Microsoft knew about the critical flaw in Internet Explorer (IE) that was recently exploited by hackers to gain entry to the systems of Google and at least 20 other big name companies.
Redmond issued an out-of-cycle patch for the flaw yesterday, urging all IE users to apply the fix as soon as possible.
However, Microsoft security programme manager Jerry Bryant said in a blog posting yesterday: "When the attack discussed in Security Advisory 979352 was first brought to our attention on Jan 11, we quickly released an advisory for customers three days later.
"As part of that investigation, we also determined that the vulnerability was the same as a vulnerability responsibly reported to us and confirmed in early September."
The news will come as no surprise to many security watchers, although it highlights what some have described as an inadequate system of 'responsible disclosure'.
Last week, little-known Russian security firm Intevydis said that it will publish details of zero-day exploits in business software every day for the rest of January in protest at the private disclosure system which it believes is flawed.
Intevydis claimed that vendors usually sit on vulnerabilities that are disclosed in private, whereas they act faster if the disclosure is made public without prior knowledge.
"Our position on responsible disclosure policy has evolved, and now we do not support it because it is enforced by vendors and it allows vendors to exploit security researchers to do quality assurance work for free," the firm added in a blog posting.
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