Forks enable innovation and improvements to digital currency and we believe that we will see an increasing number of forks in the future. We expect this to be a vibrant and innovative community.
When a digital currency forks, it creates a new digital asset. Adding new digital assets to Coinbase must be approached with caution. Not every asset is immediately safe to add to Coinbase from a technical stability, security, or compliance point of view.
Their priority is the safety of customer funds and we spend extensive time designing, building, testing and auditing our systems to ensure that the digital asset we support remains safe and secure. They may not always be first in adding an asset, but if they would do it, you can be sure that they’ve invested significant time and care into supporting it securely. They believe this is the best approach for us to maintain customer trust.
In the case of Bitcoin cash, we made clear to our customers that we did not feel we could safely support it on the day it was launched. For customers who wanted immediate access to their Bitcoin cash, they advised them to withdraw their Bitcoin from the Coinbase platform. However, there are several points we want to make clear for our customers:
They operate by the general principle that their customers should benefit to the greatest extent possible from hard forks or other unexpected events.
Over the last several days, they've examined all of the relevant issues and have decided to work on adding support for Bitcoin cash for Coinbase customers. They are going to make a decision based on factors such as the security of the network, customer demand, trading volumes, and regulatory considerations.
They are planning to have support for Bitcoin cash by January 1, 2018, assuming no additional risks emerge during that time.
Once supported, customers will be able to withdraw Bitcoin cash.
https://blog.coinbase.com/what-is-a-bitcoin-fork-cba07fe73ef1