A Russian hacker dramatically demonstrated one of the most common security weaknesses in the Ruby on Rails web application language. By doing so, he took full control of the databases GitHub uses to distribute Linux and thousands of other open-source software packages.

Egor Homakov exploited what's known as a mass assignment vulnerability in GitHub to gain administrator access to the Ruby on Rails repository hosted on the popular website. The weekend hack allowed him to post an entry in the framework's bug tracker dated 1,001 years into the future. It also allowed him to gain write privileges to the code repository. He carried out the attack by replacing a cryptographic key of a known developer with one he created. While the hack was innocuous, it sparked alarm among open-source advocates because it could have been used to plant malicious code in repositories millions of people use to download trusted software.

Homakov launched the attack two days after he posted a vulnerability report to the Rails bug list warning mass assignments in Rails made the websites relying on the developer language susceptible to compromise. A variety of developers replied with posts saying the vulnerability is already well known and responsibility for preventing exploits rests with those who use the language. Homakov responded by saying even developers for large sites for GitHub, Poster, Speakerdeck, and Scribd were failing to adequately protect against the vulnerability.

In the following hours, participants in the online discussion continued to debate the issue. The mass assignment vulnerability is to Rails what SQL injection weaknesses are to other web applications. It's a bug that's so common many users have grown impatient with warnings about them. Maintainers of Rails have largely argued individual developers should single out and "blacklist" attributes that are too sensitive to security to be externally modified. Others such as Homakov have said Rails maintainers should turn on whitelist technology by default. Currently, applications must explicitly enable such protections.

A couple days into the debate, Homakov responded by exploiting mass assignment bugs in GitHub to take control of the site. Less than an hour after discovering the attack, GitHub administrators deployed a fix for the underlying vulnerability and initiated an investigation to see if other parts of the site suffered from similar weaknesses. The site also temporarily suspended Homakov, later reinstating him.

"Now that we've had a chance to review his activity, and have determined that no malicious intent was present, @homakov's account has been reinstated," a blog post published on Monday said. It went on to encourage developers to practice "responsible disclosure."

For an excellent analysis of the GitHub hack, and the underlying Rails vulnerability that made it all possible, see this post from Rob Graham of Errata Security.

Updated to differentiate between Ruby and Rails and to add link from Errata Security.