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Enumerations in C++ come in two forms: scoped enumerations , where in which the underlying type is fixed , and unscoped enumerations where in which the underlying type may or may not be fixed. The range of values that can be represented by either form of enumeration may include enumerator values not specified by the enumeration itself. The range of valid enumeration values for an enumeration type is defined by the C++ Standard, [dcl.enum], paragraph 8 [ISO/IEC 14882-2014]:

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To avoid operating on unspecified values, the arithmetic value being cast must be within the range of values the enumeration can represent. When dynamically checking for out-of-range values dynamically, it checking must be performed prior to before the cast expression.

Noncompliant Code Example (Bounds Checking)

This noncompliant code example attempts to check whether a given value is within the range of acceptable enumeration values. However, it is doing so after casting to the enumeration type, which may not be able to represent the given integer value. On a two's complement system, the valid range of values that can be represented by enum_type are :  [0..3], so if a value outside of that range were passed to f(), the cast to enum_type would result in an unspecified value, and using that value within the if statement results in unspecified behavior.

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This compliant solution uses a scoped enumeration, which has a fixed underlying type of type int by default, allowing any value from the parameter to be converted into a valid enumeration value. It does not further restrict the converted value to one for which there is a specific enumerator value, but it could do so by using the previous compliant solution.

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Similar to the previous compliant solution, this compliant solution uses an unscoped enumeration , but provides a fixed underlying type of type int, allowing any value from the parameter to be converted into to a valid enumeration value:

enum enum_type : int {
  E_A,
  E_B,
  E_C
};

void f(int int_var) {
  enum_type enum_var = static_cast<enum_type>(int_var);
}

While Although similar to the previous compliant solution, the previous compliant solution differs from the noncompliant code example in the way the enumerator values are expressed in code and what implicit conversions are allowed. The previous compliant solution requires a nested name specifier to identify the enumerator (e.g.for example, enum_type::E_A) , and will not implicitly convert the enumerator value to int. As with the noncompliant code example, this compliant solution does not allow a nested name specifier and will implicitly convert the enumerator value to int.

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Unexpected behavior can lead to a buffer overflow and the execution of arbitrary code by an attacker. This behavior is most likely if the program in one case checks the value correctly and then fails to do so later. Such a situation could allow an attacker to avoid verification of a buffer's length, and so on.

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Related Vulnerabilities

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

Related Guidelines

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Bibliography

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