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Microsoft has released a patch to fix critical flaws within Windows Help Facility which could enable attackers to execute arbitrary code on a victim's PC.

The HTML Help facility in Windows includes an ActiveX control which provides much of its functionality. One of the functions exposed via the control contains an unchecked buffer, Microsoft says, warning that the flaw poses a critical risk for all Windows users.

Attack mechanisms are all too familiar. The flaw could be exploited by a web page hosted on an attacker's site or by a spot of social engineering and a maliciously-constructed HTML mail (although users of the latest Outlook clients and those who have applied the post-Love Bug Outlook security patch enjoy some protection from this).

Security researchers at PivX Solutions, which discovered the flaw, say the fix omits their recommendation to set the "Kill Bit" of the flawed ActiveX control. This effectively allows a cracker to re-introduce the vulnerable control, it argues.

A second vulnerability exists because of flaws associated with the handling of compiled HTML Help (.chm) files that contain shortcuts. Te security context of these files aren't checked properly - an oversight which might allow all sorts of mischief
But this is difficult to exploit, according to Microsoft, which grades this flaw, fixed with the same patch, as a moderate risk.

That's as maybe but Windows Help vulnerabilities can be very nasty, and example of which is a flaw in Win XP - silently fixed with Service Pack 1 - which allowed a hacker to create a link that, when selected, could easily delete contents on any directory of a user's PC.

Microsoft has issued four new security alerts in total. As well as the Windows help bug 'fix', there's also a patch for flaws in handling file decompression. Microsoft describes this flaw, which involves the way that Windows PCs handle Zip files, as a moderate risk. The problem arises from coding mistakes which have resulted, yet again, in a buffer overflow vulnerability that could be manipulated to inject malicious code onto a user's PC.

There's more information in Microsoft's advisory here.

Redmond's other overnight advisories relate to a cumulative patch for SQL Server and apparently less serious flaw involving the Services for Unix 3.0 Interix SDK. ®

Internet Security Threat Report 2014

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