CWE-843 Base Incomplete

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

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.

Definition

What is CWE-843?

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.
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.
Real-world impact

Real-world CVEs caused by CWE-843

  • 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.

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

    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.

  3. 3

    As a result, modification of buf.nameID - an int - can effectively modify the pointer that is stored in buf.name - a string.

  4. 4

    Execution of the program might generate output such as:

  5. 5

    ``` Pointer of name is 10830 Pointer of name is now 10831 Message: ello World ```

Vulnerable code example

Vulnerable C

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

Vulnerable 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);
  		}
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-843

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-843

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-843 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-843?

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.

How serious is CWE-843?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-843?

MITRE lists the following affected platforms: C, C++.

How can I prevent CWE-843?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-843?

Plexicus's SAST engine matches the data-flow signature for CWE-843 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-843?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/843.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-843

CWE-704 Parent

Incorrect Type Conversion or Cast

This vulnerability occurs when software incorrectly changes data from one type to another, leading to unexpected behavior or security flaws.

CWE-1389 Sibling

Incorrect Parsing of Numbers with Different Radices

This vulnerability occurs when software processes numeric input expecting standard decimal numbers (base 10), but fails to handle inputs…

CWE-588 Sibling

Attempt to Access Child of a Non-structure Pointer

This vulnerability occurs when code incorrectly treats a pointer to a basic data type (like an integer) as if it points to a structured…

CWE-681 Sibling

Incorrect Conversion between Numeric Types

This vulnerability occurs when a program converts a value from one numeric type to another (like a 64-bit integer to a 32-bit integer) and…

CWE-119 Can precede

Improper Restriction of Operations within the Bounds of a Memory Buffer

This vulnerability occurs when software accesses a memory buffer but reads from or writes to a location outside its allocated boundary.…

Resources

Further reading

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