Microsoft was alerted to the programming bug earlier this week and assured that it would have a fix by the end of today, according to the Associated Press.
The flaw in its MSN Messenger software allows anyone with access to another person's computer to read and send email from that person's Hotmail account without his or her knowledge. Hotmail is Microsoft's free, Web-based email service.
The bug can be exploited by stopping the Hotmail email page from loading and viewing the underlying HTML software code, exposing the user's email name and password, according to the report.
The problem is most easily exploited in an office setting where coworkers share or access each other's computers. The bug's effects are blunted for home computer users since actual physical access is necessary to take advantage of the flaw.
The battle between Microsoft and AOL ignited almost immediately after the software giant launched its MSN Messenger last month. One of the key features of the new service was its ability to connect with AOL's popular Instant Messenger service, commonly known as AIM. AOL, however, countered by blocking the rival service from accessing its Buddy Lists.
Industry analysts have noted how crucial it is for rival IM services to be able to connect to AIM given its 40 million members.
The two companies have since fired volleys back and forth, with MSN offering "fixes" to punch holes in AOL's blockades and AOL erecting new barriers.
Earlier this week, Microsoft said it will publish the protocol for its MSN Messenger service, hoping to enlist the help of third parties to help push its efforts in the messaging wars.