Apple’s operating systems have a reputation for security, but sometimes, even a small mistake in the code can open up huge vulnerabilities. In this post, we'll break down CVE-2023-36495—an integer overflow bug Apple patched in mid-2023—and show you how such issues can be exploited for kernel-level code execution. We'll keep it simple, walk through the details, show you relevant code concepts, and give tips for defense.

tvOS 16.6

Summary:  
If you send specially crafted data to a vulnerable device, you can make the system do math that produces a value "too big" for its type. This can confuse the system, leading to memory corruption and potentially letting a rogue app run its own code as the system kernel.

Apple’s note:  
> "An integer overflow was addressed with improved input validation. This issue is fixed in … [the versions listed above]. An app may be able to execute arbitrary code with kernel privileges." (Apple Security Update – CVE-2023-36495)

Breaking It Down: What Is an Integer Overflow?

An integer overflow happens when a math operation tries to create a number larger than what the variable can store. For example, imagine you have:

uint16_t x = 65535;
x = x + 1;
// x becomes , because 65536 cannot fit in 16 bits

Why does this matter? Many low-level checks assume arithmetic 'just works'—if it "wraps around", a smart attacker can use this to break out of expected memory bounds.

Suppose part of the macOS/iOS kernel—say, a driver or messaging API—looks like this

// Pseudocode of vulnerable pattern
uint32_t n = user_input_size;
uint32_t total_bytes = n * sizeof(struct thing);

void *data = malloc(total_bytes);
memcpy(data, user_supplied, total_bytes); // Oops!


If the attacker sends a very large n, this multiplication (n * sizeof(struct thing)) could overflow, making total_bytes actually much smaller than intended. If the system then copies more data than there is memory for, it will overwrite important stuff—sometimes even kernel structures.

Privilege Escalation

If an attacker manages to overwrite nearby memory—especially in the kernel—they can "smuggle in" their own code, changing how the operating system works. This style of exploit is how attackers can escape app sandboxing and control the whole device.

Below is a simplified C-style PoC of the type of bug (not Apple's actual code)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

struct thing {
    char buf[16];
};

void process_input(uint32_t n, char *data) {
    uint32_t bytes = n * sizeof(struct thing);
    // Intent: allocate memory for n things
    struct thing *arr = malloc(bytes);
    if (!arr) exit(1);

    memcpy(arr, data, bytes); // Danger: bytes may be less than n * 16!
}

int main() {
    uint32_t big = xFFFFFFFF / 16 + 2; // Huge number, triggers overflow

    char evil[32] = {};
    // Fill evil[] with attacker payload
    process_input(big, evil);
}

What happens here?
- bytes overflows, wrapping to a small value (in the real kernel, this could allocate less than expected memory).

memcpy() then writes more data than was allocated, smashing adjacent memory.

On Apple, in the kernel, an attacker could then craft data that overwrites function pointers or security data structures. This could be chained into executing arbitrary code as the kernel.

Kernel privilege: An attacker can "own" your device at the system level.

- Bypasses sandboxing: Apps are supposed to be isolated; this bug lets one ‘jailbreak’ the system.

How Apple Fixed It

Apple’s fix was to add better input validation. Now, before multiplying or allocating, they check if the numbers are going to overflow.

Example "safe pattern" in C

if (n > UINT32_MAX / sizeof(struct thing)) {
    // handle error, don't allocate
    return -1;
}

This prevents an overflow before ever doing the math.

Be extra careful with memory sizes, especially in kernel or native code.

- Use helper functions for safe arithmetic: for example, Apple's os_mul_overflow() or C11’s __builtin_mul_overflow() for overflow checks.

References and Further Reading

- Apple Security Update – CVE-2023-36495
- Apple Security Advisories
- Integer overflow bugs in kernel code (CWE-190)
- Project Zero “Exploiting Kernel Vulnerabilities”

Conclusion

CVE-2023-36495 is a perfect example of how even a small arithmetic mistake in system code can lead to major risks. Always keep your devices updated, and if you’re a developer, be skeptical with all user input—even simple integers.

Timeline

Published on: 07/28/2023 05:15:10 UTC
Last modified on: 08/03/2023 16:59:42 UTC