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

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

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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:.

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

In this example, the first read will not overflow, but could fill bufOne with a truncated string. Furthermore, the second read still could overflow bufTwo. The C++ Standard, [istream.extractors], paragraphs 7–9  [ISO/IEC 14882-2014], describes the behavior of operator>>(basic_istream &, charT *) and, in part, and states in part [ISO/IEC 14882-2014]states the following:

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).

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The best solution for ensuring that data is not truncated and for guarding against buffer overflows 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 stringOne, stringTwo;
  std::cin >> stringOne >> stringTwo;
}

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#include <fstream>
#include <string>
 
void f(std::istream &in) {
  char buffer[32];
  try {
    in.read(buffer, 32sizeof(buffer));
  } catch (std::ios_base::failure &e) {
    // Handle error
  }
 
  std::string str(buffer);
  // ...
}

Compliant Solution

This compliant solution continues to assume assumes that the input from the file is exactly at most 32 characters, and instead . Instead of inserting a null terminator, it constructs the std::string object based on the sizethe number of characters read from the input stream. If the size of the input is uncertain, it is better to use std::basic_istream<T>::readsome() or a formatted input function, depending on need.

#include <fstream>
#include <string>

void f(std::istream &in) {
  char buffer[32];
  try {
    in.read(buffer, 32sizeof(buffer));
  } catch (std::ios_base::failure &e) {
    // Handle error
  }
  std::string str(buffer, 32in.gcount());
  // ...
}

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

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