Redmond has confirmed that a single superfluous character in its own development code is responsible for the bug that has let hackers exploit Internet Explorer (IE) since early July.

A pair of German researchers who analysed a vulnerability in a Microsoft-made ActiveX control came to the same conclusion three weeks ago.

"The bug is simply a typo," Michael Howard, a principal security program manager in Microsoft's security engineering and communications group, said in a post to the Security Development Lifecycle (SDL) blog. Howard, who is probably best known for co-authoring Writing Secure Code, went on to say that the typo -- an errant "&" character - is the "core issue" in the MSVidCtl ActiveX control.

That video-streaming control was created by Microsoft using a modified version of an older edition of a code "library," dubbed Active Template Library (ATL), that Microsoft admitted contained multiple vulnerabilities. Microsoft also patched Visual Studio, the company's development platform that contains ATL. Those patches, however, do not automatically fix software that was developed using the buggy ATL. Instead, vendors - Microsoft as well as third-party firms - must use the patched Visual Studio to recompile their code, then distribute the new, secure software to users.

Howard said that the bug in the MSVidCtl ActiveX control was introduced by an internal version of ATL, not one that was available to outside developers.

Others at Microsoft drew a direct line between the ATL bug Howard described to the public exploits hackers have been using for much of this month, including drive-by attacks conducted from thousands of compromised websites.

"This public exploit took advantage of the fact that MSVidCtl uses a modified version of vulnerable ATL headers," said Fermin Serna, an engineer in the Microsoft Security Research Center (MSRC), in a blog post. "In this specific instance, the vulnerability allows an attacker to corrupt memory which may lead to a remote code execution," added Serna.

The extra "&" had been fingered by a pair of German researchers - Thomas Dullien, the CEO and head of research at Zynamics GmbH, and Dennis Elser - on 9 July, when the former posted their findings in a blog post. The mistake, said Dullien, was in an ATL function called "ATL::CComVariant::ReadFromStream."

That function led the two researchers to several other Microsoft-made components, including several critical to important Windows applications, such as IE, Windows Media Player and Terminal Services. "The bug is actually much 'deeper' than most people realize," said Dullien at the time, "[and] the kill-bit fix is clearly insufficient, as there are bound to be many other ways of triggering the issue."

The "kill-bit" fix Dullien alluded to was one that Microsoft had issued in the form of a stop-gap tool on 6 July to disable the flawed ActiveX control. On 14 July, Microsoft pushed the same kill-bit settings to Windows users as an update on its regular monthly patch day.

Microsoft has now acknowledged that the kill-bit solution was insufficient because of the bugs in ATL. In response, it has also updated IE, which now tries to block attackers from exploiting the ATL flaws in ActiveX controls.