Someone accidentally stopped the ongoing WannaCry attacks by registering a domain.
In certain sandbox environments traffic is intercepted by replying to all URL lookups with an IP address belonging to the sandbox rather than the real IP address the URL points to, a side effect of this is if an unregistered domain is queried it will respond as it it were registered (which should never happen).
Source: How to Accidentally Stop a Global Cyber Attacks | MalwareTech