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what doubts do you have?
you ask if XOOPS is safe & secure? that's like asking if your house is secure.
but to answer your question, it's secure compared to the likes of php-nuke and other scripts. but if you're under the impression of it being 100% secure then don't be.. nothing is secure and a knowledgable and determined hacker will most likely break the security.. but it's enough to keep the vast majority of script kiddies, wannabe;s who just got a new tool etc away..