From today's Casey Newton's newsletter:

In early December, a number of Twitter’s security certificates are set to expire — particularly those that power various back-end functions of the site. (“Certs,” as they are usually called, serve to reassure users that the website they are visiting is authentic. Without proper certs, a modern web browser will refuse to establish the connection or warn users not to visit the site). Failure to renew these certs could make Twitter inaccessible for most users for some period of time.

We’re told by some members of Twitter’s engineering team that the people responsible for renewing these certs have largely resigned — raising concerns that Twitter’s site could go down without the people on hand to bring it back. Others have told us that the renewal process is largely automated, and such a failure is highly unlikely. But the issue keeps coming up in conversations we have with current and former employees.