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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 checking for out-of-range values dynamically, it must be performed prior to 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.

enum enum_type {
  E_A,
  E_B,
  E_C
};

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

  if (enum_var < E_A || enum_var > E_C) {
    // Handle error
  }
}

Compliant Solution (Bounds Checking)

This compliant solution checks that the value is within the range of acceptable enumeration values before the conversion to guarantee there is no unspecified result. It further restricts the converted value to one for which there is a specific enumerator value.

enum enum_type {
  E_A,
  E_B,
  E_C
};

void f(int int_var) {
  if (int_var < E_A || int_var > E_C) {
    // Handle error
  }
  enum_type enum_var = static_cast<enum_type>(int_var);
}

Compliant Solution (Scoped Enumeration)

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 could do so by using the previous compliant solution.

enum class enum_type {
  E_A,
  E_B,
  E_C
};

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

Risk Assessment

Unexpected behavior can lead to a buffer overflow and the execution of arbitrary code by an attacker. This 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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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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INT18-CPP. Evaluate integer expressions in a larger size before comparing or assigning to that size      04004. Integers (INT)      05005. Floating Point Arithmetic (FLP)