The House of Representatives’ Republican campaign organisation - known as the National Republican Congressional Committee (NRCC) - believes that thousands of sensitive emails were exposed as a result of the attack.
It has told the FBI that the email accounts of four top aides at the NRCC, which oversees the selection of candidates and fundraising for them, were compromised for months in an intrusion detected in April.
The news site said while the issue became a major headache for the NRCC, which hired a leading Washington DC law firm and public relations company to handle its response to the intrusion, senior Republicans including Paul Ryan, the speaker, and House majority leader Kevin McCarthy were reportedly not told about it.
“The NRCC can confirm that it was the victim of a cyber intrusion by an unknown entity. The cybersecurity of the Committee’s data is paramount, and upon learning of the intrusion, the NRCC immediately launched an internal investigation and notified the FBI, which is now investigating the matter,” Ian Prior, a spokesman for Mercury Public Affairs, which is representing the NRCC, said in a statement.
He added: “To protect the integrity of that investigation, the NRCC will offer no further comment on the incident.”
In March 2018, NRCC Chairman Steve Stivers said the committee hired multiple cybersecurity staffers to work with its candidates and promised to do more, with cybersecurity around elections having become a hot button issue.
A number of congressional committees are looking into the issue of meddling in the 2016 presidential election, including from Russia, while Special Counsel Robert Mueller's federal investigation is doing the same as part of its expansive remit.
“We're starting to advise campaigns, but we're not ready to roll the whole thing out. We're working on it,” Stivers said at the time. “We're working on the technology-based stuff to try and make sure that we know what's out there — which is hard, too — and then we try to defend against it the best we can.”
In August, Microsoft alerted the public to attempts by government-backed Russian hackers to target US conservatives' email by creating fake website. It also confirmed an attempt similarly attributed to Russian hackers to infiltrate the Senate computer network of Sen. Claire McCaskill, the Missouri Democrat who lost a re-election bid in November.
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