The German government issued its warning after malicious code suspected in recent attacks against Google was published online, the BBC reported Monday.
Certa, a French agency that examines cyber threats, cautioned against using all versions of the Microsoft program, the BBC said. Microsoft responded, calling its IE8 version "the most secure browser on the market" and recommending that users upgrade.
Microsoft has only seen malicious code that targets the older version of its browser, IE6, a company spokesman said. Security experts say that could soon change.
"Microsoft themselves admit there is a vulnerability, even in IE8," Graham Cluley of Sophos, a security firm, said.
Since details of the vulnerability were now online, hackers could alter the code to target other versions of the browser, Cluley said.
But changing to another browser such as Firefox or Google Chrome could bring new problems, he added.
"Switching away will get away from this particular problem, but all browsers have security flaws," he said.