Access of Resource Using Incompatible Type ('Type Confusion')

The program 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 program 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.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

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

#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; = defaultMessage;
  printf("Pointer of is %p\n",;
  /* 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 is now %p\n",;
  if (buf.msgType == NAME_TYPE) {
    printf("Message: %s\n",;
  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 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 - 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 was changed, even though 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 could contain an arbitrary pointer, leading to out-of-bounds reads or writes.

Example Two

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

$value = $_GET['value'];
$sum = $value + 5;
echo "value parameter is '$value'<p>";
echo "SUM is $sum";

When called with the following query string:


the program calculates the sum and prints out:

SUM is 128

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


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 Three

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).

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.

See Also

SEI CERT C Coding Standard - Guidelines 03. Expressions (EXP)

Weaknesses in this category are related to the rules and recommendations in the Expressions (EXP) section of the SEI CERT C Coding Standard.

Type Errors

Weaknesses in this category are caused by improper data type transformation or improper handling of multiple data types.

Comprehensive CWE Dictionary

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Weaknesses without Software Fault Patterns

CWE identifiers in this view are weaknesses that do not have associated Software Fault Patterns (SFPs), as covered by the CWE-888 view. As such, they represent gaps in...

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