How do I restrict external users to view other projects?

[Salman Naqvi]

I do not want external users to view other projects. Like we have a projectA, what I want is that an external user when he logs in can only see ProjectA and nothing else and he does not even have the option to go to other projects using the drop down that appears at the top left side.

Please let me know how to do this, as this is urgent?

[Elvir Hadzic]

You'll want to have a look at this: http://confluence.atlassian.com/display/JIRA/Managing+Project+Permissions

If a user has no permission for a project he also won't see it in the drop down.

Edit: The permission you'll have to use/check is "Browse Projects". Depending on your standard security scheme and groups, external users might already not be able to see projects they're not directly involved with.