There is a lot of buzz going around lately about the vulnerabilities that AJAX can open up on your site. The same technology that allows for a seamless Web experience and is widely responsible for the Web 2.0 surge can allow worms and malware to spread at a ferocious pace - all without the end user having to do anything out of the ordinary.
So what exactly is the issue? Most experts agree that the culprits are inexperienced programmers. In a mad rush to include the technology, many site developers are not properly installing the software - leading to major security holes that hackers can use to secretly install damaging code and spread it fast. If you're using AJAX, be sure that it was properly installed and you may want to have an expert reassure you.
Some good security information can be found at Ziff Davis Internet's Security IT Hub.