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

Description

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.

Potential Impact

Availability, Integrity, Confidentiality

Read Memory, Modify Memory, Execute Unauthorized Code or Commands, DoS: Crash, Exit, or Restart

Demonstrative Examples

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

Bad

#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:

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.

Bad

$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:

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:

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

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.

Detection Methods

Automated Static Analysis High — 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 sea

Real-World CVE Examples

CVE ID	Description
CVE-2025-32352	Type confusion in PHP app allows authentication bypass when users have passwords whose MD5 hashes can be interpreted as numbers
CVE-2010-4577	Type confusion in CSS sequence leads to out-of-bounds read.
CVE-2011-0611	Size inconsistency allows code execution, first discovered when it was actively exploited in-the-wild.
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.

Taxonomy Mappings

CERT C Secure Coding: EXP39-C — Do not access a variable through a pointer of an incompatible type

Frequently Asked Questions

What is CWE-843?

CWE-843 (Access of Resource Using Incompatible Type ('Type Confusion')) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. 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.

How can CWE-843 be exploited?

Attackers can exploit CWE-843 (Access of Resource Using Incompatible Type ('Type Confusion')) to read memory, modify memory, execute unauthorized code or commands, dos: crash, exit, or restart. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-843?

Follow secure coding practices, conduct code reviews, and use automated security testing tools (SAST/DAST) to detect this weakness early in the development lifecycle.

What is the severity of CWE-843?

CWE-843 is classified as a Base-level weakness (Medium abstraction). It has been observed in 4 real-world CVEs.