CWE-843¶

Access of Resource Using Incompatible Type (‘Type Confusion’). [Type-Errors, Improper-Control-Of-A-Resource-Through-Its-Lifetime]

Required inputs: IR

The product allocates or initializes a resource such as a pointer, object, or variable using one type, but it later accesses that resource using a type that is incompatible with the original type.

When the product accesses the resource using an incompatible type, this could trigger logical errors because the resource does not have expected properties. In languages without memory safety, such as C and C++, type confusion can lead to out-of-bounds memory access.

While this weakness is frequently associated with unions when parsing data with many different embedded object types in C, it can be present in any application that can interpret the same variable or memory location in multiple ways.

This weakness is not unique to C and C++. For example, errors in PHP applications can be triggered by providing array parameters when scalars are expected, or vice versa. Languages such as Perl, which perform automatic conversion of a variable of one type when it is accessed as if it were another type, can also contain these issues.

Demonstrative Examples

Example 1

The following code uses a union to support the representation of different types of messages. It formats messages differently, depending on their type.

Example Language:C
    #define NAME_TYPE 1
    #define ID_TYPE 2

    struct MessageBuffer
    {
        int msgType;
        union {
            char *name;
            int nameID;
        };
    };


    int main (int argc, char **argv) {
        struct MessageBuffer buf;
        char *defaultMessage = "Hello World";

        buf.msgType = NAME_TYPE;
        buf.name = defaultMessage;
        printf("Pointer of buf.name is %p\n", buf.name);
        /* This particular value for nameID is used to make the code architecture-independent. If coming from untrusted input, it could be any value. */

        buf.nameID = (int)(defaultMessage + 1);
        printf("Pointer of buf.name is now %p\n", buf.name);
        if (buf.msgType == NAME_TYPE) {
            printf("Message: %s\n", buf.name);
        }
        else {
            printf("Message: Use ID %d\n", buf.nameID);
        }
    }

The code intends to process the message as a NAME_TYPE, and sets the default message to "Hello World." However, since both buf.name and buf.nameID are part of the same union, they can act as aliases for the same memory location, depending on memory layout after compilation.

As a result, modification of buf.nameID - an int - can effectively modify the pointer that is stored in buf.name - a string.

Execution of the program might generate output such as:

Pointer of name is 10830

Pointer of name is now 10831

Message: ello World

Notice how the pointer for buf.name was changed, even though buf.name was not explicitly modified.

In this case, the first "H" character of the message is omitted. However, if an attacker is able to fully control the value of buf.nameID, then buf.name could contain an arbitrary pointer, leading to out-of-bounds reads or writes.

Example 2

The following PHP code accepts a value, adds 5, and prints the sum.

Example Language:PHP (Unsupported language for documentation only)
    $value = $_GET['value'];
    $sum = $value + 5;
    echo "value parameter is '$value'<p>";
    echo "SUM is $sum";

When called with the following query string:

value=123

the program calculates the sum and prints out:

SUM is 128

However, the attacker could supply a query string such as:

value[]=123

The "[]" array syntax causes $value to be treated as an array type, which then generates a fatal error when calculating $sum:

Fatal error: Unsupported operand types in program.php on line 2

Example 3

The following Perl code is intended to look up the privileges for user ID's between 0 and 3, by performing an access of the $UserPrivilegeArray reference. It is expected that only userID 3 is an admin (since this is listed in the third element of the array).

Example Language:Perl (Unsupported language for documentation only)
    my $UserPrivilegeArray = ["user", "user", "admin", "user"];

    my $userID = get_current_user_ID();

    if ($UserPrivilegeArray eq "user") {
        print "Regular user!\n";
    }
    else {
        print "Admin!\n";
    }

    print "\$UserPrivilegeArray = $UserPrivilegeArray\n";

In this case, the programmer intended to use "$UserPrivilegeArray->{$userID}" to access the proper position in the array. But because the subscript was omitted, the "user" string was compared to the scalar representation of the $UserPrivilegeArray reference, which might be of the form "ARRAY(0x229e8)" or similar.

Since the logic also "fails open" (CWE-636), the result of this bug is that all users are assigned administrator privileges.

While this is a forced example, it demonstrates how type confusion can have security consequences, even in memory-safe languages.

Possible Messages

Key	Text	Severity	Disabled
accessing_inactive_member	Accessing inactive union member.	None	False
cast_changes_type_inside_category	Disallowed type conversion	None	False
type_confusion_cast	Possible type confusion, cast to ‘{}’ of pointer pointing to ‘{}’ type.	None	False

Options¶

This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity
The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

allow_reads_through_arrays_of_character_types¶

allow_reads_through_arrays_of_character_types : bool = True

Do not report aliasing reads through character types, as they are guaranteed by the C Standard to result in a valid individual-byte access.

check_explicit_casts¶

check_explicit_casts : bool = True

Whether explicit casts should be checked and reported.

check_implicit_casts¶

check_implicit_casts : bool = True

Whether implicit casts should be checked and reported.

look_through_casts¶

look_through_casts : bool = False

If true, operand after stripping casts is used.

only_complex_expressions¶

only_complex_expressions : bool = False

Whether all operands or only those deemed complex should be inspected.

show_operand_in_entity¶

show_operand_in_entity : bool = False

Whether entity should be "from->to" or "(from->to)operand".

type_category¶

type_category : set[TypeCategory] = {'object_pointer_types'}

Selection of type categories to consider.

type_system¶

type_system : bauhaus.ir.common.types.type_systems.TypeSystem = <bauhaus.ir.common.types.type_systems.CompilerTypeSystem object at 0x7f6f1c5fd510>

Which type system to use: compiler types, underlying types, essential types.

Option Types¶

These types are used by options listed above:

TypeCategory¶

Base class for the different type categories.

signed_types

unsigned_types

float_types

char_types

plain char.

bool_types

bool, _Bool and special expressions.

enum_types

void_types

void_pointer_types

incomplete_pointer_types

function_pointer_types

object_pointer_types

null_pointer_types

other_types

Those not covered by other categories.

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