Microsoft has removed access to dozens of its open source projects on GitHub due to a potential security breach, where hackers injected password-stealing malware into the code. The affected projects are related to Microsoft's cloud service Azure and AI development tools. The company is investigating the incident and has notified a small number of customers who may have been affected. The breach is an example of a 'supply chain' attack, where hackers target code used by many software products or specific types of users.