Page History

Copying data into an array a container that is not large enough to hold that data results in a buffer overflow. To prevent such errors, data copied to the destination array container must be restricted based on the size basis of the destination arraycontainer's size, or , preferably, the destination array container must be guaranteed to be large enough to hold the data to be copied.

Vulnerabilities that result from copying data to an undersized buffer often can also involve null-terminated character arrays (NTCA)strings. Consult STR31STR50-CPP. Guarantee that storage for character arrays strings has sufficient space for character data and the null terminator for  for specific examples of this rule that involve NTCA.

Noncompliant Code Example (Array)

Improper use of functions that limit copies with a size specifier, such as memcpy(), may result in a buffer overflow. In this noncompliant code example, an array of integers is copied from src to dest using memcpy(). However, the programmer mistakenly specified the amount to copy based on the size of src, which is stored in len, rather than the space available in dest. If len is greater than 256, then a buffer overflow will occur.

enum { WORKSPACE_SIZE = 256 };

void func(const int src[], size_t len) {
  int dest[WORKSPACE_SIZE];
  memcpy(dest, src, len * sizeof(int));
  /* ... */
}

Compliant Solution (Array)

The amount of data copied should be limited based on the available space in the destination buffer. This can be accomplished by adding a check to ensure the amount of data to be copied from src can fit in dest.

enum { WORKSPACE_SIZE = 256 };

void func(const int src[], size_t len) {
  int dest[WORKSPACE_SIZE];
  if (len > WORKSPACE_SIZE) {
      /* Handle Error */
  }
  memcpy(dest, src, sizeof(int)*len);
  /* ... */
}

Noncompliant Code Example (Vector)

strings.

Copies can be made with the std::memcpy() function. However, the std::memmove() and std::memset() functions can also have the same vulnerabilities because they overwrite a block of memory without checking that the block is valid. Such issues are not limited to C standard library functions; standard template library (STL) generic algorithms, such as std::copy(), std::fill(), and std::transform(), also assume valid output buffer sizes [ISO/IEC 14882-2014].

Noncompliant Code Example

STL containers Vectors can be subject to the same vulnerabilities as array data types. The copy function std::copy() algorithm provides no inherent bounds checking , and can lead to a buffer overflow. In this noncompliant code example, a vector of integers is copied from src to dest using std::copy(). Since Because std::copy() does nothing to expand the dest vector, thus the program will overflow the buffer on copying the first element.

#include <algorithm>
#include <algorithm><vector>

void funcf(const std::vector<int> &src) {
  std::vector<int> dest;
  std::copy( src.begin(), src.end(), dest.begin());
  /*/ ... */
}

This hazard applies to any algorithm that takes a ' destination ' iterator, expecting to fill it with values. Most of the STL algorithms expect the destination container to have sufficient space to hold the values provided.

Compliant Solution (

...

Sufficient Initial Capacity)

The proper way to use std::copy() is to ensure the destination container can hold all the elements being copied to it. This code example compliant solution enlarges the capacity of the vector before starting the copyoprior to the copy operation.

#include <algorithm>
#include <vector>
void funcf(const std::vector<int> &src) {
  vector<int> dest;// Initialize dest with src.size() default-inserted elements
  std::vector<int> dest.resize( src.size());
  std::copy( src.begin(), src.end(), dest.begin());
  /*/ ... */
}

Compliant Solution (

...

Per-Element Growth)

An alternative safe approach is to supply a std::back_insert_iterator as the destination argument. This iterator expands the destination container by one element for each element supplied by the algorithm. This , which guarantees the destination container will become sufficiently large enough to hold the elements provided.

#include <algorithm>
#include <iterator>
#include <algorithm><vector>

void funcf(const std::vector<int> &src) {
  std::vector<int> dest;
  std::copy( src.begin(), src.end(), std::back_inserter( dest));
  //* ... */
}

The front_insert_iterator works in a similar fashion to the back_insert_iterator, but it automatically pushes new elements to the front of the container, causing them to be listed in the container in reverse order. Since it uses the member push_front() method, it is not available for vectors.

Also note that using insert iterators is less efficient than using resize() becuase they expand the destination container one element at a time.

}

Compliant Solution (Assignment)

The simplest solution is to construct dest from src directly, as in this compliant solution.

#include <vector>

void f(const std::vector<int> &src) {
  std::vector<int> dest(src);
  // ...
}

Noncompliant Code Example

In this noncompliant code example, std::fill_n() is used to fill a buffer with 10 instances of the value 0x42. However, the buffer has not allocated any space for the elements, so this operation results in a buffer overflow.

#include <algorithm>
#include <vector>

void f() {
  std::vector<int> v;
  std::fill_n(v.begin(), 10, 0x42);
}

Compliant Solution (Sufficient Initial Capacity)

This compliant solution ensures the capacity of the vector is sufficient before attempting to fill the container.

#include <algorithm>
#include <vector>

void f() {
  std::vector<int> v(10);
  std::fill_n(v.begin(), 10, 0x42);
}

However, this compliant solution is inefficient. The constructor will default-construct 10 elements of type int, which are subsequently replaced by the call to std::fill_n(), meaning that each element in the container is initialized twice.

Compliant Solution (Fill Initialization)

This compliant solution initializes v to 10 elements whose values are all 0x42.

#include <algorithm>
#include <vector>

void f() {
  std::vector<int> v(10, 0x42);
}

Risk Assessment

Copying data to a buffer that is too small to hold that the data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code.

Automated Detection

Fortify SCA Version 5.0 can detect violations of this rule.

Splint Version 3.1.1 can detect violations of this rule.

Compass/ROSE can detect some violations of this rule.

...

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as ARR33-C. Guarantee that copies are made into storage of sufficient size.

References

\[[ISO/IEC PDTR 24772|AA. C++ References#ISO/IEC PDTR 24772]\] "XYB Buffer Overflow in Heap," "XYW Buffer Overflow in Stack," and "XYZ Unchecked Array Indexing"
\[[Meyers 01|AA. C++ References#Meyers 01]\] Item 30: Make sure destination ranges are big enough
\[[MITRE 07|AA. C++ References#MITRE 07]\] [CWE ID 119|http://cwe.mitre.org/data/definitions/119.html], "Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer"
\[[Seacord 05a|AA. C++ References#Seacord 05]\] Chapter 2, "Strings"
\[[VU#196240|AA. C++ References#VU196240]\]

Related Guidelines

Bibliography

...

Image Added Image Added Image AddedImage Removed      06. Arrays and the STL (ARR)      VOID ARR34-CPP. Ensure that array types in expressions are compatible