How Hackers Break Into the OS Kernel: Methods, Exploits, and Defenses

How to Attack the Operating System Kernel?

This is a technically detailed discussion of the ways attackers can obtain kernel‑level execution rights, why the kernel is the ultimate security boundary, and what real‑world exploits look like.

Four Legitimate Ways to Enter Kernel Mode

Programs move between user space and kernel space through mechanisms that the operating system pre‑defines. The four official entry points are:

Interrupts :

Interrupts are divided into hardware (hard) interrupts and software (soft) interrupts. A hard interrupt is generated by a device, while a soft interrupt is triggered by the CPU executing an int instruction (e.g., int 2e on Windows or int 80 on Linux). In both cases the CPU saves the current context and jumps to an interrupt handler recorded in the IDT.

Exceptions :

Exceptions occur when the CPU encounters an illegal operation, such as division by zero or an invalid memory access. Like interrupts, they are handled by functions registered in the IDT.

System Calls :

System calls are the programming interfaces that let user programs request services such as file I/O, networking, process management, or memory allocation. Early x86 systems used software interrupts for system calls; modern CPUs provide dedicated instructions (e.g., sysenter , syscall , swi ) and registers for faster transitions.

Driver Loading :

Loading a kernel driver requires high privileges, so this path is not described in detail here.

These are the only “official” routes; once code runs in kernel mode, the operating system decides what code is executed, leaving no room for arbitrary user‑controlled behavior.

Zero‑Address (NULL) Exploits

In C, the address 0x0 represents a NULL pointer. If a kernel routine dereferences a pointer without checking for NULL, an attacker can map the first 4 KB page at address zero and place malicious data there. When the kernel later uses the unchecked pointer, the attacker’s code runs with kernel privileges.

Real‑world example: CVE-2014-4113 on Windows.

Use‑After‑Free (UAF) Attacks

Use‑after‑free occurs when a program frees memory but continues to use the dangling pointer. An attacker can allocate the same memory region with crafted data, causing the original pointer to reference attacker‑controlled code.

Typical vulnerability: CVE-2016-0728 on Linux.

Illustrative example (simplified): two objects of the same size are allocated; the first is freed, the pointer is not cleared, and a fake object is allocated in the same slot. Subsequent calls through the stale pointer invoke the fake object's functions, hijacking execution flow.

Integer Overflow + Out‑of‑Bounds Writes

Many kernel structures (e.g., system‑call tables, IDT) are stored in arrays. If a kernel routine writes to an array without proper bounds checking and the index can be controlled from user space, an attacker can overwrite function pointers in these tables.

Typical vulnerability: CVE-2013-2094 on Linux, which allowed arbitrary kernel‑mode code execution by overwriting the IDT.

Security Defenses

Modern CPUs and operating systems have added mitigations:

Intel SMEP (Supervisor Mode Execution Prevention) on 3rd‑generation Core CPUs blocks execution of user‑space code while in kernel mode.

ARM’s PXN (Privileged Execute Never) provides similar protection.

Windows 8.1 disables allocation of the zero page.

Linux kernel 2.6.26 introduced vm.mmap_min_addr to raise the minimum user‑space address, preventing zero‑page usage.

Simple programming mistakes such as null‑pointer dereferences, use‑after‑free, array out‑of‑bounds writes, and integer overflows become catastrophic when they occur in kernel code, underscoring the importance of rigorous coding practices.