Off-by-one buffer overflow in server allows remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2004-0342This is an interesting example that might not be an off-by-one.
CVE-2001-0609An off-by-one enables a terminating null to be overwritten, which causes 2 strings to be merged and enable a format string.
CVE-2002-1745Off-by-one error allows source code disclosure of files with 4 letter extensions that match an accepted 3-letter extension.
CVE-2002-1816Off-by-one buffer overflow.
CVE-2002-1721Off-by-one error causes an snprintf call to overwrite a critical internal variable with a null value.

Showing 15 of 18 observed examples.

CWE-682: Incorrect Calculation

The product performs a calculation that generates incorrect or unintended results that are later used in security-critic...

CWE-617: Reachable Assertion

The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an applicatio...

CWE-170: Improper Null Termination

The product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminato...

CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer'...

Taxonomy Mappings

  • PLOVER: — Off-by-one Error
  • CERT C Secure Coding: STR31-C — Guarantee that storage for strings has sufficient space for character data and the null terminator

Frequently Asked Questions

What is CWE-193?

CWE-193 (Off-by-one Error) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.

How can CWE-193 be exploited?

Attackers can exploit CWE-193 (Off-by-one Error) to dos: crash, exit, or restart, dos: resource consumption (cpu), dos: resource consumption (memory), dos: instability. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-193?

Key mitigations include: When copying character arrays or using character manipulation methods, the correct size parameter must be used to account for the null terminator that needs to be added at the end of the array. Some e

What is the severity of CWE-193?

CWE-193 is classified as a Base-level weakness (Medium abstraction). It has been observed in 18 real-world CVEs.