STR39-CPP. Range check element access

The std::string index operators const_reference operator[](size_type) const and reference operator[](size_type) return the character stored at the specified position, pos. When pos >= size(), a reference to an object of type charT with value charT() is returned. The index operators are unchecked (no exceptions are thrown for range errors), and attempting to modify the resulting out-of-range object results in undefined behavior.

Similarly, the std::string::back() and std::string::front() functions are unchecked as they are defined to call through to the appropriate operator[]() overload without throwing.

Do not pass an out of range value as an argument to std::string::opperator[](). Similarly, do not call std::string::back(), or std::string::front() on an empty string. This rule is a specific instance of CTR30-CPP. Guarantee that container indices and iterators are within the valid range.

Noncompliant Code Example

In this noncompliant code example, the value i may be greater than the number of elements stored in the string, resulting in undefined behavior:

#include <string>
 
extern std::size_t get_index();
 
void f() {
  std::string s("01234567");
  s[get_index()] = '1';

}

Compliant Solution (try/catch)

This compliant solution uses the std::basic_string::at() function, which behaves in a similar fashion to the index operator[] but throws a std::out_of_range exception if pos >= size():

#include <string>

extern std::size_t get_index();

void f() {
  std::string s("01234567");
  try {
    s.at(get_index()) = '1';
  } catch (std::out_of_range &) {
    // Handle error
  }
}

Compliant Solution (Range Check)

This compliant solution checks that the value returned by get_index() is within a valid range before calling operator[]():

#include <string>

extern std::size_t get_index();

void f() {
  std::string s("01234567");
  std::size_t i = get_index();
  if (i < s.length()) {
    s[i] = '1';
  } else {
    // Handle error
  }
}

Noncompliant Code Example

This noncompliant code example attempts to replace the initial character in the string with a capitalized equivalent. However, if the given string is empty, the behavior is undefined.

#include <string>
#include <locale>

void capitalize(std::string &S) {
  std::locale L;
  S.front() = std::use_facet<std::ctype<char>>(L).toupper(S.front());
}

Compliant Solution

In this compliant solution, the call to std::string::front() is only made if the string is not empty:

#include <string>
#include <locale>

void capitalize(std::string &S) {
  if (S.empty()) {
    return;
  }

  std::locale L;
  S.front() = std::use_facet<std::ctype<char>>(L).toupper(S.front());
}

Risk Assessment

Unchecked element access can lead to out-of-bounds reads and writes and write-anywhere exploits. These exploits can in turn lead to the execution of 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