Sony chief executive Sir Howard Stringer has sidestepped calls to stand down in the wake of the devastating PlayStation Network hacking attack that affected 100 million gamers.

Speaking at the company's annual shareholders meeting on Monday night, a combative Stringer said the electronics giant was targeted simply because it tried to protect its intellectual property.

"We believe that we first became the subject of attack because we tried to protect our IP , our content – in this case video games," the Welsh-born executive said. "These are our corporate assets, and there are those that don't want us to protect them, they want everything to be free."

Stringer sidestepped one shareholder's call to step down over the breach, a call greeted by scattered applause from some other shareholders.

Sony was forced to shut down its popular PlayStation Network on 20 April after the attack, thought to be the largest in internet history. The company has estimated that recovery from the huge breach, which saw the personal information of as many as 100 million gamers stolen, will cost about $173m (£105m).

No group has yet claimed responsibility for the attack. The notorious hacking clan Anonymous refuted allegations that it was to blame after Sony claimed to have found evidence pointing to the loose-knit collective.

Stringer has fought back at critics who accused the Japanese electronics giant of being too slow to warn gamers that their personal information – including email addresses and credit card details – might have been compromised.

He said in May: "This was an unprecedented situation. Most of these breaches go unreported by companies – 43% [of companies] notify victims within a month. We reported in a week. You're telling me my week wasn't fast enough?"

The network, which allows gamers to play virtually against each other online, has slowly limped back into life after it was reopened in a reduced form on 16 May.

The US Homeland Security Department on Monday unveiled a new guidance system to help large corporations stave off attacks from internet pranksters, the Financial Times reported. The guidance includes a list of the top 25 programming weakspots which hackers have been known to target.