Morgan Stanley says one of its employees exposed the personal details of 900 über-rich clients.

The financial titan announced the data breach on Monday, saying that hundreds of its wealth management clients' names and account numbers were briefly posted on the Internet.

A Morgan Stanley (MS) employee close to the situation told CNNMoney the bank thinks it stopped an insider from covertly selling the private information on the black market.

Like most banks, Morgan Stanley routinely scans criminal havens online to search for exposed customer data. On the morning of Dec. 27, the bank discovered that 900 wealth management clients appeared on one of those websites, according to the company.

The bank immediately took down the post, then it reviewed internal logs and found one employee who had accessed the records of 350,000 Morgan Stanley customers -- 10% of the bank's clients.

The bank said it fired him last week and has called in law enforcement to investigate the incident.

Although the batch of stolen documents was huge, only 900 appeared online -- all were wealth management customers. Clients who opt for that kind of service are typically wealthy individuals and families whose vast savings and investments grant them exclusive and preferred treatment from the bank's financial advisers.

Consider that altogether, the bank's portfolio of those special clients' assets was worth more than $2 trillion last year.

Morgan Stanley said the 900 clients did not face any economic loss due to the data breach.

Related: How safe are you? CNN's cybersecurity magazine