Guninski sent an advisory about the issues to security e-mail lists and posted it on his Web site on Monday (1 April).
The first vulnerability, which affects Outlook XP, would allow an attacker to embed "active" content in an e-mail, he wrote. Active content contains both an object and a script. The content embedded in the e-mail would then execute when the e-mail is forwarded or replied to, wrote Guninski, who has uncovered a number of vulnerabilities in Microsoft products in the past.
The vulnerability could force a user to visit a Web page designated by the attacker, Guninski wrote.
The second security hole, which affects the spreadsheet component of Office XP, can be used in conjunction with the first vulnerability to place executable files in a user's start-up directory, which could lead to a takeover of the target machine, he wrote.
Guninski, who lives in Bulgaria, included sample code in his advisory to demonstrate how to exploit both vulnerabilities.
Guninski wrote that he notified Microsoft of the bugs on 17 March, but that the company did not produce a patch in two weeks, triggering his advisory. In the past, Microsoft has criticised Guninski for releasing his vulnerability data too quickly, calling his actions "irresponsible".
Workarounds for the vulnerability include disabling all "active" content in Internet Explorer (which is used by parts of Outlook) and fully deleting the spreadsheet component of Office XP. Guninski also wrote that "the solution is to get a real mail client and office applications".
In a statement Microsoft said that it is investigating the matter, but also acknowledged the accuracy of the first vulnerability.
As a workaround for that flaw, Microsoft recommends disabling HTML e-mail and not selecting Microsoft Word as the e-mail editor.
As for the second vulnerability, Microsoft said it does "not as yet have a workaround," but "that even in the worst case it could only be used to create files - not to execute them or take any other action on the user's computer".
Microsoft added: "We are concerned that this report has gone public before we've had a fair chance to investigate it. Its publication may put our customers at risk or at the very least cause customers needless confusion and apprehension.
"Responsible security researchers work with the vendor of a suspected vulnerability issue to ensure that countermeasures are developed before the issue is made public and customers are needlessly put at risk."