CWE-193: Off-by-one Error

How it's found

Off-by-one Error is typically found by tracing untrusted input from where it enters the system to the point where it is used without the check or neutralization this weakness describes, combining manual code review with dynamic testing.

Vulnerable vs. safe

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

char firstname[20];char lastname[20];char fullname[40];fullname[0] = '\0';strncat(fullname, firstname, sizeof(fullname)-strlen(fullname)-1);strncat(fullname, lastname, sizeof(fullname)-strlen(fullname)-1);

Consequences

• 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.
• 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.
• 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.

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().

Where this fits in a TurboPentest engagement

This weakness is not covered by the automated black-box pentest. IntegSec pentesters cover it in a manual engagement.

Frequently asked questions

Related CWEs

• Pillar weakness
  CWE-682: Incorrect Calculation