Earlier this morning, GitHub announced that it had changed its revision control website to use SSL only; however, a significant flaw in the implementation means that session cookies can still be captured by Firesheep and other network sniffing tools.
Firesheep brought session hijacking to the masses when it was released last month. Ironically, its own GitHub repository includes a github.js handler, which was designed to capture unencrypted session cookies from GitHub users. This allowed novice attackers to monitor shared network traffic (such as public WiFi) and hijack those sessions.
A day after its release, Firesheep’s author stated that a basic expectation of privacy should not be a premium feature, referring to the fact that, at the time, you had to pay GitHub if you wanted to use full-session SSL. GitHub’s move to SSL this morning should have eliminated the session hijacking vulnerability, rendering Firesheep useless; however, the session cookies used by the site are not always handled securely.
When a user logs in to GitHub, the server sets a
_gh_ses session cookie in the client browser. This cookie is not marked with the Secure flag, which means it will be transmitted unencrypted if the user subsequently visits http://github.com, even though that page immediately redirects the user to https://github.com. This means the site’s users may still be vulnerable to sniffing tools such as Firesheep.
Netcraft successfully hijacked a session from the GitHub site by sniffing the cookies that were sent via unencrypted HTTP. Many legacy URLs will still point to the HTTP version of the site, so an attacker may not even need to entice a victim into visiting the HTTP site. Once a session has been hijacked, the attacker can freely create repositories, delete/add email addresses and change passwords, so it looks like the sidejack prevention that GitHub implemented a week ago (which did use a Secure cookie) has been undone.
Although GitHub’s move to SSL has not yet been implemented securely, it is at least a step in the right direction for Firesheep’s author, Eric Butler. When he released the tool on 24 October 2010, he said:
Websites have a responsibility to protect the people who depend on their services. They’ve been ignoring this responsibility for too long, and it’s time for everyone to demand a more secure web. My hope is that Firesheep will help the users win.
GitHub announced the SSL-only change on Twitter this morning, and is expected to publish a blog post about it soon.
GitHub has since fixed the session cookie to be secure. Now that it can only be transmitted over encrypted connections, this makes the site invulnerable to Firesheep.