Microsoft is warning of two bugs in its software that could potentially give unauthorised control or access over a person's computer, while a third problem has been highlighted by a security research company.

One vulnerability revisits the WMF debacle at the start of the year, but impacts fewer users. The bug is in Internet Explorer (IE) 5.01 Service Pack 4 on the Windows 2000 Service Pack 4 OS and IE 5.5 Service Pack 2 on Windows Millennium, Microsoft said.

An attacker could gain control if a user opened a malicious e-mail attachment or if a user were persuaded into visiting a website that had a specially-crafted WMF image, Microsoft said.

A patch has not been issued, but Microsoft said the issue is under investigation, and an out-of-cycle patch could be provided depending on customer needs. Microsoft typically issues patches on the second Tuesday of the month, due this month on 14 February.

A second vulnerability could allow a person with low-user privileges gain higher-level access, Microsoft said. Proof-of-concept code that has been released attempts to exploit overly permissive access controls on third-party application services, along with the default services of Windows XP Service Pack 1 and Windows Server 2003, the company said. No attacks have been reported.

Microsoft said several factors diminish the threat of the problem. Those running Windows XP Service Pack 2 and Windows Server 2003 Service Pack 1 - the latest updates of the software - are not affected, and someone who launches an attack would need authenticated access to the affected OS.

Security vendor Secunia detailed a third vulnerability involving Microsoft's HTML Help Workshop, software that can create online help for a software application or website content.

Secunia said the problem "is caused due to a boundary error within the handling of a '.hhp' file that contains an overly long string in the 'contents file' field. This can be exploited to cause a stack-based buffer overflow and allows arbitrary code execution when a malicious '.hhp' file is opened."

The bug could allow arbitrary code to be executed on a computer, Secunia said. An exploit has been released, and Secunia advised that untrusted .hhp files not be opened.