When you browse the web using Firefox, you expect it to keep you safe. But sometimes, even the most popular web browsers have serious security flaws. Today, we’ll break down CVE-2023-5175—a use-after-free vulnerability in Firefox that could let attackers take control of your computer. We'll keep things simple, show you the bug in action, and explain why updating is so important.

What Is CVE-2023-5175?

CVE-2023-5175 is a security issue found and patched in Firefox versions before 118. The problem happens during process shutdown, when an ImageBitmap can be created and later used after it’s already been freed (deleted) from another part of the code. This kind of bug is called a *use-after-free*, and it can crash your browser or even let cyber criminals run their own code.

> Affected software:
> Firefox versions before 118 (Mozilla advisory MFSA 2023-40)

How Did This Happen? (Technical Background)

ImageBitmap is a web API that allows fast and easy manipulation of images in web applications, often used in canvas operations.

In Firefox, if the browser starts shutting down (maybe you’re closing a tab or the whole browser), but certain images are still being processed, it’s possible for the browser to destroy (free) the memory for those images before the process is really done using them. If a malicious web page can time actions just right, it could trick the browser into using memory that’s already been freed, which could lead to code execution.

Here's a high-level version of what went wrong in the Firefox source code (simplified for clarity)

// Imagine this simplified flow
std::shared_ptr<ImageBitmap> safeImageBitmap;

// 1. ImageBitmap created
safeImageBitmap = CreateImageBitmap(someImage);

// 2. Process shutdown starts (possibly in another thread or event)
ShutdownProcess();

// 3. safeImageBitmap is freed in shutdown
safeImageBitmap.reset();

// 4. But from another callback, we try to use it again!
safeImageBitmap->Draw(context);  // Oops! It was already freed!

This leads the program to access memory that is now invalid, which in some cases could be exploited by attackers to inject malicious code.

Exploit Details

While exploiting use-after-free bugs is notoriously tricky, skilled attackers can often use JavaScript and Web APIs to reliably trigger the bug. Here is a proof-of-concept JavaScript snippet (demonstration only!) that shows how an attacker might try to trigger the vulnerability by creating and using an ImageBitmap at the moment the page or browser is closing:

// This code simulates creating an ImageBitmap, then shutting down.
let canvas = document.createElement('canvas');
canvas.width = 10;
canvas.height = 10;
let context = canvas.getContext('2d');

// Draw something
context.fillRect(, , 10, 10);

// Create an ImageBitmap from the canvas
createImageBitmap(canvas).then(imageBitmap => {
    // Register a 'beforeunload' event to trigger during shutdown
    window.addEventListener('beforeunload', () => {
        // Try to use the ImageBitmap during (or after) shutdown
        // Firefox < 118 could use freed memory here!
        context.drawImage(imageBitmap, , );

        // In a real exploit, an attacker would try to corrupt memory here!
    });
});

// Trigger shutdown by reloading or closing the tab/window

In an actual attack, the malicious script would time these operations to coincide exactly with the browser’s internal shutdown process—a tough, but not impossible, job for determined hackers.

Real-World Impact

If you’re still running Firefox 117 or older, a malicious website could crash your browser, steal data, or even run their own programs on your computer—all just by getting you to visit a dangerous web page. These attacks could be automated and require no interaction beyond you simply browsing to the site.

How to Stay Safe

Update to Firefox 118 or later right away. Mozilla patched this bug in the September 2023 release. Updating is fast and easy—just head to Mozilla’s website or use Firefox’s built-in update tool.

References

- Mozilla Security Advisory MFSA 2023-40
- CVE Details: CVE-2023-5175
- Mozilla Bugzilla: Bug 1847421

Conclusion

Memory bugs like CVE-2023-5175 don’t just crash your browser—they can put your whole device at risk. Make sure you’re running the latest version of Firefox, and be careful when browsing unknown websites. For developers, always stay alert to process shutdown logic and proper memory management, especially with asynchronous APIs like ImageBitmap.

Stay safe out there, and keep your software updated!

Timeline

Published on: 09/27/2023 15:19:42 UTC
Last modified on: 09/29/2023 13:46:01 UTC