Microsoft has decided to stop its scrap with AOL about messaging services, claiming that to continue would pose a serious security risk for its users. The security claim was made by Yusuf Mehdi, Microsoft director of internet marketing, who said that AOL was using a "very serious security bug" to stop MSN users trying to send messages to AOL Instant Messenger users. Mehdi's point is that AOL's ability to identify MSN users constitutes a security bug. The two companies have been engaged in an arms race over instant messaging since Microsoft introduced its own service with AOL interoperability. This has been an an-off feature as AOL's programmers introduced mechanisms to block MSN users, then Microsoft's programmers worked to get around AOL's routines, and so on. But in the case of instant messaging Microsoft finds itself in an unusual position. AOL is the market leader here, and holds the proprietary cards. Microsoft wants an open industry standard that will allow instant messages to be sent to and from different systems, but under the circumstances this doesn't look like something AOL is going to rush into. So for the moment Microsoft has retired to its tent, issuing Version 2.0 of its MSN Messenger Service this week, with the attempts at interoperability taken out. ® Related stories Mail wars: the Microsoft versus AOL square-off Open source supremo backs MS as Apple sides with AOL