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.

Primary

Resultant

Object Type Confusion

Implementation

Availability
Integrity
Confidentiality
Read Memory
Modify Memory
Execute Unauthorized Code or Commands
DoS: Crash, Exit, or Restart
When a memory buffer is accessed using the wrong type, it could read or write memory out of the bounds of the buffer, if the allocated buffer is smaller than the type that the code is attempting to access, leading to a crash and possibly code execution.

Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
High

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;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.

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:

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

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.

CVE-2025-32352
Type confusion in PHP app allows authentication bypass when users have passwords whose MD5 hashes can be interpreted as numbers
https://www.cve.org/CVERecord?id=CVE-2025-32352

CVE-2010-4577
Type confusion in CSS sequence leads to out-of-bounds read.
https://www.cve.org/CVERecord?id=CVE-2010-4577

CVE-2011-0611
Size inconsistency allows code execution, first discovered when it was actively exploited in-the-wild.
https://www.cve.org/CVERecord?id=CVE-2011-0611

CVE-2010-0258
Improperly-parsed file containing records of different types leads to code execution when a memory location is interpreted as a different object than intended.
https://www.cve.org/CVERecord?id=CVE-2010-0258

EXP39-C
Do not access a variable through a pointer of an incompatible type
Exact

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

This weakness is possible in any type-unsafe programming language.

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.
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.

CWE Content Team
MITRE
2011-05-15
1.13
2011-06-01

CWE Content Team
MITRE
2012-05-11
updated References

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms, Taxonomy_Mappings

CWE Content Team
MITRE
2019-01-03
updated Relationships

CWE Content Team
MITRE
2019-06-20
updated Relationships

CWE Content Team
MITRE
2020-02-24
updated Relationships

CWE Content Team
MITRE
2020-06-25
updated Common_Consequences, Relationships

CWE Content Team
MITRE
2022-04-28
updated Research_Gaps

CWE Content Team
MITRE
2023-01-31
updated Description

CWE Content Team
MITRE
2023-04-27
updated References, Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2023-10-26
updated Demonstrative_Examples

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Detection_Factors, Observed_Examples, Weakness_Ordinalities