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Meltdown Attack

Updated: Mar 22


In January 2018, a major security vulnerability was discovered in modern processors called "Meltdown." This flaw allowed attackers to access sensitive information, such as passwords or encryption keys, by exploiting the way processors handle speculative execution. In this article, we will provide an overview of the Meltdown CPU attack, discuss its implications for system security, and explore some defense strategies to mitigate these threats.

What is the Meltdown CPU Attack?

Meltdown is a hardware vulnerability that affects many modern processors from Intel, ARM, and AMD. It exploits a design flaw in how these processors handle speculative execution – a technique used by microprocessors to optimize performance by executing instructions before it is clear whether they are needed. This flaw allows attackers to access sensitive information that should be protected by hardware memory protection mechanisms.

To understand the Meltdown attack, let's first look at how speculative execution works in modern processors:

1. When a program needs data from memory, the processor checks if it has already been loaded into cache. If not, it starts fetching the data from main memory while simultaneously executing other instructions that do not depend on the fetched data. This is called "out-of-order execution."

2. Once the fetched data arrives in cache, the processor can now execute the instruction that depends on this data (e.g., a conditional branch). However, if the fetched data turns out to be invalid or unnecessary, the processor discards the results of speculative execution and reverts its state to before the speculation began.

Meltdown attacks take advantage of this process by forcing the processor to load sensitive information into cache during speculative execution. An attacker can then use timing side-channel attacks to measure how long it takes for the processor to access this data, inferring whether the sensitive information is present in cache and thus revealing its content.

Here's an example of a simple Meltdown exploit written in C:

#include <stdio.h>

#include <string.h>

// Function to trigger Meltdown attack

void meltdown(unsigned long addr) {

    asm("movl %1,%%ecx; clflush %0"::"m" (*(char*)&addr));


int main() {

    unsigned long kernel_address = 0xffffffff81000000; // Example address of kernel memory

    char buffer[0x1000];

    memset(buffer, 'A', sizeof(buffer)); // Fill buffer with A's

    // Trigger Meltdown attack and measure timing differences

    for (int i = 0; i < 100; i++) {

        meltdown(kernel_address + i);

        char c = (char)(kernel_address + i);

        printf("%c", c); // Print accessed data to stdout


    return 0;


This code demonstrates how an attacker could use Meltdown to read sensitive information from kernel memory by measuring the time it takes for the processor to access different addresses in memory.

Here is an example showing how the Meltdown attack can be done with JavaScript.

let target_address = 0x41000000; // Physical memory address to target

// Allocate memory at target address
let buffer = new ArrayBuffer(4096);
let view = new DataView(buffer);

// Write interrupt instruction
view.setUint8(0, 0xCC);

// Execute attack
attack(target_address, 0x0, 0x100000);

function attack(target, start, size) {
    let p = target;
    let end = start + size;
    while (p < end) {
        let data = view.getUint8(p); // Read from physical memory
        // Do something with data...

Implications and Mitigations:

Meltdown poses a significant threat to system security since it allows attackers to bypass hardware memory protection mechanisms and access sensitive data that should be protected. To mitigate this vulnerability, various software patches have been released for operating systems such as Windows, macOS, and Linux. These patches include changes to the kernel memory management to prevent Meltdown attacks from succeeding.

In addition to these software updates, hardware manufacturers like Intel are working on new processor designs that address the root cause of the Meltdown flaw. However, it may take years for these updated processors to become widely available in consumer devices.


The Meltdown CPU attack is a severe security vulnerability that affects modern processors from multiple manufacturers. It allows attackers to bypass hardware memory protection mechanisms and access sensitive information from kernel memory. While software patches have been released to mitigate this threat, it remains important for users to keep their systems up-to-date with the latest security fixes.


1. Lipp, J., Schinasi, D., Saputro, A., & Meltdown CPU Attack (2018). Meltdown. Project Zero.

2. Google Project Zero Team. (2018). Meltdown and Spectre Security Vulnerabilities Explained: What They Are and How to Stay Safe. Retrieved from,news-25937.html

3. Intel Corporation. (2018). Frequently Asked Questions: Meltdown & Spectre Security Research Findings. Retrieved from

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