AVG Technologies is offering a free software tool to protect Twitter users from malicious sites that lurk behind shortened URLs.

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LinkScanner checks out URLs before browsers actually connect to Web sites, stripping out any malware they may contain. It first matches URLs against an internal black list of known harmful sites, then filters traffic as it comes in from Web sites, says Roger Thompson, chief research officer at AVG.

It looks for 800 to 900 exploits that are used over and over again by hackers. The software connects with AVG’s network periodically to update its library.

URL shortening tools such as TinyURL make it possible to post a URL of any length in 25 characters so they fit within a 140-character Twitter message. But that obscures the name of the actual site, which might be a clue that the site is dangerous.

The AVG tool screens any Web site, not just those being reached through Twitter.

Plug ins for browsers Firefox and Internet Explorer reveal the underlying URLs so users can bail out of connecting with them if they recognize them as malicious or suspect they are. But these plug ins don’t actually filter traffic from these sites for malware.

LinkScanner works in concert with anti-virus software, Thompson says. The free tool isn’t sufficient to protect Internet connected machines, so he hopes that by getting LinkScanner for free that users will consider buying AVG’s broader security suite that includes anti-virus software.