|The hacker brought down the websites of the Tel Aviv Stock Exchange and the national carrier El Al [EPA]
A hacker brought down the websites of Israel's national carrier El Al and the Tel Aviv Stock Exchange (TASE).
Both sites were affected early in the day on Monday, posting messages saying they had been taken down for "maintenance." By early afternoon the El Al site was back up, although the TASE website was still inaccessible.
The websites of two small banks were also attacked, Israeli media reported.
It was the latest incident in a series of attacks over the past two weeks, which have seen details of tens of thousands of Israeli credit cards posted online and websites defaced by hackers claiming to be from Saudi Arabia or Gaza.
A spokeswoman for the stock exchange confirmed the site had come under attack but said only the website and not the trading systems had been affected.
"There is someone that has been attacking the Tel Aviv Stock Exchange website since this morning," Idit Yaaron told the AFP news agency, describing what appeared to be a distributed denial of service attack (DDOS).
The sites crashed several hours after an alleged Saudi hacker who calls himself "0xOmar" warned he was going to target both websites, Israeli media reported.
Praise from Hamas
The cyber attacks were hailed by Sami Abu Zuhri, a spokesman for Gaza's Hamas rulers, who said it was "a sign of the Arab youth's creativity in inventing new forms of Arab and Islamic resistance against the Israeli occupation."
"Hamas praises the Arab hackers and calls on the Arab youth to play their role in cyberspace in the face of Israeli crimes," he said.
Danny Dolev, professor of computer science at the Hebrew University, said Monday's attacks were unlikely to have any serious impact.
"Most of these attacks appear right now to be on the surface, attacking only the websites that present information to the public, but happily it didn't touch the internal information systems," he said.
"Right now, these attacks don't seriously damage any function, either economic or otherwise.
"I hope that small incidents like that will continue for some time because it helps to raise awareness of the issue and many companies will take the threat more seriously," he added.
Cyber security expert Ron Porat said Israel's military and government sites were protected by an advanced level of security, with corporate and civilian websites more open to attack.
"El Al and the Israeli Stock Exchange are some of the bodies that were well protected," he told AFP, saying that even the best-defended online systems were at risk.
"There is not one organisation in Israel and around the world that cannot be hacked -- it's a matter of effort," he said.
But lone hackers like 0xOmar were unlikely to pose a threat to Israel's critical infrastructure, he said.
"This cannot be done by one person, it has to be a well-organised group of people like in a terror act or warfare," he said.
Meanwhile, hackers also brought down several government websites in Azerbaijan, Israel's main Muslim ally.
They hacked the interior and communications ministries, that of its governing party and the constitutional court, leaving threats and anti-Israeli messages.
0xOmar first struck on January 3 when he claimed to have posted details of 400,000 Israeli-owned cards online.
Three days later, he said he had published another 11,000 card details, but the links turned out to contain malware that infected anyone who downloaded the information.
Israel's main credit card companies said about 20,000 valid cards had been affected.
In response, an Israeli hacker called "0xOmer" published details of more than 200 Saudi-owned cards, sparking a spate of tit-for-tat attacks.
Last week, Gaza-based activists hacked the website of Israel's deputy foreign minister Danny Ayalon, and on Friday they hacked the website of the Israel Fire and Rescue Services, posting a message saying "Death to Israel."
"This is the beginning of cyberwar against Israel, you are not safe any more," 0xOmar wrote in a posting on Pastebin.com last week, urging hackers from across the Arab and Muslim world to target Israel.
"We'll hack Israeli servers for different purposes like leaking Israeli data, sensitive and hidden information extraction and defacing websites," he wrote.
Yossi Melman, a security expert and commentator, said the attacks would serve as "a wake-up call," prompting Israel to address gaps in its cyber security.
"The gaps are being addressed. The government is aware of the problem," he said.