Business-to-business payments provider Billtrust is still recovering from a ransomware attack that began last week.  The company said it is in the final stages of bringing all of its systems back online from backups.

With more than 550 employees, Lawrence Township, N.J.-based Billtrust is a cloud-based service that lets customers view invoices, pay, or request bills via email or fax. In an email sent to customers today, Billtrust said it was consulting with law enforcement officials and with an outside security firm to determine the extent of the breach.

“Our standard security and back-up procedures have been and remain instrumental in our ability to execute the ongoing restoration of services,” the email reads. “Out of an abundance of caution, we cannot disclose the precise ransomware strains but will do so as soon as prudently possible.

In an interview with KrebsOnSecurity on Monday evening, Billtrust CEO Steven Pinado said the company became aware of a malware intrusion on Thursday, Oct. 17.

“We’re aware of the malware and have been able to stop the activity within our systems,” Pinado said. “We immediately started focusing on control, remediation and protection. The impact of that was several systems were no longer available to our customers. We’ve been fighting the fight, working on restoring services and also digging into the root cause.”

A report from BleepingComputer cites an unnamed source saying the ransomware strain that hit Billtrust was the BitPaymer ransomware, but that information could not be confirmed.

One of Billtrust’s customers has published a day-by-day chronology of the attack and communications from the company here (h/t @gossithedog).

Pinado said Billtrust had restored most of its systems, and that it was in the process now of putting additional security measures in place. He declined to discuss anything related to the ransomware attack, such as whether the company paid a ransom demand in exchange for a key to unlock files scrambled by the malware, although he allowed Billtrust does have cybersecurity insurance for just such occasions.

Billtrust recently teamed up with Visa to launch the Billtrust Business Payments Network, an effort to digitize payments between businesses.

Cloud service providers are a favorite target of attackers who deal in ransomware. In August, Wisconsin-based PerCSoft paid a hefty ransom to get out from beneath an attack that separated hundreds of dental offices from their patient records.

In July, attackers hit QuickBooks cloud hosting firm iNSYNQ, holding data hostage for many of the company’s clients. In February, cloud payroll data provider Apex Human Capital Management was knocked offline for three days following a ransomware infestation.

On Christmas Eve 2018, cloud hosting provider Dataresolution.net took its systems offline in response to a ransomware outbreak on its internal networks. The company was adamant that it would not pay the ransom demand, but it ended up taking several weeks for customers to fully regain access to their data.