Page History

Enumerations in C++ come in two forms: scoped enumerations in which the underlying type is fixed and unscoped enumerations 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 as follows in by the C++ Working Draft Standard:Standard, [dcl.enum], in paragraph 8 [ISO/IEC 14882-2020]:

 For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the
underlying type. Otherwise, for an enumeration where emin is the smallest enumerator and emax is the
largest, the values of the enumeration are the values in the range bmin to bmax, defined as follows: Let K
be 1 for a two’s complement representation and 0 for a one’s complement or sign-magnitude representation.
bmax is the smallest value greater than or equal to max(|emin| ? K, |emax|) and equal to 2^M ? 1, where
M is a non-negative integer. bmin is zero if emin is non-negative and ?(bmax + K) otherwise. The size of
representable by a hypothetical integer type with minimal width M such that all enumerators can be represented. The width of the smallest bit-field large enough to hold all the values of the enumeration type is max(M, 1) if bmin is
zero and M + 1 otherwise. It is possible to define an enumeration that has values not defined by any of its
enumerators. If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a
single enumerator with value 0.

The C++ Standard, [expr.static.

...

cast], paragraph 10, states the following:

A value of integral or enumeration type can be explicitly converted to a complete enumeration type. If the enumeration type has a fixed underlying type, the value is first converted to that type by integral conversion, if necessary, and then to the enumeration type. If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values (9.7.1), and otherwise, the behavior is undefined. A value of floating-point type can also be explicitly converted to an enumeration type. The resulting value is the same as converting the original value to the underlying type of the enumeration (7.3.10), and subsequently to the enumeration type.

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

According to the same standard, when converting an arithmetic or enumeration type to an enumeration type, the value will be unchanged if it is in the range of enumeration values of the enumeration type. Otherwise, the value is unspecified.

Therefore, to avoid unexpected behavior, the value being converted must be inside of the range of enumeration values. Futhermore, if it is necessary to check for out-of-range values dynamically, it checking must be done performed before the conversioncast expression.

Noncompliant Code Example (Bounds

...

Checking)

This noncompliant code exmample is trying to check for an out-of-bounds condition. Howeverexample attempts to check whether a given value is within the range of acceptable enumeration values. However, it is doing so after the conversion, so the result of the conversion is unspecified and the statement may have no effectcasting 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 EnumType are [0..3], so if a value outside of that range were passed to f(), the cast to EnumType would result in an unspecified value, and using that value within the if statement results in unspecified behavior.

enum et1EnumType {
   E_AFirst,
  Second,
  E_BThird
};

void f(int intVar) i1{
 = -1;
et1 e1EnumType enumVar = static_cast<et1>cast<EnumType>(i1intVar);

  if (e1enumVar < 0 First || enumVar > Third) {
    /*/ Handle error
 condition */}
}

Compliant

...

Solution (Bounds

...

Checking)

This compliant solution checks for an out-of-bounds condition before that the value can be represented by the enumeration type before performing the conversion to guarantee there is no unspecified resultthe conversion does not result in an unspecified value. It does this by restricting the converted value to one for which there is a specific enumerator value.

enum et1EnumType {
  First,
  E_ASecond,
  E_BThird
};

void f(int intVar) i1{
 = -1;

if (i1intVar < 0First || i1intVar > E_CThird) {
    //* Handle error
 condition */
}

et1 e1  EnumType enumVar = static_cast<et1>cast<EnumType>(i1intVar);
}

Noncompliant Code Example (Switch-statement)

Compliant Solution (Scoped Enumeration)

This compliant solution uses a scoped enumeration, which has a fixed underlying int type by default, allowing any value from the parameter to be converted into a valid enumeration value. It does not restrict the converted value to one for which there is a specific enumerator value, but it could do so as shown in the previous compliant solutionThis noncompliant code may result in a truncation of the value of i1 when converted to type et1, thus causing the program to execute case E_A.

enum et1class EnumType {
  First,
  E_ASecond,
  E_BThird
};

void f(int i1 = 5;

switch(static_cast<et1>(i1))intVar) {
  case E_A:
    /* some action A */
  case E_B:
    /* some action B */
}

Compliant Code Example (Switch-statement)

EnumType enumVar = static_cast<EnumType>(intVar);
}

Compliant Solution (Fixed Unscoped Enumeration)

Similar to the previous compliant solution, this compliant solution uses an unscoped enumeration but provides a fixed underlying int type allowing any value from the parameter to be converted to a valid enumeration valueThis compliant solution checks for an out-of-bounds condition before the conversion to guarantee there is no unspecified values, and therefore, no truncation.

enum EnumType et1: int {
  E_AFirst,
  Second,
  E_BThird
};

int i1 = 5;

if(i1 < 0 || i1 > E_Cvoid f(int intVar) {
  /*EnumType errorenumVar condition */
}

switch(= static_cast<et1>cast<EnumType>(i1intVar)) {
  case E_A:
    /* some action A */
  case E_B:
    /* some action B */
}

Risk Assessment

;
}

Although similar to the previous compliant solution, this compliant solution differs from the noncompliant code example in the way the enumerator values are expressed in code and which implicit conversions are allowed. The previous compliant solution requires a nested name specifier to identify the enumerator (for example, EnumType::First) 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.

Risk Assessment

It is possible for unspecified values to result in a buffer overflow, leading to Unexpected behavior can lead to a buffer overflow and the execution of arbitrary code by an attacker. However, because enumerators are rarely used for indexing into arrays or other forms of pointer arithmetic, it is more likely that this scenario will result in data integrity violations rather than arbitrary code execution.

References

Automated Detection

Related Vulnerabilities

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

Bibliography

...

Image Added Image Added Image AddedTodo.