The top three banks in the Netherlands have been targeted in rolling multiple cyber attacks over the past week, blocking access to websites and internet banking services, they said on Monday.
The Dutch Revenue Service and a national digital signature system were also hit on Monday by similar attacks, but services were quickly restored, spokesmen for both organisations said.
The top Dutch bank by assets, ING, experienced a so-called distributed denial of service (DDoS) attack on Sunday evening while the eurozone nation's third largest lender, ABN Amro, suffered three attacks over the weekend in a total of seven over the last week, Dutch media reported.
Rabobank, the country's number two lender, saw its internet banking services go down early Monday.
"We have been targeted by a DDoS attack since 9:10 am (0810 GMT) this morning and our clients don't have access or very little access to online banking," Rabobank spokeswoman Margo van Wijgerden said.
"We are working to resolve the problem as quickly as possible," she told AFP, with Dutch news reports later saying the problem had been fixed by 11:00 am (1000 GMT).
Also on Monday, the Dutch Revenue Services saw its website go down for about 10 minutes due to an attack, spokesman Andre Karels said.
"Things are running as normal and we are investigating the incident," Karels told AFP.
Later in the afternoon, yet another attack briefly hit DigiD, the Dutch official online signature system used by more than 12 million citizens in the country of 17 million.
"This was not a hack and people's private details were never in danger," Rick Bron, a spokesman for DigiD said.
ING, which has some eight million private clients, experienced an attack on Sunday evening.
"During the DDoS attack ING's internet site was blasted with data traffic causing our servers to overload and which put pressure on the availability of online banking," ING said on its website, adding services had been restored.
ABN Amro experienced a similar attack but also said services were restored. The banks all stressed that client details were not compromised or leaked.
It is not the first time Dutch banks have been targeted in a DDoS attack, central bank chief Klaas Knot told a TV news programme Sunday.
"I think these (recent) attacks are serious, but our own website is being attacked thousands of times per day," Knot told the Buitenhof talk show.
"That is the reality in 2018," he said.
Dutch anti-terror chief Dick Schoof said "DDoS attacks are becoming more advanced, we need to continue to invest in digital safety."
But Schoof would not be drawn into suggestions linking the latest attacks to revelations last week that Dutch intelligence services may have provided "crucial evidence" to US counterparts about Russian online meddling in the 2016 elections.
Erik de Jong, chief research officer at the Delft-based cyber security firm Fox-IT said the best defence against cyber attacks was to ensure proper systems were in place.
Since a previous large-scale attack in 2013 "banks have made enormous investments in beefing up their systems," he said.
"But it remains a cat-and-mouse game between putting defences in place and what attackers will do next."
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