A major accounting software and cloud services company has been hit by malware, affecting their many clients across the US.

Wolters Kluwer, a major provider of tax accounting software and cloud services, has been hit by malware. The many financial software services they offer to clients across the country have been down since Monday, May 6.

The software provided by Wolters Kluwer is extremely popular in the US accounting industry. Users include every one of the top 100 American accounting firms, as well as 90% of the top banks worldwide, and 90% of Fortune 500 companies.

This malware attack comes at an especially vulnerable time when many accounting firms (and their clients) are intending to file their taxes. With their primary accounting systems offline, they won’t be able to do so, or at least not with Wolters Kluwer software.

However, it’s not as simple as just using different accounting software. Wolters Kluwer also provides cloud services to their clients, which means that necessary client financial data is stored in their servers, and inaccessible by the accounting firms during this outage.

Since the attack began Monday morning, Wolters Kluwer took many of its systems offline to slow the spread of the malware. According to representatives, they have since been working non-stop to try to eliminate the malware and bring their systems back online. They have contacted authorities and third-party forensic teams to investigate the attack.

“We’re working around the clock to restore service, and we want to provide [clients] the assurance that we can restore service safely,” said Elizabeth Queen, vice president of risk management for Wolters Kluwer, to CNBC. “We’ve made very good progress so far.”

However, end-users have still not been able to access their tax documents that are stored in Wolters Kluwers cloud servers. The many systems that Wolters Kluwer took offline on Monday include the customer services lines that end users have relied on to get info from the software provider.

When a backup customer service number was finally provided, users were told that there is no estimated window in which the services will be fully restored. For the time being, thousands of accountants at numerous firms across the US are being expected to wait and see.