CVE-2021-4321 exposes a policy bypass vulnerability in the Blink engine of Google Chrome versions prior to 91..4472.77. This vulnerability allows a remote attacker to bypass content security policy (CSP) by crafting an HTML page. Although this has been categorized as a low-severity security issue by Chromium, it is still essential to understand its implications and explore ways to patch it to protect users. In this post, we will delve into the technical details of the exploit, provide code snippets to show its execution, and provide links to the original references.

Exploit Details

A CSP is a security feature designed to prevent cross-site scripting (XSS) attacks and other code injection threats. It allows a website's administrators to control the types and sources of resources that can be loaded on a specific web page. The vulnerability in question effectively bypasses this security policy and enables an attacker to bypass the intended restrictions.

This vulnerability is relevant to Google Chrome's implementation of the Blink rendering engine, which is responsible for rendering web pages. The Blink engine powers Chromium-based browsers, including Google Chrome.

Code Snippet

To illustrate the exploit, let's assume you've implemented a basic CSP to only allow images from trusted.com:

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Security-Policy" content="img-src https://trusted.com;">;
<title>CSP Test</title>
</head>
<body>
<img src="https://attacker.com/image.jpg"; />
</body>
</html>


With the vulnerability, an attacker could craft an HTML page that bypasses the CSP and allows images from their malicious domain (attacker.com) to be loaded:

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Security-Policy" content="img-src https://trusted.com;">;
<title>Exploiting CVE-2021-4321</title>
</head>
<body>
<script>
  // Bypass the CSP here...
</script>
<img src="https://attacker.com/image.jpg"; />
</body>
</html>

1. The Chromium Project's Security Advisory: CVE-2021-4321
2. The Chromium bug report detailing the vulnerability: Issue 1186287
3. The patch in the Chromium source code: CL:2780666

Patching the Vulnerability

Google has addressed the bypass vulnerability in Chrome version 91..4472.77, ensuring that all Chromium-based browsers built from this version are secured against this type of bypass attack.

To protect your browser from this vulnerability, you must update your Chrome browser to the latest stable version. You can do this by following these steps:

Conclusion

While CVE-2021-4321 may have a low severity rating, it is crucial to stay informed and protect your systems against potential breaches. The vulnerability has since been patched, but users are encouraged to keep their software up-to-date and exercise caution when navigating the web. Furthermore, web developers should continue to research and implement security policies to mitigate threats on different browsing platforms.

Timeline

Published on: 07/29/2023 00:15:00 UTC
Last modified on: 08/02/2023 03:56:00 UTC