Some points before the answer.
You may rethink your strategy on frameworks. Modern frameworks like Zend Framework and also Symfony2 are loosely coupled. This means: you dont need to download/use the whole framework.
Another reason to rethink it, is that these frameworks are often robust, widely tested and used. In terms of performance they are not always of huge impact, especially with apc and other optimization methods, so are you affraid you will use too much harddisk space?
Your question is about something specific but easily done. This is exactly the type of stuff that frameworks are good at. Codeigniter, Zend Framework, Symfony, Symfony2 (am I missing one?), all offer code that will allow you to do this in maximum a matter of hours.
Libraries I would consider
Zend_Auth (has only zend_exception as hard depencencies). It's lightweight and has many options for integration with openauth and such. (requires about 10 php files) http://framework.zend.com/manual/en/zend.auth.html
Apache ZetaComponents (before ezcomponents)
http://incubator.apache.org/zetacomponents/documentation/trunk/Authentication/tutorial.html
Pear LiveUser
http://pear.php.net/package/LiveUser/download/
If you were to consider codeigniter, check this out: http://codeigniter.com/wiki/Category:Libraries::Authentication/
