Base weakness

Draft

CWE-193: Off-by-one Error

Also known as: off-by-five

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

Recorded CVEs

With this primary weakness

KEVs

In the CISA KEV catalog

Not rated

Likelihood of exploit

Per MITRE

Base

Abstraction

MITRE classification

Overview

CWE-193 (Off-by-one Error) is a base-level software weakness catalogued by MITRE in the Common Weakness Enumeration (CWE). It describes a recurring type of mistake that can lead to exploitable security vulnerabilities.

Real-world CVEs

Code examples

Illustrative examples from MITRE showing how the weakness appears in code.

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.

Vulnerable example

int i;

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.

Vulnerable example

char firstname[20];

Safe example

char firstname[20];

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.

Vulnerable example

#define PATH_SIZE 60

Safe example

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.

Vulnerable example

int setFilename(char *filename) {

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.

Illustrative examples

Real CVEs that MITRE cites as examples of this weakness.

CVE-2003-0252 — 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.
CVE-2001-1391 — Off-by-one vulnerability in driver allows users to modify kernel memory.
CVE-2002-0083 — Off-by-one error allows local users or remote malicious servers to gain privileges.
CVE-2002-0653 — 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.
CVE-2002-0844 — Off-by-one buffer overflow in version control system allows local users to execute arbitrary code.
CVE-1999-1568 — Off-by-one error in FTP server allows a remote attacker to cause a denial of service (crash) via a long PORT command.
CVE-2004-0346 — Off-by-one buffer overflow in FTP server allows local users to gain privileges via a 1024 byte RETR command.
CVE-2004-0005 — Multiple buffer overflows in chat client allow remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2003-0356 — Multiple off-by-one vulnerabilities in product allow remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2001-1496 — Off-by-one buffer overflow in server allows remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE-2004-0342 — This is an interesting example that might not be an off-by-one.
CVE-2001-0609 — An off-by-one enables a terminating null to be overwritten, which causes 2 strings to be merged and enable a format string.
CVE-2002-1745 — Off-by-one error allows source code disclosure of files with 4 letter extensions that match an accepted 3-letter extension.
CVE-2002-1816 — Off-by-one buffer overflow.
CVE-2002-1721 — Off-by-one error causes an snprintf call to overwrite a critical internal variable with a null value.
CVE-2003-0466 — 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.
CVE-2003-0625 — Off-by-one error allows read of sensitive memory via a malformed request.
CVE-2006-4574 — Chain: security monitoring product has an off-by-one error that leads to unexpected length values, triggering an assertion.

CWE-193: Off-by-one Error

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Terminology & mappings

Alternate terms

Mapped taxonomies

Frequently asked questions

What is CWE-193?

What CVEs are caused by CWE-193?

How do you prevent CWE-193?

How is CWE-193 detected?

What are the consequences of CWE-193?

Is CWE-193 actively exploited?

References

Stay ahead of CWE-193

Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Code examples

Illustrative examples

Related weaknesses

Parent

Can precede

Terminology & mappings

Alternate terms

Mapped taxonomies

Frequently asked questions

What is CWE-193?

What CVEs are caused by CWE-193?

How do you prevent CWE-193?

How is CWE-193 detected?

What are the consequences of CWE-193?

Is CWE-193 actively exploited?

References

Stay ahead of CWE-193