Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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CWE-193: Off-by-one Error

Weakness ID: 193
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.
+ Alternate Terms
An "off-by-five" error was reported for sudo in 2002 (CVE-2002-0184), but that is more like a "length calculation" error.
+ Relationships

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.

+ Relevant to the view "Research Concepts" (CWE-1000)
+ Relevant to the view "Development Concepts" (CWE-699)
ChildOfClassClass682Incorrect Calculation
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

+ Applicable Platforms
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.


Class: Language-Independent (Undetermined Prevalence)

+ Common Consequences

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.


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

This weakness will generally lead to undefined behavior and therefore crashes. In the case of overflows involving loop index variables, the likelihood of infinite loops is also high.

Technical Impact: Modify Memory

If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the wrap around results in other conditions such as buffer overflows, further memory corruption may occur.
Access Control

Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism

This weakness can sometimes trigger buffer overflows which can be used to execute arbitrary code. This is usually outside the scope of a program's implicit security policy.
+ Demonstrative Examples

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:
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;

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 strncat and snprintf functions are used 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:
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:
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), "%s", name);

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:
#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:
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:
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
Off-by-one error allows remote attackers to cause a denial of service and possibly execute arbitrary code via requests that do not contain newlines.
Off-by-one vulnerability in driver allows users to modify kernel memory.
Off-by-one error allows local users or remote malicious servers to gain privileges.
Off-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.
Off-by-one buffer overflow in version control system allows local users to execute arbitrary code.
Off-by-one error in FTP server allows a remote attacker to cause a denial of service (crash) via a long PORT command.
Off-by-one buffer overflow in FTP server allows local users to gain privileges via a 1024 byte RETR command.
Multiple buffer overflows in chat client allow remote attackers to cause a denial of service and possibly execute arbitrary code.
Multiple off-by-one vulnerabilities in product allow remote attackers to cause a denial of service and possibly execute arbitrary code.
Off-by-one buffer overflow in server allows remote attackers to cause a denial of service and possibly execute arbitrary code.
This is an interesting example that might not be an off-by-one.
An off-by-one enables a terminating null to be overwritten, which causes 2 strings to be merged and enable a format string.
Off-by-one error allows source code disclosure of files with 4 letter extensions that match an accepted 3-letter extension.
Off-by-one buffer overflow.
Off-by-one error causes an snprintf call to overwrite a critical internal variable with a null value.
Off-by-one error in function used in many products leads to a buffer overflow during pathname management, as demonstrated using multiple commands in an FTP server.
Off-by-one error allows read of sensitive memory via a malformed request.
Chain: security monitoring product has an off-by-one error that leads to unexpected length values, triggering an assertion.
+ Potential Mitigations

Phase: 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().
+ Memberships
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.
+ Notes


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.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVEROff-by-one Error
CERT C Secure CodingSTR31-CGuarantee that storage for strings has sufficient space for character data and the null terminator
+ References
[REF-155] Halvar Flake. "Third Generation Exploits". presentation at Black Hat Europe 2001. <>.
[REF-156] Steve Christey. "Off-by-one errors: a brief explanation". Secprog and SC-L mailing list posts. 2004-05-05. <>.
[REF-157] klog. "The Frame Pointer Overwrite". Phrack Issue 55, Chapter 8. 1999-09-09. <>.
[REF-140] Greg Hoglund and Gary McGraw. "Exploiting Software: How to Break Code". Chapter 7, "Buffer Overflow". Addison-Wesley. 2004-02-27. <>.
[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 5: Buffer Overruns." Page 89. McGraw-Hill. 2010.
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 5, "Off-by-One Errors", Page 180. 1st Edition. Addison Wesley. 2006.
+ Content History
Submission DateSubmitterOrganization
Modification DateModifierOrganization
2008-09-08CWE Content TeamMITRE
updated Alternate_Terms, Common_Consequences, Relationships, Observed_Example, Relationship_Notes, Taxonomy_Mappings
2008-11-24CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2009-12-28CWE Content TeamMITRE
updated Demonstrative_Examples, Potential_Mitigations
2010-02-16CWE Content TeamMITRE
updated Demonstrative_Examples
2010-12-13CWE Content TeamMITRE
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2011-09-13CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Observed_Examples, References, Relationships
2014-06-23CWE Content TeamMITRE
updated References
2014-07-30CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2017-05-03CWE Content TeamMITRE
updated Demonstrative_Examples
2017-11-08CWE Content TeamMITRE
updated Applicable_Platforms, References, Taxonomy_Mappings

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Page Last Updated: January 18, 2018