In May 2022, Apple patched a serious vulnerability—CVE-2022-26738—that affected several Apple platforms, including iOS, iPadOS, macOS, and tvOS. The flaw was characterized as an “out-of-bounds write” in the kernel, meaning malicious applications could write data where they shouldn’t, possibly leading to arbitrary code execution with kernel-level privileges. This post takes a deep dive into how this vulnerability worked, how it was fixed, and what it could mean for attackers and defenders.

What Is an Out-of-Bounds Write?

Let’s begin by understanding some basics. In software, arrays and buffers are allocated with a fixed size. An out-of-bounds write happens when a program unintentionally—or through malicious intent—writes data past the end (or before the beginning) of the allocated memory space. Such bugs can corrupt memory, crash a program, or more dangerously, allow attackers to inject their own code and potentially take control.

Impact: Arbitrary code execution with kernel privileges

- Affected: iOS/iPadOS 15.x, macOS Monterey before 12.4, tvOS before 15.5
- Patched: iOS/iPadOS/tvOS 15.5, macOS Monterey 12.4
- Apple Security Advisory (HT213252)

The Apple advisory states:  
> “An out-of-bounds write issue was addressed with improved bounds checking. An application may be able to execute arbitrary code with kernel privileges.”

How Did the Vulnerability Work?

Apple didn’t provide detailed code, but based on security research and public disclosures, CVE-2022-26738 is likely related to improper validation of buffer sizes during certain kernel operations.

An application, possibly with crafted input, could trigger a scenario where data written to a kernel buffer would exceed its intended bounds.

Here's a conceptual example in C-like pseudo-code (not the original Apple code)

// Vulnerable version
char buf[256];
void vulnerable_copy(const char* user_input, size_t input_len) {
    memcpy(buf, user_input, input_len); // No bounds check!
}

If input_len is larger than 256, the memcpy overruns buf into adjacent memory, causing an out-of-bounds write.

Fixed version

// Safe version
void safe_copy(const char* user_input, size_t input_len) {
    if (input_len > sizeof(buf)) {
        input_len = sizeof(buf); // Clamp to buffer size
    }
    memcpy(buf, user_input, input_len);
}

Real-World Exploitation

In the real vulnerability, the attacker might use a crafted app to call a system interface, like an IOService or Mach port, sending data chunks larger than allowed. The unchecked size leads the kernel to overwrite memory structures, potentially controlled by the attacker.

Let’s imagine how an attacker could abuse this, step by step

1. Craft Exploit App: Write an app that interacts with a vulnerable kernel API, sending oversized data.
2. Trigger the Vulnerability: Overflow the buffer to overwrite critical kernel data, such as function pointers or security check variables.
3. Execute Arbitrary Code: Redirect execution to attacker’s code payload, running with kernel (root) privileges.

Why Kernel Privileges Matter

Kernel privilege is the highest level on an operating system. If an attacker gets code to run in the kernel, they can:

Patch and Mitigation

Apple fixed CVE-2022-26738 by improving bounds checking—meaning all data copied into kernel buffers must fit their allocated size.

Advice:  
- Update all Apple devices to iOS/iPadOS 15.5, macOS Monterey 12.4, tvOS 15.5 or later.

Original References & Further Reading

- Apple Security Advisory HT213252
- NVD - CVE-2022-26738
- Project Zero Exploit Write-ups (General)

Final Thoughts

CVE-2022-26738 is a classic example of why robust input validation and bounds checking are so vital, especially in operating system kernels. Even a simple oversight can let attackers take full control of a device. Always keep your systems updated, and stay tuned for more deep dives on high-impact CVEs!


If you want more detailed technical info or proof-of-concept code, keep an eye on security researchers’ blogs and Apple’s advisories, as these vulnerabilities often get more public analysis after they are patched.

Timeline

Published on: 05/26/2022 19:15:00 UTC
Last modified on: 06/07/2022 15:37:00 UTC