A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value. 1000 699 Weakness ChildOf 682 1000 Weakness CanPrecede 617 1000 Weakness CanPrecede 170 1000 Weakness CanPrecede 119 734 Category ChildOf 741 868 Category ChildOf 875 888 Category ChildOf 907 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. 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. Implementation Availability DoS: crash / exit / 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. Integrity 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. Confidentiality Availability Access_Control 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. 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(). 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. C 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. 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. The following C/C++ example demonstrates the Off-by-one error in the main method of a pattern matching utility that looks for a specific pattern within a specific file. The main method uses the string copy method, strncpy, to copy the command line user input file name and pattern to the Filename and Pattern character arrays respectively. C int main(int argc, char **argv) { char Filename[256]; char Pattern[32]; /* Validate number of parameters and ensure valid content */ ... /* copy filename parameter to variable, may cause off-by-one overflow */ strncpy(Filename, argv[1], sizeof(Filename)); /* copy pattern parameter to variable, may cause off-by-one overflow */ strncpy(Pattern, argv[2], sizeof(Pattern)); printf("Searching file: %s for the pattern: %s\n", Filename, Pattern); Scan_File(Filename, Pattern); } However, the calls to strncpy use the sizeof method call for the size parameter that does not take into account that the strncpy will add a null terminator to each character array. Therefore if a user enters a filename or pattern that are the same size as (or larger than) their respective character arrays a null terminator will be added beyond the end of the buffer for the character arrays creating an off-by-one buffer overflow. In addition to creating a buffer overflow that may cause a memory address to be overwritten, if the character arrays are output to the user through the printf method the memory addresses at the overflow location may be output to the user. To fix this problem, be sure to subtract 1 from the sizeof() call to allow room for the null byte to be added. C /* copy filename parameter to variable, no off-by-one overflow */ strncpy(Filename, argv[2], sizeof(Filename)-1); /* copy pattern parameter to variable, no off-by-one overflow */ strncpy(Pattern, argv[3], sizeof(Pattern)-1); Similarly, this example uses the strncat and snprintf functions 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. C 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. C 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)-1), "%s", name); The Off-by-one error can also be manifested when reading characters of a character array using a for loop that has the incorrect size as a continuation condition and attempts to read beyond the end of the buffer for the character array as shown in the following example. C #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(); } C 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. C 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. 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. 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. Halvar Flake Third Generation Exploits presentation at Black Hat Europe 2001 http://www.blackhat.com/presentations/bh-europe-01/halvar-flake/bh-europe-01-halvarflake.ppt Steve Christey Off-by-one errors: a brief explanation Secprog and SC-L mailing list posts 2004-05-05 http://marc.theaimsgroup.com/?l=secprog&m=108379742110553&w=2 klog The Frame Pointer Overwrite Phrack Issue 55, Chapter 8 1999-09-09 http://kaizo.org/mirrors/phrack/phrack55/P55-08 G. Hoglund G. McGraw Exploiting Software: How to Break Code (The buffer overflow chapter) Addison-Wesley February 2004 Michael Howard David LeBlanc John Viega 24 Deadly Sins of Software Security "Sin 5: Buffer Overruns." Page 89 McGraw-Hill 2010 Mark Dowd John McDonald Justin Schuh The Art of Software Security Assessment Chapter 5, "Off-by-One Errors", Page 180. 1st Edition Addison Wesley 2006 Off-by-one Error Guarantee that storage for strings has sufficient space for character data and the null terminator STR31-C Guarantee that storage for character arrays has sufficient space for character data and the null terminator STR31-CPP PLOVER CWE Content Team MITRE 2008-09-08 updated Alternate_Terms, Common_Consequences, Relationships, Observed_Example, Relationship_Notes, Taxonomy_Mappings CWE Content Team MITRE 2008-11-24 updated Relationships, Taxonomy_Mappings CWE Content Team MITRE 2009-12-28 updated Demonstrative_Examples, Potential_Mitigations CWE Content Team MITRE 2010-02-16 updated Demonstrative_Examples CWE Content Team MITRE 2010-12-13 updated Demonstrative_Examples CWE Content Team MITRE 2011-06-01 updated Common_Consequences CWE Content Team MITRE 2011-06-27 updated Common_Consequences CWE Content Team MITRE 2011-09-13 updated Relationships, Taxonomy_Mappings CWE Content Team MITRE 2012-05-11 updated Common_Consequences, Observed_Examples, References, Relationships