Off-by-one Error
Draft Base
Structure: Simple
Description
An off-by-one error occurs when a program incorrectly calculates a boundary, such as a loop counter or array index, by being one unit too high or too low. This often leads to buffer overflows, memory corruption, or unexpected program behavior.
This common programming mistake typically happens when developers use incorrect comparison operators (like <= instead of <) or miscalculate loop termination conditions. It's especially prevalent when dealing with zero-based indexing in arrays, strings, or memory buffers, where the boundary between the last valid element and the first invalid one is easy to misjudge. To prevent off-by-one errors, carefully review all loop conditions and boundary checks, paying close attention to whether your logic uses inclusive or exclusive upper limits. Using standardized container iteration methods (like iterators in C++ or for-each loops in other languages) instead of manual index calculations can eliminate many of these errors entirely.
Common Consequences 3
Scope: Availability
Impact: DoS: Crash, Exit, or RestartDoS: 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.
Scope: Integrity
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.
Scope: ConfidentialityAvailabilityAccess Control
Impact: Execute Unauthorized Code or CommandsBypass 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.
Detection Methods 1
Automated Static AnalysisHigh
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 searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)Potential Mitigations 1
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().
Demonstrative Examples 4
ID : DX-20
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.Code Example:
Bad
C
cHowever, this code contains an off-by-one calculation error (Off-by-one 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 (Incorrect Calculation of Buffer Size). So if the user ever requests MAX_NUM_WIDGETS, there is an out-of-bounds write (Out-of-bounds Write) 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.
Code Example:
Bad
C
cWhen 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.
Code Example:
Good
C
cThe 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.
Code Example:
Bad
C
cIf 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.
Code Example:
Good
C
cAs 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.
Code Example:
Bad
C
cHowever 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 18
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.
CVE-2003-0466Off-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.
CVE-2003-0625Off-by-one error allows read of sensitive memory via a malformed request.
CVE-2006-4574Chain: security monitoring product has an off-by-one error that leads to unexpected length values, triggering an assertion.
References 6
Third Generation Exploits
Halvar Flake
presentation at Black Hat Europe 2001
ID: REF-155
Off-by-one errors: a brief explanation
Steve Christey
Secprog and SC-L mailing list posts
05-05-2004
ID: REF-156
The Frame Pointer Overwrite
klog
Phrack Issue 55, Chapter 8
09-09-1999
https://phrack.org/issues/55/8(2025-07-24)
ID: REF-157
Exploiting Software: How to Break Code
Greg Hoglund and Gary McGraw
Addison-Wesley
27-02-2004
ID: REF-140
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Applicable Platforms
Languages:
C : UndeterminedNot Language-Specific : Undetermined
Modes of Introduction
Implementation
Alternate Terms
off-by-five
An "off-by-five" error was reported for sudo in 2002 (CVE-2002-0184), but that is more like a "length calculation" error.Related Weaknesses
ChildOf:
Incorrect Calculation (CWE-682)
ChildOf:
Incorrect Calculation (CWE-682)
CanPrecede:
Reachable Assertion (CWE-617)
CanPrecede:
Improper Null Termination (CWE-170)
Taxonomy Mapping
- PLOVER
- CERT C Secure Coding
Notes
RelationshipThis 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.