It is believed the male was successfully prosecuted for the abuse, but Croydon Council declined to reveal his sentence.
The report found there were failures in communication, problems with newly qualified staff and their managers and insufficient assessment of foster carers.
One finding seen as particularly worrying was that Claire was kept at the foster carers' home even when a doctor had expressed concerns she could have an STI.
CSCB independent chair Sarah Baker, said: "All [staff], without exception, believed at the time of this incident they were providing Claire with a safe and secure home.
"The review found no evidence that anything at the time could have indicated this foster carer was likely to sexually abuse a child in his care.
"However, there are still lessons to be learned and actions were immediately taken to tighten up a variety of processes."
Claire's maternal family, who gave her up "to be cared for by the state", told how they were "deeply distressed" by revelations of how she had suffered.