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CWE-843: Access of Resource Using Incompatible Type ('Type Confusion')

Weakness ID: 843
Abstraction: Base
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

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.

Extended Description

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.

+ Alternate Terms
Object Type Confusion
+ Time of Introduction
  • Implementation
+ Applicable Platforms





Type-unsafe Languages

+ 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.

(Bad Code)
Example Language:
#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 2

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

(Bad Code)
Example Language: PHP 
$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 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).

(Bad Code)
Example Language: Perl 
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.

+ Observed Examples
Type confusion in CSS sequence leads to out-of-bounds read.
Size inconsistency allows code execution, first discovered when it was actively exploited in-the-wild.
Improperly-parsed file containing records of different types leads to code execution when a memory location is interpreted as a different object than intended.
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class704Incorrect Type Conversion or Cast
Development Concepts (primary)699
Research Concepts (primary)1000
CanPrecedeWeakness ClassWeakness Class119Improper Restriction of Operations within the Bounds of a Memory Buffer
Research Concepts1000
+ Research Gaps

Type confusion weaknesses have received some attention by applied researchers and major software vendors for C and C++ code. Some publicly-reported vulnerabilities probably have type confusion as a root-cause weakness, but these may be described as "memory corruption" instead. This weakness seems likely to gain prominence in upcoming years.

For other languages, there are very few public reports of type confusion weaknesses. These are probably under-studied. Since many programs rely directly or indirectly on loose typing, a potential "type confusion" behavior might be intentional, possibly requiring more manual analysis.

+ References
Mark Dowd, Ryan Smith and David Dewey. "Attacking Interoperability". "Type Confusion Vulnerabilities," page 59. 2009. <>.
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 7, "Type Confusion", Page 319.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submission DateSubmitterOrganizationSource
2011-05-15MITREInternal CWE Team
Modification DateModifierOrganizationSource
2012-05-11CWE Content TeamMITREInternal
updated References

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Page Last Updated: January 11, 2017