WASHINGTON — Hillary Clinton's private e-mail server, containing an electronic inventory of some 55,000 pages of e-mails from her stint as secretary of state, was repeatedly hit by attempted cyberattacks originating in China, South Korea, and Germany in 2014, according to a congressional document obtained by the Associated Press.
At least five cyberattack tries were apparently blocked by a ''threat monitoring'' product that was connected to her network in October 2013, eight months after she left office. But for more than three months earlier that year, the security product had not been installed.
Clinton's server operated without its threat monitoring protection between June and October 2013, according to a letter sent by Senator Ron Johnson, Republican of Wisconsin, chairman of the Homeland Security and Government Affairs Committee. That means her server was possibly vulnerable to cyberattacks during that time.
It was not immediately clear whether the attempted intrusions into Clinton's server were serious espionage threats or the sort of nuisance attacks that hit computer servers the world over. But the revelations underscore the extent to which any private e-mail server is a target, raising further questions about Clinton's decision to undertake sensitive government business over private e-mail stored on a homemade system.
Johnson's letter to Victor Nappe, chief executive of SECNAP, the company that provided the threat monitoring product, seeks a host of documents relating to the company's work on Clinton's server and the nature of the cyber intrusions detected. Johnson's committee is investigating Clinton's e-mail arrangement.
Clinton has not said what, if any, firewall or threat protection was used on her server before June 2013, including the time she was secretary of state from 2009 to 2013 and the server was kept in her home in the New York City suburbs.
SECNAP reported in February 2014 that malicious software based in China ''was found running an attack against'' Clinton's server. In total, Senate investigators have found records describing three such attempts linked to China, one based in Germany, and one originating in South Korea. The attacks occurred in 2013 and 2014.
The letter describes four attacks, but investigators have since found records about a fifth attempt, said officials who were not authorized to discuss the matter publicly.
Any hackers who got access to her server in 2013 or 2014 could have stolen a trove of sensitive traffic involving the foreign relations of the United States. Thousands of Clinton e-mails made public under the Freedom of Information Act have been heavily redacted for national security and other reasons.
Clinton ''essentially circumvented millions of dollars' worth of cybersecurity investment that the federal government puts within the State Department,'' said Justin Harvey, chief security officer of Fidelis Cybersecurity.
''She wouldn't have had the infrastructure to detect or respond to cyberattacks from a nation-state,'' he said. ''Those attacks are incredibly sophisticated and very hard to detect and contain. And if you have a private server, it's very likely that you would be compromised.''
A spokesman for the Clinton campaign did not answer detailed questions about the cyber intrusions. Instead, the spokesman, Brian Fallon, attacked Johnson by linking him to the House Benghazi committee inquiry, which the campaign dismissed in a recent media ad as politically motivated.
''Ron Johnson is ripping a page from the House Benghazi Committee's playbook and mounting his own taxpayer-funded sham of an investigation with the sole purpose of attacking Hillary Clinton politically,'' Fallon said by e-mail. ''The Justice Department is already conducting a review concerning the security of her server equipment, and Ron Johnson has no business interfering with it for his own partisan ends.''
The FBI is investigating whether national security was compromised by Clinton's e-mail arrangement.
In June 2013, after Clinton had left office, the server was moved from her Chappaqua, N.Y., home to a data center in New Jersey, where it was maintained by a Denver technology company, Platte River Networks, records show.
In June 2013, Johnson's letter says, Platte River hired SECNAP Network Security Corp. to use a product called CloudJacket SMB, which is designed to block network access by ''even the most determined hackers,'' according to company literature. But the product was not up and running until October, according to Johnson's letter, raising questions about how vulnerable Clinton's server was during the interim.