The leak, underscoring what critics have long charged is Japan's vulnerability to cyber-attacks, coincide with Abe's push to legislate a big shift from post-war pacifism, keep a nascent recovery on track and craft plans to rein in public debt.
News of the hacking came days after the United States and Japan announced Washington would extend its cyber-defence umbrella over its ally to help it cope with the growing threat of attacks against military bases and infrastructure.
Japan Pension Service staff computers were improperly accessed by an external email virus, allowing the leak of some 1.25 million cases of personal data, its president told a news conference on Monday. The leaks involved names, identification numbers, birth dates and addresses.
"We will make every effort to keep this from causing inconvenience to those whose data was leaked and to review the issue and take preventive measures," Abe's top aide, Chief Cabinet Secretary Yoshihide Suga, told a news conference.
While Abe will probably weather the storm, a risk to his ratings, now relatively robust, could not be ruled out, analysts said.
"The conditions are much more favourable to him (than in his first term)," said Sophia University professor Koichi Nakano, citing opposition weakness and Japanese media's tendency not to antagonise a government that could last several more years.
"On the other hand, even though his support levels are still relatively high, his policies are not widely supported," Nakano said. "It depends on how he is seen as coping."
Opposition officials were quick to go on the attack.
"This is a situation which undermines the foundations of livelihoods and shakes confidence in pensions," Kyodo news agency quoted Democratic Party executive Yoshiaki Takaki as saying.
Takaki said the party would grill officials in parliament.
Public outrage over the loss of 50 million records of pension premium payments by the Japan Pension Service's predecessor was a major factor in a devastating defeat suffered by Abe's Liberal Democratic Party in a 2007 upper house election.
Abe, whose first cabinet lost several ministers to other scandals and gaffes, including one who committed suicide, quit in September 2007 due to political deadlock and ill health.
© Thomson Reuters 2015