More often than not, a "leaking" filter basket isn't actually leaking. Rather, water coming out of this part of the machine typically stems from a poor seal between the filter basket and the group gasket. A common cause of this in most espresso machines is simply an over-filled basket. As locking the filter in also raises it up slightly, the tamped grounds in a slightly over-filled basket would press into the shower screen before the rim of the filter basket can form a seal against the group gasket. With a weak seal, pressurized water can then slowly trickle out from any of the sides. A quick test for this is to attach an empty portafilter (with a basket in place) to the machine and tighten it firmly (making a note of how far it turns), then try again with normally tamped grounds. If you can turn the handle further to the right without coffee than with, you're over-filling the basket. Similarly, a dirty or old group gasket can also lead to a poor seal where water can force its way through. This should be wiped down once a month and replaced once every one to two years.