How it was found

Hi,

I found the following issue in my own Gitlab installation. This is a request forgery that reveals the Rails authenticity_token remotely, which in turn allows mounting state-changing CSRF attacks.

Vulnerability

The web app code relies on location.pathname in a number of places to create new relative URLs.

Here is one: https://gitlab.com/gitlab-org/gitlab-ce/blob/f78ecda55d5431f9a74ab2892b516ecb45f24d80/app/assets/javascripts/environments/folder/environments_folder_view.js#L21

And below, it sends a request to such a URL: https://gitlab.com/gitlab-org/gitlab-ce/blob/f78ecda55d5431f9a74ab2892b516ecb45f24d80/app/assets/javascripts/environments/folder/environments_folder_view.js#L86

Now, a forged link containing //namespace/repo/ will make these URLs absolute because of the leading double slashes. I was able to create an attack.com namespace which translates to https://attack.com/repo/. The app will then send a request including the anti-CSRF token to an attacker controlled domain. Which it definitely should not, ever (see point 2. below).

Impact

A malicious web page and the coordinated server will be able to perform actions through the victim's browser in a second.

In addition, this opens up a new attack surface as the app will process untrusted replies as trusted content. There is high risk of injecting javascript somewhere, maybe here: https://gitlab.com/gitlab-org/gitlab-ce/blob/f78ecda55d5431f9a74ab2892b516ecb45f24d80/app/assets/javascripts/environments/components/environment_terminal_button.js#L33

Recommendations

Issues of this kind should be fixed globally, since the code is recent and developers are likely to introduce more document.location dependencies regularly.

1. Validate or normalize requested URLs against //, /../, and co. This can be done in Rails or in JS, or both.

2. Forbid cross-domain requests in all Vue.http and $.ajax helpers, like this one: https://gitlab.com/gitlab-org/gitlab-ce/blob/f78ecda55d5431f9a74ab2892b516ecb45f24d80/app/assets/javascripts/vue_shared/vue_resource_interceptor.js#L21

Best regards, Aurel