STR35-CPP. Guarantee that storage for strings has sufficient space for character data and the null terminator

Copying data to a buffer that is not large enough to hold that data results in a buffer overflow. Buffer overflows occur frequently when manipulating strings [Seacord 2013b]. To prevent such errors, either limit copies through truncation or, preferably, ensure that the destination is of sufficient size to hold the character data to be copied and the null-termination character. This rule is a C++-specific instance of STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator.

Noncompliant Code Example

Since the input is unbounded, the following code could lead to a buffer overflow:

#include <iostream>
 
void f() {
  char buf[12];
  std::cin >> buf;
}

Noncompliant Code Example

To solve this problem, it may be tempting to use the std::ios_base::width() method, but there still is a trap as shown in this noncompliant code example. In this example, the first read won't overflow, but the second still could. The C++ Standard, [istream.extractors], paragraphs 7-9 describe the behavior of operator>>(basic_istream &, charT *), and states in part:

operator>> then stores a null byte (charT()) in the next position, which may be the first position if no characters were extracted. operator>> then calls width(0).

Thus, it is necessary to call width() prior to each operator>> call passing a bounded array.

#include <iostream>
 
void f() {
  char buf_one[12];
  char buf_two[12];
  std::cin.width(12);
  std::cin >> buf_one;
  std::cin >> buf_two;
}

Noncompliant Code Example

The following noncompliant code example calls std::ios_base_width() prior to each call to operator>>(). However, it still does not account for the input being truncated. Only the first 11 characters are read from the standard input stream, and a null terminator is then appended. The input could therefore be truncated, leading to information loss or a possible vulnerability.

#include <iostream>
 
void f() {
  char buf_one[12];
  char buf_two[12];
  std::cin.width(12);
  std::cin >> buf_one;
  std::cin.width(12);
  std::cin >> buf_two;
}

Compliant Solution

The best solution for ensuring that data is not truncated, and buffer overflows are guarded against, is to use std::string instead of a bounded array, as in this compliant solution:

#include <iostream>
#include <string>
 
void f() {
  std::string input;
  std::string string_one, string_two;
  std::cin >> string_one >> string_two;
}

Risk Assessment

Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code with the permissions of the vulnerable process.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography