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A Firefox plugin called Firesheep was released this week that makes it easy to capture and exploit other user's HTTP session cookies sent over insecure connections on untrusted networks. This vulnerability makes it possible to login to other people's accounts on a variety of services by spying on network traffic and extracting those cookies. Wikipedia has a decent article on this technique: Session Hijacking.

GitHub was susceptible to this attack, but we have now taken measures to protect you and your data. As a result, you will be prompted to re-login to the site on your next visit. This is necessary in order to switch you to the more secure system.

The Technical Details

Today, we're rolling out the first in a series of measures we have planned to make GitHub more resilient to session hijacking attacks. The basic approach revolves around setting a second cookie (in addition to the normal session cookie) that is marked as secure. Cookies marked secure, are sent only over SSL requests and are omitted on non-SSL requests. They look like this on the wire:

Set-Cookie: somename=somevalue; path=/; secure

When someone logs on, we set the secure cookie (_github_sec in our case) to the checksum of the user's id and password hash along with a secret (known only to the application) salt value. Then, on subsequent SSL requests, we verify that the cookie's value matches the same checksum. If the values are different, authentication fails.

Most browsers have a way of viewing cookie details. Here, we see the cookie is sent only over secure requests:

cookie inspector

Since we already require SSL for all sensitive requests, including browsing/admin'ing private repositories and viewing/editing account details, this extra bit of verification ensures that sidejacked session cookies cannot be used to gain access to any sensitive information.

Broader Solutions

Sidejacking is still a problem for many sites on the internet. The only way to protect yourself on every site you visit is to secure and encrypt your connection for all requests. SheepSafe from Nick Sieger is an easy to install solution for Mac that relies on an SSH tunnel and SOCKS proxy to encrypt every HTTP request you make, no matter what site you're on.

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