When iterating over elements of a container, the iterators used must iterate over a valid range. An iterator range is a pair of iterators that refer to the first and past-the-end elements of the range respectively.

A valid iterator range has all of the following characteristics:

• Both iterators refer into the same container.
• The iterator representing the start of the range precedes the iterator representing the end of the range.
• The elements iterated over do not have unspecified values.
• The iterators are not invalidated, in conformance with CTR51-CPP. Use valid references, pointers, and iterators to reference elements of a container.

An empty iterator range (where the two iterators are valid and equivalent) is considered to be valid.

Accessing two iterators that do not refer into the same container or accessing invalidated iterators results in undefined behavior.

Several generic standard template library (STL) algorithms, such as std::remove() and std::unique(), remove instances of elements from a container without shrinking the size of the container. Instead, these algorithms return a ForwardIterator to indicate the partition within the container after which elements are no longer valid. The elements in the container that precede the returned iterator are valid elements with specified values, whereas the elements that succeed the returned iterator are valid but have unspecified values. Accessing unspecified values of elements iterated over results in unspecified behavior. Frequently, the erase-remove idiom is used to shrink the size of the container when using these algorithms.

Noncompliant Code Example

In this noncompliant example, the two iterators that delimit the range point into the same container, but the first iterator does not precede the second. On each iteration of its internal loop, std::for_each() compares the first iterator (after incrementing it) with the second for equality, and as long as they are not equal, it will continue to increment the first iterator. Incrementing the iterator representing the past-the-end element of the range results in undefined behavior.

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(const std::vector<int> &c) {
  std::for_each(c.end(), c.begin(), [](int i) { std::cout << i; });
}

Invalid iterator ranges can also result from comparison functions that return true for equal values. See CTR57-CPP. Provide a valid ordering predicate for more information about comparators.

Compliant Solution

In this compliant solution, the iterator values passed to std::for_each() are passed in the proper order:

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(const std::vector<int> &c) {
  std::for_each(c.begin(), c.end(), [](int i) { std::cout << i; });
}

Noncompliant Code Example

In this noncompliant code example, iterators from different containers are passed for the same iterator range. Although many STL implementations will compile this code and exhibit reasonable behavior, there is no requirement that an STL implementation treat a default-initialized iterator as a synonym for end().

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(const std::vector<int> &c) {
  std::vector<int>::const_iterator e;
  std::for_each(c.begin(), e, [](int i) { std::cout << i; });
}

Compliant Solution

In this compliant solution, the proper iterator generated by a call to end() is passed:

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(const std::vector<int> &c) {
  std::for_each(c.begin(), c.end(), [](int i) { std::cout << i; });
}

Noncompliant Code Example

In this noncompliant code example, elements matching 42 are removed from the given container. The contents of the container are then printed to the standard output stream. However, if any elements were removed from the container, the range-based for loop iterates over an invalid iterator range, resulting in unspecified behavior.

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(std::vector<int> &c) {
  std::remove(c.begin(), c.end(), 42);
  for (auto v : c) {
    std::cout << "Container element: " << v << std::endl;
  }
}

Compliant Solution

In this compliant solution, elements removed by the standard algorithm are subsequently erased from the given container. This technique ensures that a valid iterator range is used by the range-based for loop.

#include <algorithm>
#include <iostream>
#include <vector>
 
void f(std::vector<int> &c) {
  c.erase(std::remove(c.begin(), c.end(), 42), c.end());
  for (auto v : c) {
    std::cout << "Container element: " << v << std::endl;
  }
}

Risk Assessment

Using an invalid iterator range is similar to allowing a buffer overflow, which can lead to an attacker running arbitrary code.

Automated Detection

Related Vulnerabilities

In Fun with erase(), Chris Rohlf discusses the exploit potential of a program that calls vector::erase() with invalid iterator ranges.

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

Related Guidelines

Bibliography