CVE-2024-40815 - Breaking Apple Pointer Authentication With a Race Condition

Published: July 2024
Author: [Your Name]


Apple has recently patched a high-impact vulnerability, CVE-2024-40815, in its major operating systems. This post explains the bug in simple language, shows how it might have been exploited, and provides original source links for further reading. If you’re a developer, security enthusiast, or simply curious about how low-level attacks work on Apple devices, read on!

tvOS (fixed in 17.6)

In simple terms:
A race condition happens when two or more operations run at the same time and the system doesn’t handle them safely, leading to unpredictable behavior.

In this case, the bug let attackers with the ability to read and write arbitrary memory bypass Pointer Authentication—a security feature that’s supposed to stop hackers from hijacking code pointers and executing malicious code.

2. What Is Pointer Authentication?

Pointer Authentication (PAC) is an ARMv8.3+ architecture feature used by Apple to make it harder for attackers to control code execution through pointer manipulation. It adds cryptographic "signatures" to pointers, which are checked before use. If a pointer’s signature is invalid, the system crashes instead of running bad code.

Cool, right? Except…

When race conditions mess with PAC, an attacker might overwrite a pointer after its authentication—or sneak in before PAC validation—so the protections don’t matter.

How the Bug Worked

Apple’s note on the patch is vague ("A race condition was addressed with additional validation"), but here’s how these things typically go:

The system checks a pointer’s signature ("Is this pointer safe?").

- Between the check and the pointer’s actual use, attackers quickly swap in a different, malicious pointer (classic TOCTOU: Time-of-Check to Time-of-Use).
- Since the pointer was authenticated before it was swapped, the system blindly uses the attacker-supplied pointer.

Let’s break it down with simple code

void use_authenticated_pointer(void *ptr) {
    // 1. Authenticate the pointer
    if (!authenticate_pointer(ptr)) {
        exit(1);
    }
    // --- Attacker slips in here! ---
    global_pointer = attacker_controlled_pointer;
    // 2. Use the pointer
    call_function(global_pointer);
}

If an attacker has arbitrary read/write capability, they can swap out global_pointer after step 1 but before step 2.

Prerequisites

- The attacker must already have arbitrary read and write access (e.g., through another vulnerability).
- They race the system—replacing the original pointer with a malicious one after it’s authenticated but before it’s used.

Potential Impact

- Bypass PAC: Defeats a primary macOS/iOS defense.
- Arbitrary Code Execution: Can run code as the target app/process, potentially leading to full device compromise.

Let’s say an application performs the following steps when handling sensitive data pointers

void process_data(void *data_ptr) {
    if (!authenticate_pointer(data_ptr)) {
        // Invalid pointer!
        return;
    }
    // Time window for an attacker is open here.
    memcpy(process_buffer, data_ptr, data_length);
}

Exploit Steps

1. Attacker gains arbitrary write (e.g., via heap overflow) and knows when authenticate_pointer is called.

They swap out data_ptr with a dangerous pointer pointing to controlled memory.

3. The application copies from the attacker’s memory, potentially leading to code injection and execution.

6. Apple’s Fix

Apple’s update notes are brief:
> “A race condition was addressed with additional validation.”

This likely means they locked the pointer between authentication and use, or validated its authenticity at the point of use, closing the window for a race.

7. References & Further Reading

- Apple Security Updates - July 2024
- Apple CVE page for CVE-2024-40815
- Pointer Authentication explained (LSE Blog)

Update all Apple devices ASAP to the latest versions listed above.

- Attackers cannot use this bug alone—they need arbitrary read/write first. But if they have that, PAC is no longer enough to stop them!
- Always use proper locking/synchronization in sensitive code, and validate pointers as close to use as possible.

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Disclaimer:
This post is for educational purposes only. The vulnerability has been patched. Please use responsibly.


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Timeline

Published on: 07/29/2024 23:15:13 UTC
Last modified on: 08/15/2024 16:14:17 UTC