FedEx made a statement about the attack saying,

Like many other companies, FedEx is experiencing interference with some of our Windows-based systems caused by malware.  We are implementing remediation steps as quickly as possible.  We regret any inconvenience to our customers.

The so-called ransomware attack appears to exploit a weakness that was purportedly identified by the United States' National Security Agency and leaked to the internet.

It encrypts data on infected Windows computers and demands payment before the information is unencrypted, particularly focusing on older operating systems including XP. Microsoft has issued an emergency patch and remediation advice for unsupported operating systems such as Windows XP, 8.0 and Server 2003.

The first wave of the attack has now been prevented from spreading; systems newly infected by this first variant contact an internet address which is now under the control of the US authorities - who will notify system administrators of their status. It is anticipated that the start of the working week will see modified versions of the attack seek to compromise more unpatched systems.