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

One vulnerability revisits the Windows Metafile (WMF) debacle from December, but affects fewer users. The bug is in Internet Explorer 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 email 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 – the next releases are expected on 14 February.

A second vulnerability could allow a person with low user privileges to 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, it said.

Security vendor Secunia has 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 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 firm said.

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