Mortgage origination software provider Ellie Mae says that a distributed denial of service (DDoS) attack is to blame for its systems falling over earlier this week.
The company's Encompass 360 platform stopped working leaving lenders unable to process deals during the busy end-of-month period.
In a statement, Ellie Mae says it has "detected unusually high demand for services consistent with an external malicious attack".
There is no evidence of a data breach and Encompass 360 is now back up and running but forensic specialists have been called on to collect evidence.
Sig Anderman, CEO, Ellie Mae, says: "We recognise the critical role our services play in enabling our clients to serve their customers and deeply regret the inconvenience and delays".
The Federal Financial Institutions Examination Council yesterday called on the country's financial services firms to step up their readiness to repel Distributed Denial of Service Attacks, saying: "Each institution is expected to monitor incoming traffic to its public Website, activate incident response plans if it suspects that a DDoS attack is occurring, and ensure sufficient staffing for the duration of the attack, including the use of pre-contracted third-party servicers, if appropriate."