CWE-193: Off-by-one Error

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Description

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

Extended Description

N/A

ThreatScore

Threat Mapped score: 0.0

Industry: Finiancial

Threat priority: Unclassified

Observed Examples (CVEs)

CVE: 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: CVE-2001-1391

Off-by-one vulnerability in driver allows users to modify kernel memory.
CVE: CVE-2002-0083

Off-by-one error allows local users or remote malicious servers to gain privileges.
CVE: 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: CVE-2002-0844

Off-by-one buffer overflow in version control system allows local users to execute arbitrary code.
CVE: 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: CVE-2004-0346

Off-by-one buffer overflow in FTP server allows local users to gain privileges via a 1024 byte RETR command.
CVE: CVE-2004-0005

Multiple buffer overflows in chat client allow remote attackers to cause a denial of service and possibly execute arbitrary code.
CVE: 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: 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: CVE-2004-0342

This is an interesting example that might not be an off-by-one.
CVE: 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: 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: CVE-2002-1816

Off-by-one buffer overflow.
CVE: CVE-2002-1721

Off-by-one error causes an snprintf call to overwrite a critical internal variable with a null value.
CVE: 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: CVE-2003-0625

Off-by-one error allows read of sensitive memory via a malformed request.
CVE: CVE-2006-4574

Chain: security monitoring product has an off-by-one error that leads to unexpected length values, triggering an assertion.

Related Attack Patterns (CAPEC)

N/A

Attack TTPs

N/A

Modes of Introduction

Phase	Note
Implementation	N/A

Common Consequences

Impact: DoS: Crash, Exit, or Restart, DoS: Resource Consumption (CPU), DoS: Resource Consumption (Memory), DoS: Instability — Notes: 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.
Impact: Modify Memory — Notes: 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.
Impact: Execute Unauthorized Code or Commands, Bypass Protection Mechanism — Notes: 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.

Potential Mitigations

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(). (N/A)

Applicable Platforms

C (N/A, Undetermined)
None (Not Language-Specific, Undetermined)

Demonstrative Examples

Intro: 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.

Body: 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.

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);

Intro: In this example, the code does not account for the terminating null character, and it writes one byte beyond the end of the buffer.

Body: 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.

char firstname[20]; char lastname[20]; char fullname[40]; fullname[0] = '\0'; strncat(fullname, firstname, 20); strncat(fullname, lastname, 20);

Intro: 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.

Body: 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.

#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; } }

Intro: 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.

Body: 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.

int setFilename(char *filename) { char name[20]; sprintf(name, "%16s.dat", filename); int success = saveFormattedFilenameToDB(name); return success; }

Notes

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.

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