EquiLend has warned clients that services could be down for several days after the securities lending trading platform was hit by a cyber-attack.
The platform says that on 22 January it identified a "technical issue" that knocked some of its systems offline.
An investigation "identified a cyber security incident involving unauthorized access to our systems. We took immediate steps to secure our systems and are working methodically to restore the involved services as quickly as possible."
EquiLend is now working with external cybersecurity firms and other professional advisers on an investigation and to restore service but "clients have been advised that this may take several days".
EquiLend was founded in 2001 by a consortium of 10 major Wall Street firms and now has a client base of nearly 200 asset owners, agency lending banks, broker-dealers and hedge funds.
Last week it was revealed that private equity firm Welsh, Carson, Anderson & Stowe has struck a deal to buy a majority stake in the company.