By Zeba Siddiqui

SAN FRANCISCO Hackers have stolen data from the systems of a number of users of the popular file transfer tool MOVEit Transfer, U.S. security researchers said on Thursday, one day after the maker of the software disclosed that a security flaw had been discovered.

Software maker Progress Software Corp, after disclosing the vulnerability on Wednesday, said it could lead to potential unauthorized access into users' systems.

The managed file transfer software made by the Burlington, Massachusetts-based company allows organizations to transfer files and data between business partners and customers.

It was not immediately clear which or how many organizations use the software or were impacted by potential breaches. Chief Information Officer Ian Pitt declined to share those details, but said Progress Software had made fixes available since it discovered the vulnerability late on May 28.

The software's eponymous cloud-based service had also been impacted by this, he told Reuters.

"As of now we see no exploit of the cloud platform," he said.

Cybersecurity firm Rapid7 Inc and Mandiant Consulting - owned by Alphabet Inc's Google - said they had found a number of cases in which the flaw had been exploited to steal data.

"Mass exploitation and broad data theft has occurred over the past few days," Charles Carmakal, chief technology officer of Mandiant Consulting, said in a statement.

Such "zero-day," or previously unknown, vulnerabilities in managed file transfer solutions have led to data theft, leaks, extortion and victim-shaming in the past, Mandiant said.

"Although Mandiant does not yet know the motivation of the threat actor, organizations should prepare for potential extortion and publication of the stolen data," Carmakal said.

Rapid7 said it had noticed an uptick in cases of compromise linked to the flaw since it was disclosed.

Progress Software has outlined steps users at risk can take to mitigate the impact of the security vulnerability.

Pitt did not have a comment on who might have been trying to steal data by exploiting the flaw.

"We have no evidence of it being used to spread malware," he said.

MOVEit Transfer was used by a relatively "small" number of customers compared to those of the company's other software products that number more than 20, he said.

"We have forensics partners on board and we are working with them to make sure that we have an ever-evolving grasp of the situation."

(Reporting by Zeba Siddiqui in San Francisco; Editing by Christopher Cushing)