CWE-843: Access of Resource Using Incompatible Type ('Type Confusion')Weakness ID: 843 Vulnerability Mapping:
ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction: BaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. |
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. Extended Description 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. Alternate Terms Common Consequences This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.Scope | Impact | Likelihood |
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Availability Integrity Confidentiality
| Technical Impact: 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. | |
Relationships This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Research Concepts" (CWE-1000) Nature | Type | ID | Name |
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ChildOf | Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 704 | Incorrect Type Conversion or Cast | PeerOf | Base - a weakness
that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource. | 1287 | Improper Validation of Specified Type of Input | CanPrecede | Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 119 | Improper Restriction of Operations within the Bounds of a Memory Buffer |
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Software Development" (CWE-699) Nature | Type | ID | Name |
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MemberOf | Category - a CWE entry that contains a set of other entries that share a common characteristic. | 136 | Type Errors |
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore. Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (CWE-1003) Nature | Type | ID | Name |
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ChildOf | Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. | 704 | Incorrect Type Conversion or Cast |
Modes Of Introduction The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase. 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: C
#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. 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:
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 Reference | Description |
| 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. |
Memberships This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources. Vulnerability Mapping Notes Usage: ALLOWED (this CWE ID could be used to map to real-world vulnerabilities) | Reason: Acceptable-Use | Rationale: 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. | Comments: 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. |
Notes 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.
Taxonomy Mappings Mapped Taxonomy Name | Node ID | Fit | Mapped Node Name |
CERT C Secure Coding | EXP39-C | Exact | Do not access a variable through a pointer of an incompatible type |
References
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[REF-62] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 7, "Type Confusion", Page 319. 1st Edition. Addison Wesley. 2006.
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