SAN FRANCISCO, April 29 (Reuters) -

Hackers broke into UnitedHealth's ( UNH ) tech unit on Feb.

12 by exploiting a security vulnerability in software from

private IT company Citrix that allows employees remote access to

their desktop computers, the largest U.S. health insurer will

testify before a House panel this week.

UnitedHealth ( UNH ) CEO Andrew Witty's testimony before the House

Energy and Commerce Committee, slated for Wednesday, will follow

weeks of disruption to American healthcare since the insurer's

Change Healthcare unit was hacked.

On the morning of Feb. 21, the cybercriminal gang AlphV, aka

BlackCat, locked up Change Healthcare's systems and demanded a

ransom to unlock them, Witty will tell the House panel,

according to a copy of his written testimony posted to the

panel's website on Monday.

"Not knowing the entry point of the attack at the time, we

immediately severed connectivity with Change's data centers to

eliminate the potential for further infection," the testimony

says.

The criminals used compromised login credentials to remotely

access a Change Healthcare Citrix portal that did not have

multi-factor authentication, according to the testimony.

A Citrix spokesman didn't immediately respond to a request

for comment. It's unclear which specific security flaw at Citrix

was exploited by AlphV, but U.S. officials issued multiple

warnings about security loopholes in Citrix tools late last

year, some of which were being used to breach healthcare groups.

The hearing before the panel's subcommittee on oversight and

investigations will focus on the cyberattack's impact on

patients and providers.

UnitedHealth ( UNH ) has been working with the FBI and prominent

cybersecurity firms to investigate the hack. Security experts

from Google, Microsoft ( MSFT ), Cisco ( CSCO ) and

Amazon ( AMZN ) worked with teams from Mandiant and Palo Alto

Networks ( PANW ) to secure Change Healthcare's systems after

the breach, according to the testimony.

Last week, Witty said the company had paid the hackers a

ransom to ensure the decryption of Change Healthcare's systems,

although the size of the payment is not known.

The company has been scrambling to contain the hit to

healthcare payment processing across the country. Change

processes 50% of all medical claims in the United States.

As of April 26, UnitedHealth Group ( UNH ) had provided more than

$6.5 billion in accelerated payments and no-interest, no-fee

loans to thousands of health-care providers, according to

Witty's testimony.