Github, an online forum for software developers that also hosts tools used by Chinese internet users to bypass censorship, has been hit by a powerful cyber attack that security experts believe originated in China.

The San Francisco-based start-up said it was experiencing the largest denial-of-service attack in its history, as cyber criminals bombarded the site with traffic in an attempt to take it offline.

In a blog post, the company said the attack had started on Thursday and involved a “wide combination of attack vectors”, including “sophisticated new techniques that use the web browsers of unsuspecting, uninvolved people to flood github.com with high levels of traffic”.