SAN FRANCISCO - Hoping to deal with a growing problem, Twitter has quietly introduced a feature to prevent users from posting links to malicious Web sites. But security experts say that it can be easily circumvented.The feature was first noticedMonday by Mikko Hypponen, chief research officer with security company F-Secure. When someone tries to post a link to a malicious Web site, Twitter pops up a short notification saying "Oops! Your tweet contained a URL to a known malware site," and, after a few seconds, deletes the post.Twitter is using Google's Safe Browsing API to check for malicious links, a Google spokesman confirmed Monday.F-Secure says it's recommended that Twitter start doing this because the site "is increasingly targeted by worms, spam and account hijacking," according to Hypponen's blog post. A month ago, technology entrepreneur Guy Kawasaki's account was misused to post a link to a malicious Web site. In recent weeks users have been hit with links to fake, and sometimes malicious, "rogue" security software.Security experts said Monday that while Twitter's filtering is a good first step, it still needs some work.In tests, the feature blocked a URL that led to a phishing site, but it allowed the same link to post if it was shortened using services such as Tinyurl.com or Bit.ly. Because Twitter enforces a strict 140 character limit on each message, these URL shortening services are the most common way of posting links to Twitter.The filter also permitted the phishing link when the "www" subdomain was stripped from the front of the URL.Twitter did not return messages seeking comment."This is a common problem with this sort of filtering service," said Chris Boyd, director of malware research with FaceTime Security Labs.However, even if Twitter isn't blocking malicious URLS when they've been shortened, users still get some protection. That's because some of these URL-shorteners use the Google's API themselves. Bit.ly, which is used to post more than half of all Twitter links uses the API to block people from visiting malicious sites, for example.Boyd said it will probably take Twitter "a while" to get its Web filtering up and running properly, "but even some protection is better than none."