A Microsoft security expert has denied a recent security hole in Windows was in fact a secret backdoor introduced by the software giant to give it immediate access to people's PCs.

Following speculation that the hole in Windows Metafile (WMF) may have been created deliberately, Stephen Toulouse, security program manager with Microsoft's security response centre stated simply on his blog: "That speculation is wrong."

Toulouse's comment followed speculation by security researcher Steve Gibson that Microsoft had intentionally included a known vulnerability in the graphics rendering component. "The only conclusion that can reasonably be drawn is that this was a deliberate back door put into all of Microsoft's recent editions of Windows," wrote Gibson on his website last week. "Why it was put in and who knew about it, and what they were expected to use it for... we'll never know."

It's not the first time Gibson has made a name for himself and his consultancy business by extrapolating security problems with Windows into far more significant issues. In 2001, Gibson claimed that Windows XP would undermine the Internet's very stability by allowing for widespread and simple denial-of-service attacks.

Microsoft laid out a long explanation why it felt Gibson was wrong, and so far the software giant had proved to be right.

Stephen Toulouse appeared to be in the same position, stating that Microsoft has been fielding customer questions on this topic, many of which he assumed to have been triggered by Gibson's post. "We had been looking into detailing the history anyway and some customer questions drove the idea to write it up," he wrote. "We just wanted to make sure people had the history."

According to Toulouse, the vulnerability in question concerns the way that Windows processes WMF graphics files, which are used by computer-aided design programs. In the 1990s, Microsoft added a function to Windows, called SetAbortProc, that is used in processing these files. Because of a design error in the function, it can be used by hackers to take control of a Windows computer.

Gibson argues that because the SetAbortProc function could not be triggered by a correctly formed WMF file, it served no legitimate purpose, a claim that Toulouse disputes. Gibson has conceded his error. "I was wrong about this," he said. "It is more complex than that, exactly as Toulouse explained in his posting."

Nonetheless, Gibson says he stands by his conclusion that Microsoft intentionally changed the SetAbortProc function around the time of Windows NT to make systems vulnerable to the coding error. "The best way to characterise this is, it's intentionally designed code which, without question, enables back-door functionality."

Toulouse declined to comment on this claim, but in his blog posting, he wrote that it is more difficult to exploit the vulnerability in the Windows 95 and Windows 98 operating systems.

Other security researchers are also skeptical of Gibson's theory. If Microsoft truly wanted to add a back door to Windows, there are better ways the company could have done it, said Cesar Cerrudo, chief executive officer of security research firm Argeniss. "I don't think Microsoft would use that kind of back door. They don't need to do that, they could just build an exploit for some unfixed remote vulnerability on Windows."

Gibson said that, while he has no proof of Microsoft's motives, he believes that such a back door could have been created without malicious intent - perhaps as a way for Microsoft to provide assistance to users, for example. "It would be a way for Microsoft to help people who had shut their computers down, security-wise," he said.