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Ensuring that array references are within the bounds of the array is almost entirely the responsibility of the programmer. Likewise, when using STL vectors, the programmer is responsible for ensuring integer indexes are within the bounds of the vector.

Noncompliant Code Example (Arrays)

This noncompliant code example shows a function insert_in_table() that has two int paramters, pos and value, both of which can be influenced by data originating from untrusted sources. The function uses a global variable table to determine if storage has been allocated for an array of 100 integer elements and allocates the memory if it has not already been allocated.

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The function performs a range check to ensure that pos does not exceed the upper bound of the array but fails to check the lower bound for table. Because pos has been declared as a (signed) int, this parameter can easily assume a negative value, resulting in a write outside the bounds of the memory referenced by table.

Compliant Solution

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(Arrays)

In this compliant solution, the parameter pos is declared as size_t, which prevents passing of negative arguments (see INT01-CPP. Use rsize_t or size_t for all integer values representing the size of an object).

enum { TABLESIZE = 100 };

int *table = NULL;

int insert_in_table(size_t pos, int value){
  if (!table) {
    table = new int[TABLESIZE];
  }
  if (pos >= TABLESIZE) {
    return -1;
  }
  table[pos] = value;
  return 0;
}

Compliant Solution

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(Array Templates)

Specialized function templates can be used to define functions that accept an array of a generic type {{T}} of size {{n}}. The compiler can perform template argument deduction for function templates to properly deduce both the type and size. This compliant solution defines a function template for a function {{clear()}} that takes a template parameter {{array\[\]}} of {{T}} elements and an actual length parameter of type {{size_t}}. This is not particularly useful yet, as we have already seen that passing an array with an explicit size parameter is a common (but error prone) idiom. However, you can also define a specialization of the function template that includes a template parameter {{n}} of type {{size_t}} in addition to the original type parameter. The inline function {{clear()}} has one parameter: an array of type {{T}} elements of fixed length {{n}}.

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The function template and specialized function template can be used in a straightforward manner. This example declares an array of 12 integers named {{int_array}} and invokes the {{clear()}} function passing {{int_array}} as an argument. The compiler matches this invocation to {{inline void clear(T (&array)\[n\])}} because this definition most closely matches the actual argument type of array of {{int}}. The compiler deduces that the type {{T}} is {{int}} and that {{n}} is 12, separating the size {{n}} from the pointer to the array of {{int}} before invoking the function template for {{clear()}}. This use of specialized function templates guarantees that the {{clear()}} function has the correct array size.

Noncompliant Code Example (Vectors)

The above code examples perform the same when working with vectors.

enum { TABLESIZE = 100 };

vector<int> table;

int insert_in_table(int pos, int value){
  if (pos >= table.size()) {
    return -1;
  }
  table[pos] = value;
  return 0;
}

The function performs a range check to ensure that pos does not exceed the upper bound of the array but fails to check the lower bound for table. Because pos has been declared as a (signed) int, this parameter can easily assume a negative value, resulting in a write outside the bounds of the memory referenced by table.

Compliant Solution (Arrays)

In this compliant solution, the parameter pos is declared as size_t, which prevents passing of negative arguments (see INT01-CPP. Use rsize_t or size_t for all integer values representing the size of an object).

enum { TABLESIZE = 100 };

vector<int> table;

int insert_in_table(size_t pos, int value){
  if (pos >= table.size()) {
    return -1;
  }
  table[pos] = value;
  return 0;
}

Risk Assessment

Using an invalid array or vector index can result in an arbitrary memory overwrite or abnormal program termination.

Automated Detection

The LDRA tool suite Version 7.6.0 can detect violations of this rule.

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Compass/ROSE can detect some violations of this rule. In particular, if a signed index to an array is being verified, it ensures that the value is also compared against 0.

Related Vulnerabilities

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

Other Languages

This rule appears in the C Secure Coding Standard as ARR30-C. Guarantee that array indices are within the valid range.

References

\[[ISO/IEC 9899:1999|AA. C++ References#ISO/IEC 9899-1999]\] Section 6.7.5.2, "Array declarators"
\[[ISO/IEC PDTR 24772|AA. C++ References#ISO/IEC PDTR 24772]\] "XYX Boundary Beginning Violation," "XYY Wrap-around Error," and "XYZ Unchecked Array Indexing"
\[[MITRE 07|AA. C++ References#MITRE 07]\] [CWE ID 129|http://cwe.mitre.org/data/definitions/129.html], "Unchecked Array Indexing"
\[[Viega 05|AA. C++ References#Viega 05]\] Section 5.2.13, "Unchecked array indexing"

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