Microsoft has debunked claims that a new way of attacking its software has been discovered by someone looking at its recently stolen Windows source code.

The company said that on Monday it began investigating a "reported exploit" on versions of Internet Explorer, which was "allegedly discovered by an individual studying the leaked source code".

But in a statement Microsoft said the reported exploit was in fact a known, patched, flaw: "This exploit is a known issue that Microsoft had discovered internally and addressed with the latest release of Internet Explorer 6.0 Service Pack 1."

Security experts are still warning the stolen code could pose dangers, however. European security firm Ubizen warns that the code will increase the ability of hackers to identify vulnerabilities.

"The leak ultimately means that security holes can be more easily identified and exploited," said David Williamson, UK head of Ubizen.

"Without access to code, identifying a security hole is largely the result of guesswork and reverse engineering. Having access to the leaked code will mean that hackers can spot problems with the software more easily."