A DDoS attack on DNS provider UltraDNS took down Amazon and other major sites Wednesday afternoon US Time (December 23.)
Amazon users on Twitter reported problems starting on the site at around 5pm PST (noon AEDT Thursday.) Reports note that the issue also affected Amazon web services including S3 and EC2. Walmart and Salesforce are also believed to have experienced issues.
UltraDNS owner Neustar confirmed the outage to CNet, saying that the DDoS attack commenced at 4:45pm and customer sites were either offline, or sluggish for approximately one hour.
Amazon's Web Services Health Dashboard said that services had completely returned to normal as of 6:40pm PST.