A form letter sent to the California Attorney-General on 29 April, and meant to be sent to customers, said: "We currently believe that the cyber criminals had intermittent access to our network between 13 October 2018 and 8 March 2019."

The initial announcement by Stan Black, chief security and information officer, said the company, which has headquarters in both Florida and California, had been told by the FBI on 6 March of the intrusion.

Black added that specific documents that may have been accessed, however, were unknown, adding that there was no indication that the security of any Citrix product or service had been compromised.

The 29 April letter said the attackers "removed files from our systems, which may have included files containing information about our current and former employees and, in limited cases, information about beneficiaries and/or dependants".

The letter said the information taken could have included Social Security numbers and financial information.

It added: "Our investigation has not yet concluded; but out of an abundance of caution, we have sent a notice to the most recent home address on file for current and former employees who were employed by Citrix.

In an update issued on 4 April about the breach, Citrix said:

• "We identified password spraying, a technique that exploits weak passwords, as the likely method by which the threat actors entered our network.
• "We have taken measures to expel the threat actors from our systems. Additionally, we’ve performed a forced password reset throughout the Citrix corporate network and improved internal password management protocols.
• "We have found no indication that the threat actors discovered and exploited any vulnerabilities in our products or services to gain entry.
• "Based upon the investigation to date, there is no indication that the security of any Citrix product or service was compromised by the threat actors."

Soon after the initial announcement, a security firm known as Resecurity claimed that Iranian attackers were behind the breach, a claim that was given air by some tech outlets.

But Resecurity's credentials came under examination when it was revealed that the man behind the company had been in the security industry previously but landed in controversy over some of his media comments, the researcher who discovered his identity claims.

The man appears to be one Andrey Andreevich Komarov, aka Andrew Komarov, and he was identified by a researcher who uses the Twitter handle Deacon Blues, as iTWire reported.

Resecurity has not made any further claims about Citrix, or any other breach.

Thanks to TechCrunch for a link to the letter.