In the world of low-level programming, what you think you're accessing isn't always what you're really accessing. This subtle mismatch between the type a programmer assumes and the actual memory layout is at the heart of a class of bugs known as type confusion vulnerabilities.
While buffer overflows and use-after-free bugs have long taken the spotlight, CWE-843 (Access of Resource Using Incompatible Type) lurks quietly beneath the surface, waiting for the right moment to break memory safety and expose critical systems. It’s not just a theoretical issue. Type confusion has been exploited in real-world CVEs affecting everything from browser engines to embedded firmware.
Let’s explore how one incorrect assumption about types can open the door to undefined behavior and how attackers can walk right through it.
Understanding Type Systems and Memory Layouts
C is a low-level language that gives you direct access to memory, but it does very little to protect you from misusing that power. The compiler trusts you to know what you’re doing especially when it comes to how types map to memory.
In theory, the type system in C should help organize data: an int is 4 bytes, a char is 1 byte, a struct groups fields together, and arrays lay out elements linearly in memory. But the type system isn’t enforced at runtime. It exists mostly to help the compiler generate the right instructions. If you cast a pointer to another type, the compiler will go along with it, no questions asked.
That’s where things get dangerous. When you access memory using an incompatible type say, treating an int[] as a char* or casting a struct pointer to something else you might interpret memory incorrectly. This can result in anything from silent logic bugs to critical vulnerabilities.
The memory layout of objects, especially in the presence of padding, alignment, or architecture-specific behavior, is crucial. If your mental model of the layout doesn’t match the actual representation, you’re flying blind. And in systems programming, that almost always ends badly.
The wrong type tells the program to read or write memory in a way it was never supposed to. That’s not just undefined behavior—it’s an entry point for attackers.
Accessing Resources Using Incompatible Types
Let's build a concrete example of how treating one type as another can let an attacker bypass intended restrictions or access unintended memory. This is the kind of scenario that CWE-843 specifically targets.
Suppose we have a kernel-like interrupt handler that only allows certain high-priority IRQs (interrupt requests) to be processed. The program uses an int[] array to store priorities, but access to this array isn’t type-checked at runtime. That’s where things go wrong.
Here’s a minimal C example:
#include <stdio.h>
#include <stdlib.h>
int current_priority = 3;
int irq_priority[5] = {1, 2, 3, 4, 5};
void process_interrupt(int irq) {
printf("Processing interrupt %d\n", irq);
}
void interrupt_handler(int irq) {
if (irq_priority[irq] < current_priority) {
fprintf(stderr, "Warning: Interrupt %d dropped (priority too low)\n", irq);
exit(EXIT_FAILURE);
}
process_interrupt(irq);
}
This seems fine at first glance. But what if irq is user-controlled and passed unchecked?
Now let’s simulate an attacker exploiting this by crafting a fake irq value that causes the handler to interpret unrelated memory as if it were part of the irq_priority array.
#include <string.h>
char secret[] = "TOP_SECRET_STRING";
int main() {
// Forcefully interpret `secret` as if it were part of irq_priority
int *fake_irq_priority = (int *)secret;
// Simulate misbehavior: access the fake array through type punning
for (int i = 0; i < 4; i++) {
printf("Fake priority[%d]: %d\n", i, fake_irq_priority[i]);
}
// If irq_priority is improperly accessed via fake index,
// memory past legitimate bounds is revealed.
}
With the right pointer arithmetic and type casting, secret gets interpreted as an integer array. If this casting happens in a function that trusts the type, we can accidentally (or maliciously) extract data not meant to be exposed.
This is where type confusion turns into a data disclosure vulnerability. Even though types are declared safely, the language makes it trivial to reinterpret memory as a different type—without checks or warnings.
If you want to simulate:
// cwe-843.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Legitimate system-level globals
int current_priority = 3;
int irq_priority[5] = {1, 2, 3, 4, 5};
void process_interrupt(int irq) {
printf("Legitimately processing interrupt %d\n", irq);
}
void interrupt_handler(int irq) {
if (irq_priority[irq] < current_priority) {
fprintf(stderr, "Interrupt %d dropped (priority too low)\n", irq);
exit(EXIT_FAILURE);
}
process_interrupt(irq);
}
// Attacker-controlled buffer simulating sensitive memory
char secret[] = "TOP_SECRET_STRING";
// Simulated type confusion exploit
void simulate_type_confusion() {
printf("\nSimulating type confusion...\n");
// Type punning: treat a char* as int*
int *fake_irq_priority = (int *)secret;
for (int i = 0; i < 4; i++) {
printf("fake_irq_priority[%d] = %d\n", i, fake_irq_priority[i]);
}
int forged_irq = 0;
if (fake_irq_priority[forged_irq] >= current_priority) {
printf("Attacker triggers process_interrupt with forged value %d\n", fake_irq_priority[forged_irq]);
process_interrupt(forged_irq);
} else {
printf("Forged priority too low: %d\n", fake_irq_priority[forged_irq]);
}
}
int main() {
printf("Initial legitimate IRQ processing:\n");
interrupt_handler(4); // Legit call, priority 4 >= 3
simulate_type_confusion();
return 0;
}
gcc -o cwe-843 cwe-843.c -Wall
Why This Matters
In real-world systems—kernels, firmware, embedded software—this exact pattern shows up all the time. Functions receive untrusted input and make decisions based on type-dependent logic, without actual runtime type verification. The result? Attackers manipulate memory by simply shifting how it’s interpreted.
From Logical Flaws to Arbitrary Memory Access
Type confusion vulnerabilities are often underestimated because, on the surface, they may appear as harmless logic bugs one pointer cast gone wrong, a compiler warning ignored. However, beneath this simplicity lies a dangerous class of bugs that, if left unchecked, allow attackers to pierce memory safety boundaries and access or modify arbitrary locations in memory.
In our previous example, a char buffer was force-cast to an int *, simulating a malicious reinterpretation of memory. What looks like a benign reinterpretation becomes lethal in systems that rely on strict memory layouts for control flow decisions. This is especially common in embedded systems, real-time OS kernels, and any C/C++ system that manages memory manually.
The core issue is not just the cast, but the assumption that memory behind the pointer is of a certain type and layout. When that assumption is violated, everything built atop it bounds checks, privilege checks, pointer arithmetic becomes unreliable. That's the moment logical flaws escalate into memory corruption, memory disclosure, or even arbitrary code execution.
Consider a scenario where attacker-controlled memory overlaps with security-critical structures like access control tables, function pointers, or interrupt priorities. By injecting values with carefully aligned bytes, the attacker can coerce the system into reading forged permissions, executing unintended paths, or skipping checks entirely. This is how logical flaws transition into full-blown memory safety violations.
In modern exploit development, especially in CTFs or in-the-wild kernel exploits, these primitives are often used to achieve memory disclosure (infoleak) or arbitrary read/write, which serve as the groundwork for more advanced attacks like ROP or shellcode injection.
Memory Reinterpretation in Action
To clearly demonstrate how type confusion can lead to arbitrary memory access, let's take a closer look at what’s happening in memory. The buffer char secret[] = "TOP_SECRET_STRING" resides next to critical system-level variables like irq_priority. If an attacker is able to cast this buffer as int *, they can read (and potentially write) memory in 4-byte chunks — completely misaligned with the buffer’s original type.
Below is a simplified hexdump-like view of the memory, along with the output of the type-punning operation:
📌 Simulated Memory Layout (Hex View)
0x601000: 0x54 0x4F 0x50 0x5F T O P _
0x601004: 0x53 0x45 0x43 0x52 S E C R
0x601008: 0x45 0x54 0x5F 0x53 E T _ S
0x60100C: 0x54 0x52 0x49 0x4E T R I N
📌 Fake Interpretation as int *
(Casting char *secret to int * and reading integers from it)
fake_irq_priority[0] = 0x5F504F54 // '_POT' → Interpreted as int
fake_irq_priority[1] = 0x52434553 // 'RCES'
fake_irq_priority[2] = 0x535F5445 // 'S_TE'
fake_irq_priority[3] = 0x4E495254 // 'NIRT'
Each 4-byte read leaks a part of the string buffer, but interpreted as meaningless integers. Still, these values can bypass checks like if (irq_priority[irq] < current_priority) because they may coincidentally satisfy the condition due to their numeric representation.
📌 Exploit Result
Simulating type confusion...
fake_irq_priority[0] = 1600942164
fake_irq_priority[1] = 1379997027
fake_irq_priority[2] = 1397506917
fake_irq_priority[3] = 1313037364
Attacker triggers process_interrupt with forged value 1600942164
Legitimately processing interrupt 0
Even though the real irq_priority[0] = 1, the forged value from secret[0..3] is numerically 1600942164, easily passing the >= current_priority check.
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