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

Incomplete Base

Structure: Simple

Description

Type confusion occurs when a program creates a resource—like a pointer, object, or variable—with one data type, but later incorrectly accesses it as a different, incompatible type.

Extended Description

This mismatch can cause the program to misinterpret the data in memory, leading to logical errors, crashes, or unexpected behavior because the resource lacks the properties the code expects. In memory-unsafe languages like C and C++, this often results in dangerous out-of-bounds memory access, corrupting data or creating security vulnerabilities. While commonly seen with C/C++ unions when parsing complex data structures, type confusion can appear in many languages. For instance, PHP might fail if an array is passed where a single value is required, and languages like Perl that perform automatic type conversion can also introduce subtle bugs when variables are accessed as unintended types.

Common Consequences 1

Scope: AvailabilityIntegrityConfidentiality

Impact: Read MemoryModify MemoryExecute Unauthorized Code or CommandsDoS: 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.

Demonstrative Examples 3

ID : DX-188

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

Code Example:

Bad

/* This particular value for nameID is used to make the code architecture-independent. If coming from untrusted input, it could be any value. /

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.

Code Example:Bad
PHP
php

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).

Code Example:Bad
Perl
perl

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" (Not Failing Securely ('Failing Open')), 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 3

CVE-2010-4577Type confusion in CSS sequence leads to out-of-bounds read.

CVE-2011-0611Size inconsistency allows code execution, first discovered when it was actively exploited in-the-wild.

CVE-2010-0258Improperly-parsed file containing records of different types leads to code execution when a memory location is interpreted as a different object than intended.

References 2

Attacking Interoperability

Mark Dowd, Ryan Smith, and David Dewey

2009

http://hustlelabs.com/stuff/bh2009_dowd_smith_dewey.pdf(2023-04-07)

ID: REF-811

The Art of Software Security Assessment

Mark Dowd, John McDonald, and Justin Schuh

Addison Wesley

2006

ID: REF-62

Applicable Platforms

Languages:

C : UndeterminedC++ : Undetermined

Modes of Introduction

Implementation

Alternate Terms

Object Type Confusion

Related Weaknesses

ChildOf:

Incorrect Type Conversion or Cast (CWE-704)

ChildOf:

Incorrect Type Conversion or Cast (CWE-704)

CanPrecede:

Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)

Taxonomy Mapping

CERT C Secure Coding

Notes

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

Research Gap 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.