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The SecureTwitter component is wrapped into SecureBrowsing, a plug-in for either the Firefox or Internet Explorer browsers, said Yuval Ben-Itzhak, Finjan's CTO.

SecureTwitter is designed to warn people about links that people post on the micro-blogging service. Because of Twitter's 140-character limit, most of the URLs (Uniform Resource Locators) posted have been shortened using services such as Bit.ly or TinyURL.

Those services completely obscure the true destination of the link, which is dangerous since users have no idea that they could be directed straight to a site that will look for software vulnerabilities in order to infect the PC with malware.

Even if a URL isn't shortened, it's nearly impossible to tell if a site may host malware since many legitimate sites have been hacked, too.

"The hacker is taking advantage that their content is now being served by legitimate sites, and there's high traffic on these sites today," Ben-Itzhak said. "This is how they distribute their malicious code."

SecureBrowsing shows either a green check next to a link indicating that the target Web site is fine or a red "X" if it's bad. SecureBrowsing may also show a question mark if the site can't be scanned.

Upon visiting Twitter, users will see a rotating circle as Finjan is scanning the URLs. The links are sent to Finjan's data center, which performs the scan and reports back within a couple of seconds. A pop-up window contains a link that will take a user to Finjan's site for an explanation of why the site has been flagged as bad.

SecureBrowsing will also scan links in other Web-based services and social networking sites such as Bebo, Digg, Slashdot, MySpace, Gmail and for Google and Yahoo searches.

Finjan has made SecureBrowsing free for consumers since it sells an enterprise appliance under the Vital Security brand that will scan Web links, monitor Web traffic and control bandwidth usage, among other functions.

A couple of other vendors have products similar to SecureBrowsing. AVG has a free product called LinkScanner, and McAfee has one called SiteAdvisor, both of which are intended to give users more information about the safety of the sites they're considering browsing. Neither LinkScanner nor SiteAdvisor scan Twitter links.